distribution and innervation of lateral line organs in the channel catfish

Distribution and Innervation of LateralLine Organs in the Channel Catfish

R. GLENN NORTHCUTT,1* PRESTON H. HOLMES,1AND JAMES S. ALBERT2

1Neurobiology Unit, Scripps Institution of Oceanography, and Department ofNeurosciences, University of California at San Diego, La Jolla, California 92093-0201

2Nippon Medical School, Tokyo, Japan 113

ABSTRACTThe lateral line system of the channel catfish is formed by mechanoreceptive neuromasts

located within five pairs of cephalic and one pair of trunk canals, as well as superficial linesof neuromasts, termed accessory and/or pit lines. Five pairs of pit lines occur on the head, andthree pairs of superficial lines occur on the trunk. In addition to these mechanoreceptors,which are found in most teleost fishes, catfish also possess a total of over 4000 electrorecep-tive ampullary organs scattered over the entire body. The lateral line receptors are inner-vated by five pairs of lateral line nerves whose rami are secondarily associated with facial andtrigeminal fibers that innervate taste buds and the dermis of the skin, respectively. Theneuromasts of the trunk canal and the ramules of the posterior lateral line nerve thatinnervate them seem to be organized in a segmental pattern. The same is true for theintervertebral ramules of the recurrent facial ramus, which innervate the external taste budson the trunk. The fibers of the gustatory and lateral line systems may use the neural crest,the developing spinal nerves, or both, to establish this segmental pattern. In this context, itmay not be surprising that there is an intimate relationship among each of the sensorysystems in the trunk. J. Comp. Neurol. 421:570–592, 2000. © 2000 Wiley-Liss, Inc.

Indexing terms: ampullary organs; cranial nerves; electroreceptors; neuromasts

The lateral line system of many fishes and amphibiansconsists of mechanoreceptive neuromasts and electrore-ceptive ampullary organs (Hetherington and Wake, 1979;Fritzsch and Wahnschaffe, 1983; Northcutt, 1986, 1992a;Coombs et al., 1988). Neuromasts are composed of a cen-trally elongated strip of directionally sensitive hair cellssurrounded by a peripheral zone of mantle and supportcells, and they are stimulated by low frequency watermovements parallel to the major axes of the receptors(Flock, 1965; Denton and Gray, 1989). An outgroup anal-ysis (i.e., a step-wise examination of a given trait, begin-ning with the most closely related taxon and progressingto more distantly related taxa) of living and fossil fishes(Northcutt, 1989, 1997) indicates that the neuromasts ofthe earliest fishes were arrayed in lines and housed in acomplex series of grooves extending over the head andonto the trunk. These fishes apparently possessed at least11 lines of neuromasts on the head and up to three lines ofneuromasts on the trunk. The subsequent phylogenetichistory of neuromast lines is characterized by four trends:(1) the failure of grooves to form, resulting in superficiallines of neuromasts; (2) a reduction in one or more seg-ments of a line of neuromasts; (3) the loss of an entire lineof neuromasts; and (4) the origin of adjacent lines of neu-

romasts (accessory neuromast lines). Both reduction andcomplete loss of one or more neuromast lines has occurredin every group of extant fishes and amphibians. Interest-ingly, teleost fishes exhibit the most extensive reduction,loss, or both, of phylogenetically older neuromast lines butare also the only group of fishes to have evolved newaccessory neuromast lines (Coombs et al., 1988; Puzd-rowski, 1989).

Electroreceptive ampullary organs, the second class oflateral line receptors, occur widely among fishes (lam-preys, cartilaginous fishes, sturgeons and paddlefishes,and lungfishes, Northcutt, 1986; New, 1997), as well as intwo of the three orders of living amphibians (Fritzsch andWahnschaffe, 1983; Wahnschaffe et al., 1985; Northcutt,1992a). In all of these taxa, ampullary organs occur asepidermal invaginations in which a lumen, open to the

Grant sponsor: NRSA; Grant number: training grant 5 T3 GM08107-14;Grant sponsor: NIH; Grant number: research grant 5 R01 NS24669.

*Correspondence to: R. Glenn Northcutt, Neurosciences, 0201, Univer-sity of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0201. E-mail: [email protected]

Received 25 October 1999; Revised 26 January 2000; Accepted 31 Jan-uary 2000

THE JOURNAL OF COMPARATIVE NEUROLOGY 421:570–592 (2000)

© 2000 WILEY-LISS, INC.

skin surface, ends as a deep epithelial ampulla consistingof support and receptor cells. The apical surface of eachreceptor cell usually bears a single kinocilium (Jørgensen

et al., 1972; Teeter et al., 1980; Northcutt and Brandle,1995) and, frequently, modified microvilli (stereocilia).Several surveys (Fritzsch and Wahnschaffe, 1983;

Abbreviations

af adipose finagc anterior ganglionic complexAN accessory neuromastan anterior narisanf anal finao ampullary organAP accessory pit canal and lineap apical pore of ampullary organar anterior ramule of the superficial ophthalmic ramusaro anterior ramus of octaval nerveATL accessory trunk lineAV anteroventral lateral line nervebuc buccal ramus of anterodorsal lateral line nervec corium of taste budcc corpus cerebellicct cerebellar crestcf caudal finch caudal ramule of hyoid ramuscp trunk canal pored dorsal ramus of posterior lateral line nervedf dorsal findgVII dorsal subdivision of facial sensory ganglionDL dorsal trunk linedmd deep subdivision of the anteroventral lateral line nervedr dorsal ramule of lateral ramus of posterior lateral line

nervedra dorsal ramule of dorsal somatic ramusdrg dorsal root ganglion of spinal nervedsr dorsal somatic ramus of spinal nervedV descending trigeminal tractec epithelial canaleg eminentia granularisell electrosensory lateral line lobeEN ethmoid neuromastfl facial lobefr fin rays of caudal fingAD sensory ganglion of anterodorsal lateral line nervegAV sensory ganglion of the anteroventral lateral line nervegM sensory ganglion of middle lateral line nervegO sensory ganglion of otic lateral line nervegP sensory ganglion of posterior lateral line nervegV trigeminal sensory gangliongVII sensory ganglion of facial nervegIX sensory ganglion of glossopharyngeal nerveh hyoid ramus of facial nervehyo hyomandibular trunkib inner buccal ramus of anterodorsal lateral line nerveil inferior lobeim inferomedial strandIO infraorbital canalivr intervertebral ramule of facial recurrent ramusl lateral ramus of posterior lateral line nervelr lateral ramule of a ventral somatic ramuslg lateral sensory ganglia of vagal nervelll lateral line lobesm middle lateral line nervema mantle cells of lateral line organsmab mandibular barbelman mandibular ramus of trigeminal nerveMAP mandibular pit linemax maxillary ramus of trigeminal nervemeb mental barbelmg medial sensory ganglion of vagal nervemh medial ramule of hyoid ramusmon medial octavolateral nucleusMP middle pit linemr medial ramule of a ventral somatic ramusmxb maxillary barbelmyo myomeremys myoseptummVII facial motor nucleusnb nasal barbel

ne neural elements of dorsal and ventral archesO otic canalob outer buccal ramus of anterodorsal lateral line nerveoc otic capsuleon optic nerveop olfactory peduncleope opercular ramus of facial nerveor otic lateral line nerveot optic tectumpal palatine ramus of facial nervepecf pectoral finpef position of pelvic finpelf pelvic finpf position of pectoral finpn posterior narispp posterior palatine ramus of facial nervepr sensory root of the posotic lateral line nervesprf profundal nervePRM preoperculomandibular canalpro posterior ramus of the octaval nerveQJ quadratojugal pit linerh rostral ramule of hyoid ramusri ribrp sensory root of the posterior lateral line nerverr recurrent ramus of the facial nerves support cells of lateral line organssAD sensory root of anterodorsal lateral line nervesc sensory cells of lateral line organsse stratified squamous epitheliumsf sensory fibers of facial nervesl superolateral strandsm sensory macula of neuromastsmd superficial subdivision of the combined anteroventral lat-

eral line and facial nervessn first spinal nerveSO supraorbital canalso superficial ophthalmic ramus of anterodorsal lateral line

nervesp sensory papillaspc spinal cordsr sensory roots of preotic lateral line nervesst sensory fibers of trigeminal nerveT temporal canaltb taste budTC trunk canaltd dorsal ramules of terminal dorsal and ventral somatic

ramitds terminal dorsal somatic ramustel telencephalontf trigeminal fibers that anastomose with the anterior ram-

uletlc trunk lateral line canaltv ventral ramules of terminal dorsal and ventral somatic

ramitvs terminal ventral somatic ramusv ventral ramus of posterior lateral line nervevgVII ventral subdivision of facial sensory ganglionvir visceral rami of vagal nerveVL ventral trunk linevl vagal lobeVLN vertical line of neuromasts at base of the caudal finvr ventral ramules of lateral and ventral rami of posterior

lateral line nervevra ventral ramule of a dorsal somatic ramusvsr ventral somatic ramus of spinal nervevt vertebraVIII octaval nerveIX glossopharyngeal nerveX vagal nerve1b first branchial ramus of vagal nerve2b second branchial ramus of vagal nerve3b third branchial ramus of vagal nerve

571CHANNEL CATFISH LATERAL LINE SYSTEM

Wahnschaffe et al., 1985; Northcutt, 1986, 1992b; New,1997) indicate that, primitively, ampullary organs arerestricted to the head (except in lampreys and lungfishes)and exhibit their highest density on the snout and aroundthe eyes. Their distribution is not random, as they areclosely associated with the neuromast lines and arise fromthe same placodes as do the neuromasts (Northcutt andBrandle, 1995; Northcutt et al., 1995). Ampullary electro-receptors were apparently lost with the origin of neoptery-gian fishes, because they are not present in bowfins andgars, the living representatives of the oldest neopterygianradiation (McCormick, 1982; Bullock et al., 1983), nor dothey occur in most living teleosts, the last and most suc-cessful group of neopterygians to evolve. In two groups ofteleosts, some osteoglossomorph and ostariophysan fishes,however, electroreceptors seem to have independently re-evolved. Teleost electroreceptors differ extensively fromthose of nonteleost fishes and amphibians with regard totheir histology, innervation, central projections, and phys-iology (Maler et al., 1973; Szabo, 1974; Roth and Tscharn-tke, 1976; Hetherington and Wake, 1979; Munz et al.,1984; Bullock and Heiligenberg, 1986).

The cranial nerves that innervate lateral line receptorsalso exhibit considerable phylogenetic variation. Althoughlateral line receptors were initially thought to be inner-vated by rami of the facial, glossopharyngeal, and vagalnerves (Herrick, 1899; Coghill, 1902; Norris, 1925), sub-sequent descriptive and experimental studies (Maler etal., 1973; Boord and Campbell, 1977; Puzdrowski, 1989;Song and Northcutt, 1991; Northcutt, 1992a; Northcuttand Bemis, 1993; Piotrowski and Northcutt, 1996) haverevealed that they are instead innervated by up to sixpairs of distinct lateral line nerves. An outgroup analysisof variation in the number of lateral line nerves and thepattern of electroreceptor innervation indicates that fiveof the six pairs innervate electroreceptors in the primitivecondition (Northcutt, 1986, 1997). Not surprisingly, elec-troreceptors that re-evolved independently in variousgroups of teleost fishes are innervated by a variable num-ber of lateral line nerves (Maler et al., 1973; Bell andRussell, 1978; Carr and Matsubara, 1981; Braford, 1986;Northcutt and Vischer, 1988; New and Singh, 1994). Inaddition to variation in their innervation of electrorecep-tors, lateral line nerves in teleosts vary in other ways: oneor more lateral line nerves can be lost or secondarily fusedwith another lateral line nerve or closely associated bran-chiomeric nerve. However, unlike lateral line receptors, nonew lateral line nerves have evolved.

Over the past 20 years, a wealth of information regard-ing the variation in lateral line receptors and their inner-vation has been generated, and researchers have alsoidentified where in vertebrate phylogeny major changeshave occurred. However, little progress has been made indetermining how these phylogenetic changes occur. Onto-genetic studies will allow us to identify the embryonicsource(s) of lateral line receptors and nerves and discernthe mechanisms responsible for their change over time.Garstang (1922) was the first to discern the fundamentalrelationship between ontogeny and phylogeny when herealized that phylogeny is not a succession of adult forms.Rather, it is the result of successive changes over time inan ancestral life history. Thus, ontogeny does not recapit-ulate phylogeny; changes in ontogenies create phylogeny.Because the ontogeny of a common ancestor cannot beexamined, the ontogenies of an ancestor’s living descen-

dants must be examined. An outgroup analysis of multipleontogenies (Northcutt, 1992b) can determine which stagesare primitive (i.e., were present in a common ancestor)and which are derived (i.e., have arisen subsequently). Tounderstand the ontogenetic changes underlying interspe-cific variation in neuromast lines, the loss and re-evolution of electroreceptors, and the innervation of bothtypes of lateral lines, it is necessary to compare the ontog-enies of a species that develops both neuromasts andprimitive electroreceptors, a species that develops neuro-masts but not electroreceptors, and a teleost species thatdevelops neuromasts and the newly evolved electrorecep-tors. Salamanders are the only group of vertebrates inwhich it is practical to study the development of primitiveelectroreceptors, and an extensive descriptive and exper-imental literature (Stone, 1922; Smith et al., 1988, 1990;Northcutt, 1992a; Northcutt and Brandle, 1995; Northcuttet al., 1995) exists. Although numerous neopterygianfishes are suitable for examining a species that developsneuromasts but no electroreceptors, channel catfishes (Ic-talurus punctatus) are an ideal group in which to examinethe development of the newly evolved electroreceptors.Channel catfish have the advantage that they are raisedcommercially so that embryos are available in large num-bers, and its developmental stages have been described(Armstrong and Child, 1962).

Although Herrick recognized the importance of develop-mental studies, he also believed “that studies in develop-ment should be preceded by a thorough knowledge of theadult structures involved,” (Herrick, 1901, p. 179). Accord-ingly, Herrick undertook a detailed study of the cranialnerves and cutaneous sense organs of the black bullheadcatfish, Ameirus melas. Although it is possible that thereare no major neuroanatomical differences between Amei-rus and Ictalurus, developmental studies of channel cat-fishes cannot be undertaken with confidence in the ab-sence of a detailed description of their adult lateral linesystem. Furthermore, even if there are no major differ-ences in the lateral line system of these two genera, newdescriptive and experimental neuroanatomical techniquesnow exist that were not available to Herrick. For thesereasons, a detailed study of the lateral line system ofchannel catfishes was undertaken as a necessary pream-ble to the study of the development of this system.

MATERIALS AND METHODS

All experiments and observations involved juvenile andadult channel catfish, Ictalurus punctatus, obtained fromCarolina Biological Supply Company in Burlington, NorthCarolina. All procedures were approved by the UCSDAnimal Care and Use Committee and conform to NIHguidelines.

To identify the different morphological classes of lateralline receptors and map their distribution, juveniles (2.5 to11.5 cm in total length) were anesthetized in a dilutesolution of tricaine methane-sulfonate (MS222, SigmaChemical Co., St. Louis, MO) and fixed by immersion in4% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4).After fixation for at least 1 week, the animals were rinsedin phosphate buffer, and flat-mounts of the head andtrunk skin (Lannoo, 1985; Northcutt, 1992a) were pre-pared according to the following procedure. Mid-dorsaland mid-ventral incisions were made along the entirelength of the animal, and the skin was carefully separated

572 R.G. NORTHCUTT ET AL.

from the connective tissue and muscle by using number 5Dumont forceps (Roboz Surgical Instrument Co., Inc.,Washington, DC). To achieve this separation, one tine ofthe forceps was inserted into the dorsal or ventral midlineincision, parallel to the inner surface of the skin, thenslowly oscillated and advanced under the skin, separatingit from the underlying tissues. The skin was placed in 3%hydrogen peroxide for approximately 12 hours to bleachpigment cells that would otherwise obscure details of thelateral line receptors. The skin samples were then washedin 0.1 M phosphate buffer and stained in l% methylenegreen (Sigma). The samples were stained free-floating andthen dehydrated in a graded series of ethanols and cover-slipped on slides. Maps of the distribution of the receptors(Figs. 1, 3) were prepared by tracing the projected imagesof the skin samples with the aid of a camera lucida (Olym-pus Optical Co., Ltd., Tokyo, Japan).

The skin of juveniles up to 12 cm in total length remainssufficiently thin to visualize easily the details of the largerelectroreceptors and taste buds but not the neuromasts.To make more detailed histologic observations of the lat-eral line receptors, two 55-day posthatching larvae (3 to 4cm in total length) were fixed by immersion in 4% glutar-aldehyde in 0.1 M phosphate buffer (pH 7.4). After fixa-tion, the heads were washed in distilled water for 12 hoursand then dehydrated in a graded series of ethanols. Whenthe specimens reached 95% ethanol, they were infiltratedand embedded in glycol methacrylate (Leica Instruments,Heidelberg, Germany), cut into serial sections (4 to 5 mm),and stained with 1% cresyl violet (Sigma).

The course of the peripheral rami and the position of theganglia of the cranial nerves that innervate the lateralline receptors were visualized by several different ap-proaches: gross dissection, staining of the nerves in wholecleared animals by Sudan Black (Filipski and Wilson,1984), tracing the individual rami in serial transversesections of the whole head of a juvenile (7.8 cm totallength) stained by the Bodian-reduced silver procedure(Senn, 1968), or similar tracing in glycol methacrylate-embedded sections of the 55-day posthatching larvaand/or serial transverse sections stained with cresyl violetof the pre- and postotic cranial ganglia of an adult catfish.

Finally, the rami of many of the cranial nerves sus-pected to contain fibers innervating lateral line receptorswere labeled with horseradish peroxidase (HRP, Sigma,type VI) to determine whether these rami contain fibersthat terminate centrally in the electrosensory lateral linelobes (electroreceptors) or the medial octavolateralis nu-cleus (mechanoreceptors), the known primary targets ofthe afferent lateral line fibers (Finger and Tong, 1984;New and Singh, 1994).

When HRP was used to label various rami, the animalswere anesthetized in a dilute solution of MS222, a flap ofskin was reflected, and the connective tissue or musclewas retracted to expose a nerve ramus. The ramus wastransected, and a small piece of Gelfoam (Upjohn Co.),saturated with a solution of 40% HRP dissolved in 1%lysophosphatidylcholine (Sigma) in distilled water, wasapplied to the proximal stump of the ramus. The incisionwas closed and sutured. Care was taken that the HRP-soaked Gelfoam did not come into contact with other rami.After survival times of 4–14 days at 26–28°C, the animalswere reanesthetized in a dilute solution of MS222 andperfused transcardially with cold 0.1 M phosphate buffer(pH 7.4), followed by 4% glutaraldehyde dissolved in phos-

phate buffer. The brains and nerves were removed, post-fixed for 1 to 3 hours in the same fixative, then embedded,cut into 35-mm transverse sections, and reacted (Adams,1981) as free floating sections. The sections were thenmounted on chrome-alum coated slides and counter-stained with 1% neutral red (Sigma).

The following nerves, trunks, or rami were labeled withHRP to establish the sensory components carried in them:superficial ophthalmic ramus (N3), buccal and maxillaryrami (N6), mandibular ramus (N5), profundal nerve (N3),mandibular ramus (N5), hyomandibular trunk (N4), hyoidramus (N2), deep mandibular ramus (N2), superficialmandibular ramus (N2), posterior lateral line nerve (N6),recurrent facial ramus (N5), and visceral rami of vagalnerve (N2). We were not able to label the otic or middlelateral line nerves, nor did we try to label the dorsal andventral rami of the posterior lateral line nerve.

Computer graphics applications were used in the prep-aration of photographic material. Operations included ad-justment of brightness, contrast, and color tone, applica-tion of labels, and masking of background regions by usingAdobe Photoshop 5.0.2 (Adobe Systems, San Jose, CA).

RESULTS

The lateral line system of channel catfishes consists oftwo morphological classes of receptors: canal and superfi-cial mechanoreceptive neuromasts and electroreceptiveampullary organs. The morphology and distribution ofthese receptors are described first (Figs. 1–4), followed bya description of the rami and ganglia of the cranial nervesthat innervate these receptors (Figs. 5–13).

Morphology of receptors

Both canal and superficial neuromasts (Fig. 4B,C) arelarger than ampullary organs (Fig. 4A,D,E), and bothtypes of neuromasts are elliptically shaped with an elon-gated central zone of hair cells that forms a sensory mac-ula (Fig. 4B,C). The hair cells of the macula (Fig. 4C) arescattered among support cells, and the outer borders ofboth types of neuromasts are surrounded by very smallmantle cells (Fig. 4B). Measurements of the major axes ofsuperficial neuromasts, termed large pit organs by Her-rick (1901), on the head and trunk in the flat-mountedskin of a single 11.5-cm-long individual resulted in meansof 149 mm 6 SE 7.4 (N11) and 134 mm 6 SE 4.5 (N23),respectively. Thus, cephalic superficial neuromasts areslightly larger than trunk superficial neuromasts, as arethe major axes of their sensory macula: x 5 79 mm 6 SE4.4 (N11) and x 5 68 mm 6 SE 2.2 (N23), respectively.Although it is impossible to measure canal neuromasts inflat-mounts of the skin, measurements of the major axis ofcephalic canal neuromasts from serial sections of asmaller individual (7.8 cm total length) indicate that canalneuromasts are larger (x 5 205 mm 6 SE 13.0, N10) thaneither cephalic or trunk superficial neuromasts.

The smallest class of lateral line organs are the ampul-lary organs on the head and trunk (Fig. 4A,D,E), termedsmall pit organs by Herrick (1901), with a mean diameterof 100 mm 6 SE 2.7 (N20) and 110 mm 6 SE 3.6 (N20),respectively. Ampullary organs consist of a short, invagi-nated epithelial tube, open to the skin surface by an apicalpore (Fig. 4D,E) that ends as a slightly swollen sensorybulb within the epidermis. The epithelial tube is en-


Fig. 1. Camera lucida drawings showing the position of lateral linereceptors on the head of a juvenile channel catfish (6.8 cm totallength). A: Each neuromast, whether within a canal or on the surfaceof the skin, is indicated by a solid oval. The short tubes branchingfrom the main canals are pores where the canals open to the surface.

Generally only a single neuromast is located within a canal betweentwo pore openings. B: The positions of individual electroreceptiveampullary organs (black dots) are mapped in relation to the moredeeply located lateral line canals indicated in gray. For abbreviations,see list. Scale bar 5 2 mm in B (applies to A,B).


sheathed in small mantle cells (Fig. 4E), comparable tothe mantle cells of neuromasts, and the sensory bulb con-sists of sensory and support cells (Fig. 4E).

In flat-mounts of the skin (Fig. 4A), ampullary organsmight be confused with taste buds (Fig. 4A,D,F) whichalso occur in the epidermis of the head and trunk. Tastebuds, termed terminal or end buds by Wright (1884a) andHerrick (1901), are slightly smaller than ampullary or-gans; however, when measured in the same individual(the mean diameter of cephalic taste buds was 86 mm SE4.0, N20 and the mean diameter of trunk taste buds was79 mm SE 4.1, N19). Ampullary organs can also be distin-guished from taste buds in flat-mounts because ampullaryorgans possess apical pores (Fig. 4A,E), whereas tastebuds exhibit sensory papillae (Fig. 4F) that are elevatedabove the surface of the epidermis. In transverse sectionsof the skin (Fig. 4D,E,F), the histological differences be-tween ampullary organs (Fig. 4E) and taste buds (Fig. 4F)

are even more pronounced. Taste buds are solid cellularorgans composed of an inner corium and an outer layer ofstratified squamous epithelium, and the receptor cells oftaste buds are located superficially in a sensory papillarather than at the bottom of a pit as is the case forampullary organs.

Distribution of lateral line organs

Cephalic and trunk neuromasts (Figs. 1A, 2) may occurin canals or as superficial lines. On both the head andtrunk, only a single neuromast occurs between short canalpores that open to the surface. Canal neuromasts on thehead occupy five main canals (Fig. 1A). Four neuromastsoccur in a supraorbital canal (SO) located above the orbit,and six neuromasts occur in an infraorbital canal (IO)below the orbit. These canals join caudal to the orbit,forming an otic canal (O) that houses a single neuromast.Slightly more caudal, the otic canal is joined by the more

Fig. 2. Camera lucida drawings of the rostral (A) and caudal (B)trunk of the same juvenile channel catfish illustrated in Figure 1showing the position of individual canal and superficial neuromasts.Note that the major axes of the neuromasts of the short dorsal line(DL), as well as those of the trunk canal (TC) and ventral line (VL),are parallel to the longitudinal axis of the body and are sensitive to

relative movement of water only in the longitudinal axis, whereas themajor axes of the neuromasts of the short accessory lines (ATL) abovethe trunk canal are perpendicular to all the other neuromasts and aresensitive to relative movement of water in the dorsal-ventral axis. Forabbreviations, see list. Scale bar 5 4 mm in B (applies to A,B).


ventrally located preoperculomandibular canal (PRM),which houses eight neuromasts and is the only canal lo-cated on the lower jaw and cheek region. A single canal,the temporal canal (T), which also houses a single neuro-mast, continues caudally after the fusion of the otic andpreoperculomandibular canals. The temporal canal inturn is continuous with the trunk canal (TC, Fig. 2), whichhouses approximately 100 neuromasts, with a single neu-romast occurring per body segment. In addition to the fivemain cephalic canals, there is a short canal that enclosesone of the three neuromasts of the anterior pit line (AP,Fig. 1A). Pit lines usually occur as superficial lines ofneuromasts that are believed to represent phylogeneti-cally older lines that primitively were enclosed in canals(Northcutt, 1989). Among ray-finned fishes, catfishes areunusual in that part of their anterior pit line forms acanal.

The remaining neuromasts on the head occur as super-ficial neuromasts. Rostrally, a single neuromast is locatedmedial to the supraorbital line (EN, Fig. 1A), which seemto be homologous to the ethmoid line of other teleosts and

two superficial neuromasts located ventral to the anteriornaris, which seem to be the most rostral neuromasts of theinfraorbital line based on their innervation (see next sec-tion). Two additional short lines of neuromasts occur onthe lower jaw between the infraorbital and preoperculo-mandibular canals. On the basis of their topography, it ispossible that these two lines are homologous to the man-dibular and quadratojugal (QJ) pit lines (Fig. 1A). There isan additional pit line of two neuromasts dorsally, which ishomologous to the middle pit line (MP) of other fishesbased on its position and innervation (see next section).However, channel catfishes seem to have lost posteriorand gular pit lines, as well as a supratemporal line.

The superficial neuromasts of the trunk (Fig. 2) areorganized into four groups. Three neuromasts located ros-tral to the dorsal fin form an unusually short dorsal trunkline (DL, Fig. 2A). Associated with the trunk canal, thereare vertically oriented accessory neuromast lines (ATL,Fig. 2A,B), which occur at intervals of six to eight bodysegments, and horizontally oriented neuromasts, whichform a ventral trunk line (VL, Fig. 2A,B). The spacing of

Fig. 3. Camera lucida drawings of the rostral (A) and caudal (B) trunk of the same catfish, indicatingthe distribution of individual electroreceptive ampullary organs. The position of the more deeply locatedtrunk canal is indicated in gray. For abbreviations, see list. Scale bar 5 4 mm in B (applies to A,B).


Fig. 4. Photomicrographs of flat-mounted skin (A,B) and 5-mmplastic-embedded sections of ampullary organs (A,D,E), canal (C) andpit line (B) neuromasts, and taste buds (A,D,F). The ampullary organsand taste buds illustrated in D are located at the base of a maxillary

barbel, which was sectioned in the transverse plane, as were theorgans in C–F. For abbreviations, see list. Scale bar 5 100 mm in A;50 mm in B; 25 mm in C–F.


the neuromasts of the ventral trunk line is highly vari-able. A single neuromast may occur at one, two, or eventhree body segment intervals. Finally, there is a verticalline of nine horizontally oriented neuromasts at the baseof the caudal fin (VLN, Fig. 2B).

Approximately 1,000 ampullary organs occur on eachside of the head in juvenile channel catfish (Fig. 1B). Themajority of these receptors are located adjacent to theneuromast lines, with the exception of a field of ampullaryorgans located on the hypobranchial surface of the lowerjaw. Slightly fewer ampullary organs, approximately 800,occur on each side of the trunk (Fig. 3). Except for a patchof skin immediately caudal to the pectoral fin (Fig. 3A)that is devoid of ampullary organs, these organs are al-most uniformly distributed on the trunk, with approxi-mately equal numbers occurring dorsal and ventral to thetrunk canal. Unlike neuromasts, ampullary organs occuron the adipose and anal fins and on the distal surfaces ofthe caudal fin (Fig. 3B). It has not been possible to deter-mine the exact number of ampullary organs in adults,because the skin is too thick to treat as a flat-mount, butit is probable that ampullary organs continue to increasein number.

Innervation of lateral line organs

When intact heads and trunks stained with Sudan black(Fig. 5) are compared with gross dissections of the brainand sensory ganglia of the cranial nerves (Figs. 10, 11), itis possible to reconstruct the peripheral course of thenerves that innervate the lateral line organs (Figs. 6–9).Details of the sensory ganglia and peripheral rami of these

nerves were further examined in sets of serial histologicsections through the intact head and trunk. As a finalcheck on the accuracy of these reconstructions, many ofthe rami believed to contain fibers innervating lateral lineorgans were labeled with HRP. The sensory modality(ies)of each ramus that was experimentally examined wasdetermined based on the following considerations: (1) theposition of labeled neurons within the sensory ganglia ofthe cranial nerves (see Fig. 12B for an example); (2)whether or not the sensory root of a given cranial nervecontained labeled fibers; (3) whether or not the variousmedullary sensory tracts contained labeled fibers; and (4)the termination of the labeled fibers within the medulla.The axons of sensory ganglionic cells that innervate elec-troreceptive ampullary organs terminate in the elec-trosensory lateral line lobe, whereas the axons of sensoryganglionic cells that innervate mechanoreceptive neuro-masts terminate in the medial octavolateral nucleus (Fig.12A,C). It was also possible to verify experimentally thata given ramus contained somatic sensory fibers and/orspecial visceral sensory fibers of the gustatory systembased on the presence of labeled fibers entering the de-scending trigeminal tract and the facial lobe, respectively(Fig. 12D). In theory, distinguishing trigeminal sensoryfibers from facial sensory fibers in catfish could have beenconfounded by the termination of trigeminal fibers in thefacial lobe (Kiyohara et al., 1986, 1999). However, this didnot prove to be the case because trigeminal fibers thatterminate in the facial lobe initially course within thedescending trigeminal tract, rather than in the facial sen-sory tract, before leaving the descending trigeminal tract

Fig. 5. Photomicrograph of the cleared head of a juvenile channel catfish stained with Sudan Blackto reveal the peripheral course of the cranial nerves. The palatine ramus of the facial nerve is hidden inthis photomicrograph by the maxillary ramus of the trigeminal nerve. For abbreviations, see list. Scalebar 5 4 mm.


to pass dorsally into the facial lobe. Therefore, it is possi-ble to distinguish facial sensory fibers from trigeminalsensory fibers by the location of labeled neurons withinthe anterior ganglionic complex, by the location of labeledfibers within the primary sensory tracts, and by the way inwhich these fibers enter the facial lobe.

Based on these lines of evidence, as well as an outgroupanalysis of other fishes and amphibians (Song and North-cutt, 1991; Northcutt, 1992a; Northcutt and Bemis, 1993;Piotrowski and Northcutt, 1996), we conclude that there isevidence that lateral line receptors in channel catfish areinnervated by nine rami (Figs. 6, 7) that form five lateralline nerves: anterodorsal, anteroventral, otic, middle, andposterior lateral line nerves.

Anterodorsal lateral line nerve. The ganglion of theanterodorsal lateral line nerve constitutes the most dor-solateral group of sensory neurons within the anteriorganglionic complex (Figs. 10, 12B, 13A). The ganglioniccells of this lateral line nerve (Fig. 13A) can be distin-guished easily from the other neurons in the complex bytheir large size. Measurements of a random sample ofanterodorsal lateral line ganglionic cells in a single 39-cm-long individual reveal a mean diameter of 18.7 mm 6 SE0.32 (N50). Not only are these sensory neurons among the

largest ganglionic cells in the complex, they are scatteredamong fibers of the nerve trunk, which forms a separate,distinct bundle as it passes along the lateral surface of theganglionic complex until its root joins that of the antero-ventral lateral line nerve (Fig. 10). At this point, thecombined roots enter the medulla and turn dorsally toterminate within the electroreceptive lateral line lobe, themedial octavolateral nucleus, and the eminentia granu-laris (Fig. 12A,C).

Superficial ophthalmic and buccal rami (Figs. 5, 6) arisefrom the rostral pole of the ganglion of the anterodorsallateral line nerve (Fig. 10). As the superficial ophthalmicramus passes rostrally, it exits the neurocranium bymeans of a separate foramen and continues rostrallyabove the orbit (Figs. 5, 6). Almost immediately, the su-perficial ophthalmic ramus gives rise to a ramule thatinnervates the first neuromast in the supraorbital canal(Figs. 1A, 6), and it also divides into several smaller fas-cicles that ramify in the skin. The profundal nerve alsocontributes fibers to the first ramule of the superficialophthalmic ramus as this ramule emerges from the ramus(Fig. 6). Because the profundal nerve is joined by a sizablecomponent of facial sensory fibers before its exiting theneurocranium, it is likely that this nerve consists of so-

Fig. 6. Camera lucida drawing of a Sudan Black preparation of another juvenile channel catfish,illustrating details of the cranial nerves. The rami of the various lateral line nerves are indicated in red,and the canal and superficial neuromasts are indicated by solid red dots and open circles, respectively.For abbreviations, see list. Scale bar 5 3 mm.


matic and visceral sensory fibers. Both of these fiberclasses probably also join the first ramule of the superficialophthalmic ramus. If so, fibers of the first ramule inner-vate the skin and taste buds, as well as the first supraor-bital neuromast and the ampullary organs closely associ-ated with this segment of the supraorbital canal. As thesuperficial ophthalmic ramus passes dorsal to the eye, itissues a second ramule, which innervates the second su-praorbital neuromast (Fig. 1A). This ramule also dividesinto several smaller fascicles that ramify in the skin. Asthe superficial ophthalmic ramus continues rostrally be-yond the orbit to innervate the remaining two supraor-bital neuromasts and the single ethmoid neuromast (Fig.1A), it will anastomose with profundal rami at least twomore times. Thus, it is likely that each superficial oph-thalmic ramule also innervates skin and taste buds. Notsurprisingly, application of HRP to the stump of the su-perficial ophthalmic ramus after transection at a midor-bital level retrogradely labels cells in the anterodorsallateral line ganglion, as well as cells in the facial andtrigeminal ganglia whose labeled fibers can be traced intothe facial lobe and descending trigeminal tract, respec-tively.

One additional ramule that arises from the dorsocaudalsurface of the anterodorsal lateral line ganglion (Figs. 6,10) is interpreted as an anterior ramule of the superficialophthalmic ramus. In most bony fishes, this ramule arisesfrom the proximal end of the superficial ophthalmic ramusrather than the position noted in catfish. However, in thelatter, it is interpreted as the anterior ramule because itcan be seen to innervate a single neuromast within a canalthat arises from the caudal end of the supraorbital canal,as well as two superficial neuromasts (Figs. 1A, 6). Inother fishes, this line of neuromasts is termed the anteriorpit line and is always innervated by an anterior ramule ofthe superficial ophthalmic ramus. The anterior ramule isalso joined by a small bundle of fibers that arises from thedorsal surface of the facial ganglion and, a little moredistally, by an additional bundle of fibers that arises fromthe dorsal surface of the trigeminal ganglion (Fig. 10).Thus, the anterior ramule, like the other ramules of thesuperficial ophthalmic ramus, carries somatic and visceralsensory fibers that presumably innervate the skin andtaste buds in the vicinity of the anterior pit line.

The buccal ramus of the anterodorsal lateral line nerve(Figs. 6, 10) also arises from the rostral pole of the an-

Fig. 7. Camera lucida drawing of the trunk of a juvenile channelcatfish stained with Sudan Black to reveal the regions innervated bythe three rami of the posterior lateral line nerve. The trunk canal,indicated in gray, is located ventral to the lateral ramus rostrally butcomes to lie immediately lateral to the ramus more caudally. Thecanal is not drawn here because it would obscure details of the lateral

ramus. The dorsal ramus innervates only a few ampullary organs andsuperficial neuromasts rostrally adjacent to the dorsal fin, whereasthe lateral ramus innervates all the remaining lateral line organslocated dorsal to the ventral ramus. The ventral ramus innervates theneuromasts of the ventral trunk line and all ampullary organs locatedin the skin of the belly. For abbreviations, see list. Scale bar 5 5 mm.


terodorsal lateral line ganglion. Unlike the superficialophthalmic ramus, which courses dorsally to exit the neu-rocranium high on the wall of the orbit, the buccal ramusturns ventrally and exits the neurocranium as the mostdorsolateral component of the superolateral strand (Fig.10). In catfishes, most sensory and motor fibers initiallyemerge from the anterior ganglionic complex in one ofthree major bundles: the inferomedial strand, the supero-lateral strand, or the hyomandibular trunk (Fig. 10). Boththe inferomedial and superolateral strands are composedof facial and trigeminal fibers. As the strands exit theneurocranium, some fibers are exchanged, but the infero-medial strand seems to be primarily composed of fibersthat form the maxillary ramus of the trigeminal nerve andthe palatine ramus of the facial nerve, whereas the su-

perolateral strand is primarily composed of fibers thatform the buccal ramus of the anterodorsal lateral linenerve and the mandibular ramus of the trigeminal nerve.In each case, the buccal, maxillary, and mandibular ramiare composed, in part, of facial fibers that will innervateexternal taste buds. The hyomandibular trunk, the thirdmajor bundle of fibers to leave the anterior ganglioniccomplex, is formed by fibers of the anteroventral lateralline nerve, trigeminal somatic sensory fibers, and facialgustatory fibers.

The facial fibers that constitute part of the buccal ramusof the anterodorsal lateral line nerve arise from the dor-solateral surface of the superolateral strand and initiallycap the buccal ramus (possibly a part of the accessorymaxillary nerve of Herrick, 1901). The buccal ramus alsoseems to be joined by trigeminal fibers that arise from the

Fig. 8. Schematic drawing of the left side of the trunk (rostral is tothe left on the figure) of a catfish, illustrating the intimate relation-ships of the recurrent facial ramus, a spinal nerve, and the lateral andventral rami of the posterior lateral line nerve. Taste buds on thetrunk are innervated by sensory fibers that leave the facial recurrentramus to enter the dorsal root ganglia of spinal nerves by means ofintervertebral ramules. The facial fibers then enter dorsal and ventralsomatic rami of spinal nerves where they continue to the skin inconsort with the somatic sensory fibers of these nerves. There is asimilar relationship between the spinal nerves and the fibers of thelateral and ventral rami of the posterior lateral line nerve. The ram-ules of the lateral and ventral rami anastomose with some of theelements of the spinal nerves, so that somatic sensory, gustatory, andlateral line fibers ramify beneath the skin in the same fiber fascicles.For abbreviations, see list.

Fig. 9. Camera lucida drawing (A) and photomicrograph (B) of thecleared caudal fin of a juvenile channel catfish stained with SudanBlack, illustrating the terminal arborization of the lateral ramus ofthe posterior lateral line nerve. The lateral line fibers that enter andterminate within the caudal fin do so in consort with fibers of thecaudal spinal nerves and facial recurrent ramus. For abbreviations,see list. Scale bar 5 200 mm in B (applies to A,B).


ventrolateral surface of the superolateral strand andmerge with the ventral surface of the buccal ramus. Un-fortunately, all of the experimental cases that might havecorroborated these observations included both the buccaland maxillary rami. In each of these cases, retrogradely

labeled cells were observed in the facial and trigeminalganglia, as well as in the anterodorsal lateral line gan-glion, but labeled cells would have been expected in thefacial and trigeminal ganglia if only the maxillary ramuswere labeled.

Fig. 10. Camera lucida drawing of the lateral surface of the brainand anterior ganglionic complex of an adult channel catfish. Theanterior ganglionic complex, a derived feature of teleost fishes, isformed by the partial fusion of the ganglia of the preotic lateral linenerves (solid circles), the ganglion of the trigeminal nerve (open cir-cles), and the ganglion of the facial nerve (open triangles). The extentof each of the ganglia is based on analysis of serial sections of a secondindividual of approximately the same length. Only the ganglia of thepreotic lateral line nerves can be seen in entirety because of theirlateral, superficial position within the complex. The total length of themore medially located trigeminal ganglion can be seen because of the

separation of the anterodorsal and anteroventral lateral line nerves.The dorsal and ventral borders of the trigeminal ganglion coincidewith the ventral half and dorsal half of the trunks of the anterodorsaland anteroventral lateral line nerves, respectively. The facial gan-glion is the most medial and the largest cell group in the complex. Thecaudal segment of the facial ganglion extends dorsally to the enteringroots of the preoptic lateral line nerves, and it extends rostrally andventrally to occupy the ventromedial half of the entire complex. Thearrowheads at the left of the figure indicate the position where variousrami of these nerves exit the neurocranium. For abbreviations, seelist. Scale bar 5 2 mm.

Fig. 11. Camera lucida drawing of the cranial nerves and brain ofan adult catfish. The perspective is dorsolateral with respect to theanteroposterior axis of the brain. The position and extent of thesensory ganglia of the postotic branchiomeric (open circles) and lat-

eral line (solid circles) nerves are based on analysis of serial sectionsof a second individual of approximately the same length. For abbre-viations, see list. Scale bar 5 3 mm.


Figure 12 A: Photomicrograph of a Bodian-stained transversesection through the rostral medulla and cerebellum of a channelcatfish. B: Transverse section through the anterior ganglionic com-plex with anterogradely labeled sensory neurons of the facial andtrigeminal ganglia. C: Transverse section through the rostral medullaand cerebellum with anterogradely labeled sensory fibers of the pos-terior lateral line nerve entering the electrosensory lateral line lobeand medial octavolateral nucleus: C: transverse section through the

medulla and cerebellum at a level comparable to that in A, withanterogradely labeled sensory fibers of the facial and trigeminalnerves, as well as retrogradely labeled facial motor neurons, afterapplication of horseradish peroxidase to the superolateral strand (D).In each case, dorsal and lateral surfaces are to the top and left of thephotomicrographs, respectively. For abbreviations, see list. Scalebar 5 500 mm in A; 200 mm in B–D.

Figure 13 Photomicrographs of 5-mm-thick plastic-embeddedtransverse sections through sensory ganglia of various cranial nervesin the channel catfish rostral A and caudal B levels through theanterior ganglionic complex showing the positions of the anterodorsaland anteroventral lateral line ganglia relative to the ganglia of thefacial and trigeminal nerves. C: Section through the ganglion of the

middle lateral line nerve illustrating its position relative to the root ofthe posterior lateral line nerve and the medial and lateral vagalganglia. D: Section through the sensory ganglia of the posterior lat-eral line and vagal nerves at their closest apposition. Dorsal andlateral are to the top and left of each panel, respectively. For abbre-viations, see list. Scale bar 5 100 mm in A–D.


The buccal ramus exits the neurocranium adjacent tothe dorsolateral surface of the superolateral strand (Fig.11). At this point, fibers are still being exchanged betweenthe superolateral and inferomedial strands, but this ex-change stops abruptly, and the maxillary and mandibularrami of the trigeminal nerve emerge as distinctly separatefiber bundles. As the maxillary ramus continues rostrallyand ventrally, the buccal ramus remains adjacent to thedorsolateral surface of the maxillary ramus, then contin-ues rostrally and divides into outer (ob, Fig. 6) and inner(ib, Fig. 6) branches, historically termed the outer andinner buccal rami (Herrick, 1901). The outer ramus turnslaterally to pass under the m. levator arcus palatini andissues its first ramule, which innervates the first neuro-mast within the infraorbital canal (Figs. 1A, 6). Severaladditional branches issue from the first ramule (Fig. 6)and ramify within the skin innervating ampullary organsand taste buds in the vicinity of the caudal infraorbitalcanal. The remaining fibers of the outer buccal ramuscontinue rostrally to innervate the second neuromast ofthe infraorbital canal (Figs. 1A, 6). As these fibers courserostrally, numerous small twigs enter the skin so thatmost of the skin and associated sensory receptors locatedbetween the posterior half of the infraorbital and preoper-culomandibular canals are innervated by the outer buccalramus. The most rostrally coursing fibers of the outerbuccal ramus anastomose with one or two branches of theinner buccal ramus to innervate the third neuromast ofthe infraorbital canal (Figs. 1A, 6).

Whereas the outer buccal ramus initially courses later-ally and begins to innervate neuromasts of the caudalinfraorbital canal and ampullary organs adjacent to thecanal (Fig. 1B), the inner buccal ramus continues rostrallyalong the floor of the orbit, sandwiched between the max-illary and mandibular trigeminal rami. A brief anastomo-sis occurs between the inner buccal and maxillary rami ata midorbital level, after which the inner buccal ramusdivides into lateral and medial ramules. As the lateralramule courses rostrally, it issues one or two small fiberbundles, as already noted, which anastomose with theouter buccal ramus to innervate the third neuromastwithin the infraorbital canal.

Both ramules of the inner buccal ramus continue ros-trally and laterally toward the rostral orbital wall. At thislevel, the lateral ramule issues numerous small branchesthat ramify within the skin, and then a larger branch thatinnervates the fourth neuromast within the infraorbitalcanal. The smaller branches of the lateral ramule almostcertainly innervate the ampullary organs as well as thetaste buds adjacent to the rostral orbit, because the onlyother nerve within this region, the maxillary ramus, re-mains encapsulated in connective tissue and does notissue any ramules.

The lateral ramule of the inner buccal ramus continuesrostrally to innervate the fifth and sixth neuromastswithin the infraorbital canal (Figs. 1A, 6). The lateralramule then continues lateral to the olfactory capsulewhere it again forms a brief anastomosis with the maxil-lary trigeminal ramus before repeatedly dividing intosmaller branches that ramify within the skin on the lat-eral surface of the snout. The medial ramule of the innerbuccal ramus has paralleled the lateral ramule but nowpasses ventral to the olfactory capsule and does not seemto innervate any structures until it turns dorsally aroundthe rostral pole of the olfactory capsule. At this point, it

divides into two major bundles, each of which innervatesone of the two superficial neuromasts (numbers 7 and 8) ofthe infraorbital line (Figs. 1A, 6). Both bundles also giverise to numerous small fascicles that ramify within thedorsolateral surface of the snout just prior to innervatingthe superficial neuromasts.

Anteroventral lateral line nerve. The ganglion of theanteroventral lateral line nerve is located ventral to theganglion of the anterodorsal lateral line nerve and lateralto the ventral subdivision of the facial sensory ganglion(Figs. 10, 13A,B). The cells of the anteroventral lateralline ganglion have essentially the same diameter (x 5 19.5mm SE 6 0.27, N50), as the cells of the anterodorsallateral line ganglion in the 39-cm-long catfish. In additionto being slightly larger, cells of both the lateral line gan-glia have a paler cytoplasm than cells of the adjacentganglia within the anterior ganglionic complex. The sen-sory root of the anteroventral lateral line nerve, like thatof the anterodorsal nerve, passes caudally along the lat-eral surface of the ganglionic complex and fuses with thesensory root of the anterodorsal lateral line nerve beforetheir respective fibers enter the medulla (Fig. 10).

The peripherally directed fibers of the anteroventrallateral line nerve initially form a distinctly recognizablefascicle (Fig. 12A), but as they join the overlying facialsensory and motor fibers within the anterior ganglioniccomplex to form the hyomandibular trunk, they can nolonger be distinguished from the facial fibers. As the hyo-mandibular trunk separates from the more rostrallycoursing superolateral strand, a small but distinct bundleof fibers emerges from the trigeminal sensory ganglionand runs along the medial surface of the hyomandibulartrunk for a short distance (Fig. 10) before fusing with it.Thus, the hyomandibular trunk is apparently composed offacial sensory and motor fibers, lateral line fibers, andtrigeminal sensory fibers. Application of HRP to the hyo-mandibular trunk confirms this interpretation, as retro-gradely labeled cells occur in the anteroventral lateralline, facial, and trigeminal ganglia, as well as in the facialmotor nucleus. A small number of labeled cells were seenin the trigeminal motor nucleus in two of the four casesand probably result from HRP being inadvertently takenup by trigeminal fibers that were transected along with aportion of the mandibular adductor muscle, which overliesthe hyomandibular trunk. After exiting the neurocra-nium, the hyomandibular trunk courses ventrally (Fig. 6)and initially caudally as it passes along the medial surfaceof the hyomandibular bone. As the trunk approaches thedistal end of the hyomandibular bone, it passes through aforamen and emerges onto the lateral surface of thebone. The hyomandibular trunk then passes along thelateral surface of the quadrate bone and continues to passmedial to the preoperculomandibular canal where a smallramule emerges from the trunk to innervate the firstneuromast within this canal (Figs. 1A, 6). As the hyoman-dibular trunk continues ventrally it issues a variablenumber of small ramules that pass caudally over the sur-face of the opercular bone to ramify in the overlying skin(Fig. 6). These small ramules seem to innervate the amp-ullary organs located over and adjacent to the more caudalhorizontal segment of the preoperculomandibular canal(Fig. 1B).

After issuing these small ramules, the hyomandibulartrunk almost immediately divides into a ventrally directedhyoid ramus and a rostrally directed mandibular ramus


(Fig. 5), which again divides into deep and superficialsubdivisions (dmd and smd, Fig. 6). As the hyoid ramuscontinues ventrally and medially, it divides into threemain ramules, the most caudal of which ramifies in theskin overlying the ventral half of the opercular region (ch,Fig. 6). The fibers of the caudal ramule seem to innervatea small number of ampullary organs (Fig. 1B) and tastebuds located in the more rostroventral segment of theopercular region. The second ramule that arises from thehyoid ramus (mh, Fig. 6) courses medially over the hypo-branchial region to ramify in the overlying skin. Again,this ramule seems to innervate ampullary organs (Fig. 6)and taste buds near the ventral midline of the middlesegment of the hypobranchial region. The third ramule(rh, Fig. 6) of the hyoid ramus courses rostrally to inner-vate the hypobranchial muscles and overlying skin, whichalso contains ampullary organs (Fig. 1B) and taste buds.

Application of HRP to the proximal stump of the base ofthe hyoid ramus retrogradely labels sensory neurons inthe anteroventral, facial and trigeminal rami, as well as inthe facial motor nucleus. The centrally directed fibers ofthese sensory neurons could be traced into the lateral linelobe, facial lobe, and descending trigeminal tract, respec-tively. The presence of labeled cells within the trigeminalganglion after application of HRP to the hyoid ramussuggests that this ramus also carries somatic sensoryfibers that innervate the skin of the lower cheek andcaudal hypobranchial region. This innervation may beprovided by the trigeminal sensory bundle that initiallyjoined the hyomandibular trunk near its base (Fig. 10).

The deep mandibular ramus of the anteroventral lateralline nerve contains only lateral line fibers, experimentallyconfirmed, that innervate the remaining neuromasts (twothrough eight, Fig. 1A) of the preoperculomandibular ca-nal. This subdivision of the mandibular ramus is locatedventral to the articular and dentary bones and runs alongthe medial surface of the preoperculomandibular canal.Before coming to lie along the lateral surface of the artic-ular bone, the superficial subdivision of the mandibularramus passes adjacent to the ventrolateral surface of theadductor mandibular muscle for some distance and issuessizable ramules that innervate the superficial neuromastsof the quadratojugal and mandibular pit lines (Figs. 1A,6), as well as the ampullary organs and taste buds in theskin associated with the middle segment of the preoper-culomandibular line. As the superficial subdivision of thecombined anteroventral and facial ramus continues ros-trally along the lateral surface of the dentary bone, itrepeatedly issues ramules (not shown in Fig. 6) that in-nervate ampullary organs and taste buds in the skin over-lying the rostral segment of the preoperculomandibularcanal. In Bodian-stained transverse serial sections, it ispossible to follow a single ramule as it branches and in-nervates both ampullary organs and taste buds.

As the superficial subdivision of the combined mandib-ular ramus continues rostrally, adjacent to the lateralsurface of the dentary bone, a laterally and ventrallydirected ramule of the mandibular trigeminal ramus fuseswith it at approximately the level of the third infraorbitalneuromast (Fig. 6). Nearly doubled in size by this fusion,the superficial subdivision of the combined mandibularramus continues rostrally to the very tip of the lower jaw.As it approaches its termination, it divides into severalramules that innervate ampullary organs and taste buds.It is also possible that the superficial subdivision contains

somatic sensory fibers of the trigeminal nerve. After tran-section and application of HRP to its proximal stump, ashort distance from its separation from the deep subdivi-sion, retrogradely labeled cells occur in the anteroventral,facial, and trigeminal ganglia. Thus, it seems that trigem-inal fibers join the superficial subdivision of the combinedmandibular ramus as it emerges from the hyomandibulartrunk, as well as more rostrally where a ramule of themandibular trigeminal ramus fuses with the superficialsubdivision.

Otic lateral line nerve. The otic is the only lateral linenerve in the channel catfish in which a distinct and sep-arate sensory ganglion cannot be recognized. The nervebundle termed the otic lateral line nerve is the most ros-tral bundle to arise from the dorsal surface of the an-terodorsal ganglion (Figs. 6, 10). In adult catfish, a slightelevation formed by ganglionic neurons may represent thefusion of the otic ganglion with the anterodorsal lateralline ganglion (Fig. 10). We will treat the bundle of fibersthat issues from this elevation as a separate lateral linenerve, based on the fact that an outgroup analysis of bonyfishes (Northcutt and Bemis, 1993; Piotrowski and North-cutt, 1996) indicates that an otic lateral line nerve with adistinct and separate sensory ganglion represents theprimitive condition for bony fishes. In any case, a singleramus, usually termed the otic ramus, can be traced dor-sally and rostrally to exit the neurocranium by means of aseparate foramen in the sphenotic bone. After entering thesphenotic bone, the otic ramus turns caudally to courseimmediately adjacent to the ventromedial wall of the oticlateral line canal (Figs. 1A, 6). It passes under the anteriorramule of the superficial ophthalmic ramus of the an-terodorsal lateral line nerve and then continues caudallywithin the sphenotic, and a short distance rostral to thesingle neuromast in the otic canal (Fig. 1A) it divides intotwo ramules (Fig. 6). One innervates the neuromast of theotic canal, whereas the other continues caudally to enterthe operculum where it ramifies throughout the dorsalhalf of the opercular flap. In Bodian preparations, it ispossible to trace small bundles of otic fibers into the baseof ampullary organs and taste buds. It is clear from theseobservations that visceral sensory fibers arising from thefacial ganglion constitute one fiber population of the oticramus. It is possible that trigeminal somatic sensory fi-bers also join the otic ramus as it ramifies over the upperhalf of the operculum. This region is also innervated bymandibular trigeminal fibers, as well as facial opercularfibers (ope, Figs. 5, 6). Unfortunately, we were not able tolabel any of these rami with HRP and could not confirmwhether or not they include somatic sensory fibers.

Middle lateral line nerve. The ganglion of the middlelateral line nerve consists of a small number of large cells(no more than 100) with pale cytoplasm. It lies within theneurocranium, ventral to the root of the posterior lateralline nerve (Figs. 11, 13C). The root of the middle lateralline nerve is very short, and its fibers almost immediatelyjoin the root of the posterior lateral line nerve (Fig. 11).The single ramus of the middle lateral line nerve exits theneurocranium through a foramen that also houses the rootof the glossopharyngeal nerve and the trunk of the vagalnerve. The ramus of the middle lateral line nerve runsrostrally and laterally (Fig. 11), immediately beneath thefloor of the otic capsule. As it reaches the lateral edge ofthe otic capsule, the ramus turns dorsally and divides intoanterior and posterior ramules (Fig. 6). The anterior ram-


ule turns dorsally and rostrally to course along the lateralsurface of the temporal canal (Fig. 1A). As it approachesthe single neuromast within the temporal canal, the an-terior ramule divides again, with one branch innervatingthe neuromast within the temporal canal. The secondbranch continues a slight distance rostrally and dorsally(Fig. 6) to innervate the two neuromasts of the middle pitline (MP, Fig. 1A). The posterior ramule of the middlelateral line ramus turns caudally and ventrally to enterthe most caudal segment of the opercular flap. Unfortu-nately, we were unable to isolate this nerve in the rathersmall juvenile catfishes that we used for tracing studiesand could not experimentally determine whether thisnerve also contains facial and trigeminal sensory fibers.

Posterior lateral line nerve. The sensory ganglion ofthe posterior lateral line nerve lies outside the neurocra-nium, below the caudalmost portion of the otic capsule,which houses the posterior semicircular canal and dorsalto the lateral vagal ganglion (Figs. 11, 13D). Although theposterior lateral line and lateral vagal ganglia are in con-tact (Fig. 13D), each is distinct because the diameter of thecells of the lateral line ganglion (x 5 30.6 mm SE 6 0.59,N50 in the 39-cm-long catfish) is much larger than that(x 5 18.6 mm SE 6 0.53, N50) of the lateral vagal gan-glion. The cells of the posterior lateral line ganglion alsohave an extensive pale cytoplasm, as do the cells of theother lateral line ganglia, unlike the scant, darkly stain-ing cytoplasm of the ventral subdivision of the facial gan-glion and the lateral vagal ganglion.

The root of the posterior lateral line nerve enters theneurocranium with the trunk of the vagal nerve throughthe vagal foramen. As the root of the posterior lateral linenerve passes along the ventral and medial surfaces of thecrista of the posterior semicircular canal, it lies on thelateral surface of the vagal trunk and is semilunar inshape. As it continues dorsally and rostrally, it passesover the most distal segment of the posterior ramus of theoctaval nerve and the ganglion of the middle lateral linenerve where it fuses with the very short root of the latter(Figs. 6, 11). Technically, the combined fibers of the twonerves should be described as the root of the postoticlateral line nerves (pr, Figs. 6, 10, 11). As this root con-tinues rostrally, it passes slightly dorsal to the roots of theglossopharyngeal nerve and then enters the medulla im-mediately beneath the ventrolateral border of the lateralline lobes (Fig. 11).

The trunk of the posterior lateral line nerve arises fromthe distal end of the ganglion and runs caudally anddorsally (Figs. 5, 6, 11) to pass over the dorsal surface ofthe posttemporal bone. A small bundle of fibers emergesdirectly from the distal end of the ganglion or almostimmediately from the trunk (Figs. 6, 11) of the posteriorlateral line nerve. The bundle then courses dorsally andlaterally to anastomose with the dorsal somatic ramus ofthe first spinal nerve, then turns rostrally to pass alongthe lateral surface of the otic capsule. After a short dis-tance, the bundle divides into numerous twigs. One of thelarger twigs innervates the first neuromast of the trunkcanal (Fig. 1A). Other twigs seem to ramify in the skin,dorsal to the canal, and possibly innervate the first acces-sory neuromast (AN, Fig. 1A), as well as the ampullaryorgans in that region.

As the posterior lateral line trunk continues caudally, itpasses ventral to the epibranchial trunk muscles, where asecond bundle of fibers arises (Figs. 6, 11). This bundle

also turns dorsally and rostrally and innervates the sec-ond neuromast in the trunk canal (Fig. 1A). The dorsalramus of the posterior lateral line nerve (Figs. 5, 6, 11)arises from the trunk of the posterior lateral line nerve,approximately one segment more caudally. The dorsalramus turns dorsally and caudally to innervate the secondaccessory trunk line, which consists of two neuromasts,the dorsal trunk line, which consists of three neuromasts(Figs. 2, 6) and the ampullary organs associated withthese lines.

The posterior lateral line trunk divides into lateral andventral rami (Figs. 5–8) approximately two body seg-ments after the dorsal ramus arises from the trunk. Thelateral ramus innervates the neuromasts of the accessorytrunk lines, neuromasts within the trunk canal, neuro-masts at the base of the caudal fin, and all of the trunkampullary organs dorsal to the ventral trunk line. Theventral ramus innervates the neuromasts of the ventraltrunk line and all of the trunk ampullary organs ventral tothis line of neuromasts (Figs. 2, 3). All of the trunk lateralline organs are innervated by dorsal ramules, ventralramules, or both, that arise from the lateral and ventralrami (Figs. 6–8). The relationship of these ramules to thespinal nerve rami and ramules of the facial recurrentramus are very complex, however. Even though the nervesthat innervate the trunk represent three different sensorysystems (pain, temperature, and touch information bymeans of the spinal nerves; gustatory information bymeans of the recurrent facial ramus; and electrosensoryand mechanosensory lateral line information by means ofthe posterior lateral line nerve) all three systems usemany of the same ramules as their fibers approach theperiphery (Fig. 8). Analysis of Sudan-Black stained trunknerves in conjunction with Bodian-stained serial trans-verse sections of the trunk have allowed us to reconstructthe pathways used by each of the three systems.

Fibers of the facial recurrent ramus that innervate tastebuds distributed over the trunk initially exit the recurrentramus by means of segmentally repeated intervertebralramules (Figs. 6, 8) whose fibers pass through the dorsalroot ganglia of the spinal nerves to enter either theirdorsal or ventral somatic rami. The gustatory fibers thenrun toward the periphery in association with the sensoryfibers of the spinal nerves (Fig. 8). The combined somaticand gustatory sensory fibers in the somatic rami of thespinal nerves reach the skin surface in one of two ways.The dorsal somatic ramus of each spinal nerve coursestoward the surface in the horizontal septum, which sepa-rates the epaxial and hypaxial trunk muscles. As thedorsal ramus approaches the medial surface of the lateralramus of the posterior lateral line nerve, it divides intodorsal and ventral ramules (dra and vra, Fig. 8). Each ofthese ramules then turns to course within the myoseptum,and fibers of both ramules ramify among the muscle fibersof a single myotome. However, each ramule innervates theskin in a different manner. Fibers of the dorsal ramulecontinue within the myoseptum until they reach the innersurface and ramify. As the fibers of the dorsal ramuleramify, many seem to anastomose with the branchingfibers of the dorsal ramule (dr) of the lateral ramus of theposterior lateral line nerve. The lateral ramus issues asingle dorsal and ventral ramule per trunk segment, andthese ramules course dorsally or ventrally, immediatelybeneath the skin, above the myotome. As each ramulemoves away from the lateral ramus, it branches repeat-


edly, and a single ramule may span as many as four trunksegments. Although anastomoses occur only at the skinsurface between the dorsal ramule of the lateral ramusand the dorsal ramule of the dorsal somatic ramus, theventral ramule (vra) of the dorsal spinal nerve anastomo-ses almost immediately with the ventral ramule (vr) of thelateral ramus (Fig. 8) so that somatic sensory, lateral line,and gustatory fibers run in the same fiber bundles as theyramify just beneath the surface of the skin. A similarrelationship seems to characterize the lateral ramule (le)of the ventral somatic ramus and the ventral ramule (vr)of the ventral ramus (v) of the posterior lateral line nerve.

As the lateral ramus of the posterior lateral line nervereaches the caudal end of the trunk, it assumes a new setof relationships with the facial recurrent ramus and thespinal nerves as all these nerves enter the caudal fin (Fig.9). As the spinal cord approaches the base of the caudalfin, it is deflected dorsally to form a neurohaemal organ,the urophysis. At this point of transition, a terminal dor-sal somatic ramus (tds) and a terminal ventral somaticramus (tvs) arise from the spinal cord (Fig. 9B). The ter-minal dorsal somatic ramus parallels the caudal continu-ation of the urophysis and is joined by the facial recurrentramus at the point of origin of the terminal dorsal somaticramus (Fig. 9A). In contrast, the terminal ventral somaticramus continues caudally at the same dorsoventral levelas it arose from the spinal cord (Fig. 9B). Both the termi-nal dorsal and terminal ventral somatic rami continuecaudally to the bases of the fin rays (lepidotrichia). At thispoint, each ramus divides into dorsal and ventral ramulesthat form a dorsal arch and a ventral arch (Fig. 9B). Thedorsal arch is formed by the ventral ramule of the termi-nal dorsal somatic ramus and the dorsal ramule of theterminal ventral somatic ramus, whereas the ventral archis formed by the ventral ramule of the terminal ventralsomatic ramus and by four more rostral ventral somaticrami (vsr, Fig. 9A). Neural elements arise from each archand continue within the center of each fin ray. Each neuralelement is composed of somatic sensory fibers as well asspecial visceral (gustatory) sensory fibers. These sensoryfibers also will be joined by the electroreceptive fibers ofthe lateral ramus of the posterior lateral line nerve (Fig.9). As the lateral ramus approaches the base of the caudalfin, it divides into three or four ramules that continuecaudally just beneath the skin. As these ramules fan outdorsoventrally, some of their fibers innervate the verticalline of neuromasts at the base of the caudal fin (VLN, Fig.2B). The remaining fibers of the ramules of the lateralramus of the posterior lateral line nerve continue caudallyand medially to anastomose with the neural elementswithin the fin rays (Fig. 9A), and these lateral line fibersinnervate the ampullary organs located on the caudal fin(Fig. 3B).

DISCUSSION

Our observations on the lateral line system generallycorroborate earlier studies of this system in other cat-fishes but do differ in several details. These similaritiesand difference will be discussed first. We will then discussthe homology and polarity (e.g., are various features prim-itive or derived) of the neuromast lines, their associatedampullary fields, and their innervation. Finally, we willdiscuss some of the developmental implications of thedistribution of neuromasts and ampullary organs, as well

as the significance of the anastomoses of the peripheraltrunk rami of the facial, spinal and lateral line nerves.

Earlier studies

The first description of ampullary organs and externaltaste buds in catfish seems to be Wright’s study (1884a) onthe cutaneous sense organs of the white catfish, Ictalurus(Amiurus) catus. He recognized the superficial similaritybetween the catfish ampullary organs and the primitiveelectroreceptive ampullae of Lorenzeni, i.e., both organsoccur as pits in the skin in sharks and as so-called nervesacs in sturgeons, but he had no idea of the function ofeither. Although all of these organs have proved to beelectroreceptors (reviewed in Kalmijn, 1974; Finger, 1986;Zakon, 1988), the fact that catfish ampullary organs arenot homologous to primitive electroreceptors did notemerge until much later (McCormick, 1982; Bullock et al.,1983; Zakon, 1988). Wright did describe the external tastebuds in catfishes and realized their homology to the pha-ryngeal taste buds of other vertebrates but focused ontheir tactile function, apparently because of the largernumbers of these organs on the barbels. Wright also rec-ognized both canal and superficial neuromasts in catfishbut did not describe the canals in detail and recognizedonly the quadratojugal and ventral trunk lines.

The first detailed description of the canal and superfi-cial neuromasts in catfish is the study by Herrick in 1901on the cutaneous sense organs of the black bullhead cat-fish, Ameirus (Ictalurus) melas. Herrick correctly identi-fied the preotic canals, as well as the number of neuro-masts in each. The only difference in the number of preoticcanal neuromasts in A. melas and I. nebulosus is that A.melas seems to possess four superficial neuromasts ratherthan two at the rostral end of the infraorbital line. Herrickalso noted that the otic and temporal canals each containonly a single neuromast, but he did not realize that each ofthese organs and their canals arose from different pla-codes and assumed that they were part of the lateraltrunk line. Although Herrick did not describe the super-ficial neuromast lines in detail, his map of their distribu-tion, his Figure 14, is remarkably accurate consideringthat he used only a hand lens. He correctly identified all ofthe cephalic pit lines, as well as the dorsal and ventraltrunk lines. Although it is clear that he recognized some ofthe accessory trunk neuromasts, it is not clear that herealized that these neuromasts occur in vertical lines.

Herrick, like Wright (1884a), realized that catfish pos-sess an additional class of lateral line receptors, the am-pullary organs or small pit organs and that these organsoccur on both the head and trunk, but he did not attemptto map them. Peters et al. (1974) produced the first map ofcatfish ampullary organs when they and Roth (1968, 1969)realized that catfish could detect electric fields and might,therefore, use these organs to detect each other. The am-pullary map produced by Peters et al. was based on asingle individual whose length was between 18 and 21 cm.The ampullary organs were visualized macroscopically bysubmerging freshly killed animals in 1% nitric acid, whichrevealed the pore openings of the organs as small blackholes. The authors noted that the skin of each of theirspecimens was damaged and that it was necessary toestimate the number of receptors based on counts of thereceptors histologically from 1 cm2 skin samples. Unfor-tunately, approximately half of the skin surface of themapped individual was missing, and the map conveys


little information about differences in the distribution ofthe receptors. Likewise, their estimates of the number ofreceptors (2,500 to 4,000) is problematic, as the consider-ably smaller specimen (6.8 cm total length) that we mea-sured also had approximately 4,000 receptors. It would beinteresting to determine the variation in the number ofreceptors in several individuals of the same length and todetermine whether the receptors increase in number withbody length.

We arrived at our understanding of the innervation ofthe lateral line receptors in two phases: early descriptionsof the nerves (Wright, 1884b; Herrick, 1901), followed by along period, over 50 years, in which no studies were done,and a second phase of experimental studies (Knudsen,1976a,b; Finger, 1976; Finger and Bullock, 1982; Tong andFinger, 1983; Finger and Tong, 1984; New and Singh,1994) that focused primarily on the central lateral linepathways, with little attention to peripheral organization.Wright’s initial description (1884b), primarily based ongross dissection of the cranial nerves, is remarkable for itsaccuracy regarding nerve roots, ganglia, and major ramithat innervate the lateral line and gustatory systems.Wright realized that the facial (the preotic lateral lineganglia were included as part of the facial nerve) andtrigeminal nerves were fused, and he was the first torecognize and name the inferomedial and superolateralstrands. Without doubt, his observation of the fiber ex-change between the two strands was one of his remark-able observations.

In 1901, Herrick published his classic study on thecranial nerves of catfish, based on serially sectioned ma-terial, a laborious reconstruction of the course of thesenerves relative to the lateral line receptors, and a recon-struction of the anterior ganglionic complex. Our observa-tions corroborate most of Herrick’s conclusions with sev-eral exceptions. Herrick believed that the facial superficialophthalmic ramus, our superficial ophthalmic ramus ofthe anterodorsal lateral line nerve, consisted only of lat-eral line fibers. In a sense he was correct as this ramusdoes originate primarily from a distinct anterodorsal gan-glion. However, it is joined by somatic and visceral sensoryfibers derived from the trigeminal and facial ganglia, re-spectively.

Herrick’s error regarding the composition of the super-ficial ophthalmic ramus may have resulted in a seconderror. Herrick termed the profundal ramus of Wright’sand our description a trigeminal superficial ophthalmicramus. We suspect he reached this conclusion because atrigeminal superficial ophthalmic ramus does innervatethe rostral skull roof in sharks (Norris and Hughes, 1920)and primitive ray-finned fishes such as Amia (Norris,1925). However, the ramus that both Herrick (1901) andwe identify as carrying lateral line fibers also carries tri-geminal sensory fibers. Therefore, there is no basis forrecognizing a second ramus that consists of trigeminalsomatic sensory fibers (e.g., Herrick’s superficial ophthal-mic ramus of the trigeminal nerve). We believe thatWright’s interpretation (1884b) is correct because the ra-mus that we recognize as the profundal ramus does inner-vate the snout as the deep ophthalmic or profundal ramusdoes in other vertebrates. Unfortunately, it is not possibleto determine whether Wright or Herrick was correct basedon the fiber composition of each ramus. Although it isunlikely, it is even possible that our profundal ramus is acombination of part of the trigeminal superficial ophthal-

mic ramus and the profundal ramus of other fishes. Adefinitive answer awaits developmental observations, as aprofundal nerve should arise from a distinct placode thatinitially forms a sensory ganglion that may or may notsecondarily fuse with the trigeminal ganglion (Northcutt,1992a; Northcutt and Brandle, 1995).

Wright and Herrick also disagreed regarding the natureof the fibers that form the inferomedial and superolateralstrands. Wright believed that both strands were composedof both facial and trigeminal fibers, whereas Herrick be-lieved that the inferomedial and superolateral strandsconsisted only of facial and trigeminal fibers, respectively.It is impossible to determine which interpretation is cor-rect based on the application of any tracer to the periph-eral rami, because most rami carry both facial and trigem-inal fibers. We are inclined to support Wright’sinterpretation, based on the extensive exchange of fibersthat occurs between strands just before their forming themaxillary and mandibular rami. This fiber exchange wasfirst noted by Wright (1884b) and we confirm his observa-tions. As far as we have been able to tell, Herrick (1901)did not comment on this fiber exchange between strands.It should be possible to resolve the composition of thesestrands by injecting a tracer into the facial lobe and ex-amining both strands for labeled fibers. If Wright is cor-rect, labeled fibers should occur in both strands. If Herrickis correct, labeled fibers should be restricted to the infero-medial strand.

Our reconstruction of the anterior ganglionic complexagrees closely with Herrick’s reconstruction. His descrip-tion of the relative positions of the facial, lateral line andtrigeminal ganglia within the complex seems essentiallycorrect. Unfortunately, the symbols for the various gan-glia he mentions in the legend for his Figure 2 (his recon-struction) do not appear on the figure itself. In our recon-struction, we have not included Herrick’s accessorymaxillary nerve. We agree with Herrick that the fibers ofthis “nerve” do arise from the inferomedial and superolat-eral strands, but in our preparation the fibers from eachstrand almost immediately join the buccal ramus beforeits division into inner and outer segments. Herrick’s de-scription and figures indicate that the accessory maxillaryforms a distinct bundle that parallels the outer buccalramus. These observational differences may be due toeither individual or interspecific variation. In Herrick’sreconstruction, a bundle of facial fibers joins the otic ra-mus, and a trigeminal bundle passes dorsal to this ramus.Although we believe that the otic ramus contains facialfibers, because we traced fibers from the otic ramus thatinnervate taste buds, we never saw a distinct trigeminalfascicle pass over the otic ramus as indicated by Herrick.We did see a distinct trigeminal fascicle join the anteriorramule; however, before the ramule passes over the oticramus. Again, these differences may be due to errors inreconstruction or to inter- or intraspecific variation. Inany case, both the anterior ramule and the otic ramusappear to contain facial and trigeminal sensory fibers inaddition to their lateral line fibers.

Phylogenetic considerations

All of the features of the lateral line canals of the chan-nel catfish are primitive for ray-finned fishes with theexception of the following derived features: (1) the su-praorbital and infraorbital canals are not continuous ros-trally due to the commissural canal being reduced to a line


of two superficial neuromasts; (2) the first neuromast ofthe anterior pit line is enclosed in a canal that is a caudalextension of the supraorbital canal; (3) the peroperculo-mandibular canal caudally joins the postotic canals (Fig.1A) rather than ending slightly ventral to these canals; (4)catfish have lost the supratemporal commissural line ofneuromasts that is usually enclosed in a canal intercon-necting the posttemporal canals; (5) the pit line patternhas been altered by the loss of the horizontal, vertical,gular, and posterior pit lines; (6) a reduced dorsal trunkline and the elaboration of the accessory trunk lines. Asalready noted, the presence of cephalic and trunk ampul-lary organs is also a derived feature in catfishes.

In addition, many aspects of the innervation of thecephalic lateral line and other receptors are also derivedfeatures: (1) the probable loss of a distinct trigeminalsuperficial ophthalmic ramus; (2) a probable reversal inthe positions of the otic ramus and the anterior ramule; (3)the absence of a mandibular facial ramus; (4) the loss of asupratemporal lateral line nerve; (5) the presence of arecurrent facial ramus and facial fibers within almost allother branchiomeric nerves due to the presence of exter-nal taste buds; (6) unique divisions of the maxillary andmandibular rami associated with barbel development; and(7) fusion of the ganglia of the facial, preotic lateral line,and trigeminal nerves to form the anterior ganglioniccomplex. Many, if not most, of these changes in cephaliclateral line and receptor innervation seem to be correlatedwith the development of external taste buds rather thanthe development of electroreceptors.

Developmental considerations

The distribution of electroreceptors and the close asso-ciation of the posterior lateral line nerves with the facialrecurrent ramus and spinal nerves raise important devel-opmental questions, beyond the obvious questions of howexternal taste buds are developed and maintained. AsHerrick (1901) initially suggested, the ampullary organsof catfish are not randomly distributed on the head. Withthe exception of the hypobranchial ampullary field, all ofthe other ampullary organs are centered on the lateralline canals. Even though the ampullary organs of catfishare not homologous to those of axolotls, the ampullaryorgans in axolotls are even more restricted to areas adja-cent to the lateral lines (Northcutt, 1992a). The develop-ment of ampullary organs in axolotls has been extensivelydocumented (Smith et al., 1988, 1990; Northcutt et al.,1994, 1995). In these animals, the lateral line placodesinitially elongate to form sensory ridges (the posteriorlateral line placode, which migrates down the trunk, is anexception). The primordia of the neuromasts form within acentral strip of the sensory ridge flanked by the ampullaryprimordia. The distribution of ampullary organs in thechannel catfish suggests that these organs may develop inthe same way as in axolotls. However, it is possible thatthe ampullary organs of catfish are induced directly fromgeneral ectoderm by the sprouting of lateral line fibers(Vischer et al., 1989). Resolution of how these receptorsare induced and how they differentiate are critical forunderstanding how lateral line systems change throughtime.

The close association of the trunk fibers (facial, lateralline, and spinal nerves) that innervate the skin and itsspecialized sensory receptors suggests that the nerve fi-bers of one of these systems may establish the distribution

pattern for the other fibers. Fibers of the recurrent facialramus almost certainly do not establish this pattern, de-spite the ramus’s issuing segmental invertebral ramules,because there is also a close association between the lat-eral line and spinal fibers in zebrafish, for which no exter-nal taste buds occur on the trunk (unpublished observa-tions, RGN).

Partial ablation of trunk neural crest in axolotls dis-rupts the regular development of trunk neuromasts inaxolotls (Smith et al., 1994) and suggests that neural crestor its derivatives may be involved in patterning trunknerves. The migrating posterior lateral line placode depos-its neuromast primordia at regular intervals until itreaches the point on the trunk where neural crest hasbeen removed. At this point, the placode stops depositingneuromast primordia but continues to migrate. When theplacode again reaches a normal trunk segment, it resumesdepositing neuromast primordia. Smith et al. (1994) sug-gested that the migrating placode was receiving clues forthe deposition of sensory primordia from myosepta or fromspinal nerve fibers that run within the myosepta. In eithercase, the spinal fibers and their associated glial elementsare derived from neural crest, as is much of the connectivetissue that forms the myosepta. The presence of a canalneuromast and its associated innervation in each trunksegment is certainly suggestive. It is also possible that therecurrent facial ramus uses similar clues to issue inter-vertebral ramules. The embryos of axolotls, catfish, andzebrafish offer fertile ground to test this model in moredetail.

The neural crest origin of spinal nerves and their inti-mate relationship to neuromasts and the lateral line fibersthat innervate them may explain another set of develop-mental observations. When neural crest has been labeledin several species of fish, labeled cells have been detectednear neuromasts (Lamers et al., 1981) or within neuro-masts (Collazo et al., 1994) later in development. Theseobservations have resulted in the claim that neuromastshave a dual embryonic origin from neural crest and lateralline placodes (Collazo et al., 1994). The intimate relation-ship of lateral line and spinal ramules suggests anotherinterpretation, however. It is possible that some fractionof the glial cells that ensheath fibers of the lateral lineramules arise from neural crest as, apparently, do all ofthe glial cells of the spinal nerves. If this is the case, it ispossible that migrating Schwann cells of neural crest or-igin use both spinal and lateral line fibers and ensheathboth. In this context, it may be significant that Collazo etal. (1994) claim that neuromasts derived from placodesappeared to differentiate earlier than neuromasts derivedfrom neural crest. The unusual relationship of fibers ofspinal nerve origin, as well as those of cranial nerve ori-gin, suggests that there may also be very complicatedinteractions between neural crest and placodal deriva-tives.

ACKNOWLEDGMENTS

We were fortunate to have had the technical assistanceof Mariola Milik. We thank Georg Striedter for the loan ofa set of Bodian-stained transverse sections of the brain ofa channel catfish, which allowed us to photograph thesection in Figure 11A. Sue Commerford provided litera-ture retrieval, word processing, and other support ser-vices, and Mary Sue Northcutt assisted in numerous


phases of the study. P.H.H. was supported in part byNRSA training grant and R.G.N. received support from aNIH research grant.

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distribution and innervation of lateral line organs in the channel catfish

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