cocconeis distans gregory and amphicocconeis debesi

Botanica Marina 49 (2006): 438–449 � 2006 by Walter de Gruyter • Berlin • New York. DOI 10.1515/BOT.2006.055

Article in press - uncorrected proof

2006/15

Cocconeis distans Gregory and Amphicocconeis debesi(Hustedt) De Stefano comb. nov. (Bacillariophyta),an intricate taxonomical history

Mario De Stefano1,*, Ugo Sacchi2, Cecilia Totti3

and Oscar E. Romero4

1 Department of Environmental Sciences, The 2ndUniversity of Naples, Via Vivaldi 43, 81100 Caserta,Italy, e-mail: [email protected] Stazione Zoologica ‘‘A. Dohrn’’, Villa Comunale I,80121, Naples, Italy3 Department of Marine Sciences, Universita Politecnicadelle Marche, Via Brecce Bianche, 60131 Ancona, Italy4 Department of Geosciences, University of Bremen,P.O. Box 330440, 28334 Bremen, Germany

* Corresponding author

Abstract

We present a taxonomical revision of Cocconeis distansand C. debesi based on historical collections from Hus-tedt, and live material sampled from leaves of Posidoniaoceanica. Our observations clarify the specific morphol-ogy of the two taxa, which have been confused by pastand modern authors. In addition, we demonstrate thatthe valve structure of C. debesi deviates from that of thegenus Cocconeis, but agrees with that of the genusAmphicocconeis. Therefore, we propose a new nomen-clatural combination, Amphicocconeis debesi. The studyalso provides additional information on the geographicaldistribution of these taxa.

Keywords: Amphicocconeis debesi; biogeography;Cocconeis distans; Mediterranean Sea; SEM; TEM;ultrastructure.

Introduction

The first description of Cocconeis distans was publishedby Gregory from Glenshira diatomite (Gregory 1855, p.39, pl. 4, fig. 9). He presented the following diagnosticcharacters for the species: ‘‘This very beautiful form is atonce characterised by the equal size of the dots or gran-ules, and their great distance from each other, so that italmost loses the aspect of striation. The form is purelyoval’’. He also highlighted differences between C. distansand some varieties of C. scutellum Ehrenberg, but hepublished a drawing of a C. scutellum sternum valve asthe original drawing of the new species. Subsequently,Gregory acknowledged his mistake and provided a moredetailed description of C. distans, based only on a ster-num valve (Gregory 1857, p. 490, pl. 9, fig. 23; Figure 1)in which he stated: ‘‘the valve is marked by distant lines,much inclined near the apices, not reaching the medianline. These lines are about 10 in 0.001’’, and consist of

white hyaline faint bars, on which are set small and dis-tant granules. The number of only four or five in the long-est of these lines, so that the granules are very distant.They are, as nearly as possible, of equal size, and fromtheir distance, give to the valve a spotted rather than astriated aspect’’.

Greville (1861) observed the same taxon but created anew taxonomic entity by introducing Cocconeis granuli-fera (Greville 1861, p. 73, pl. 8, fig. 19). Subsequently,Schmidt (1874, vol. 2, pl. 3, figs 22, 23?; Figure 8a,b)illustrated a sternum valve of C. distans sensu Gregoryrather different from the original illustrations of Gregory(1855, 1857). Schmidt’s drawings caused some confu-sion in the delineation of C. distans, since many differenttaxa with a certain similarity with his drawings werewrongly identified as C. distans by Grunow in Van Heurck(1896, p. 287, pl. 29, fig. 817; Figure 8a,b), Grunow inSchmidt et al. (1874–1959, pl. 193, figs 29–32; Figures9–11a,b), Weissflog in Schmidt et al. (1874–1959, pl.193, figs 36, 37), Peragallo and Peragallo (1897–1908, p.14, pl. 3, figs 14–16; Figure 12a–c) and Cleve-Euler(1953, p. 8, fig. 491a; Figure 13a,b).

This historical taxonomic confusion was definitivelyresolved by Hustedt (1933) who introduced Cocconeisdebesi and synonymized all previously described taxa onthe basis of the sternum valve drawings published underC. distans sensu Schmidt (Figures 8–13). In addition,Hustedt synonymized C. granulifera with C. distans,including specimens described by Schmidt et al.(1874–1959, pl. 193, fig. 34), Peragallo (1888, pl. 4, fig.35 (as C. villosa H. Peragallo), Cleve (1895, p. 168, pl. 2,fig. 36), Peragallo and Peragallo (1897–1908, pl. 3, figs16–18; Figure 2a–c) and Cleve-Euler (1953, p. 8, fig. 491;Figure 3).

The holotype of Cocconeis distans was re-examinedby Hendey (1964) who recognized an entire frustule withmorphological characters similar to those described byGregory (1857). More recently, reports of C. distans byHustedt (1933; Figures 5a,b, 6a,b, 7a,b) and (1955; Fig-ure 5c,d), Ricard (1977), Foged (1979, 1984, 1986),Riaux-Gobin (1991a,b), Witkowski (1993), and Witkowskiet al. (2000) as well as of C. debesi by Zanon (1948),Cleve-Euler (1953), and Ognjanova-Rumenova and Zapr-janova (1998) were not accompanied by thorough inves-tigations of the valve structures.

In the present work, we provide the first detailed studyby electron microscopy of Cocconeis distans and C.debesi based on historical collections from Hustedt andlive material from the leaves of Posidonia oceanica (Lin-naeus) Delile collected in several coastal regions of theMediterranean Sea (Table 1). Our observations clarify theparticular morphology of C. distans, exhibiting a suite ofvalve characteristics that are unique within the genusCocconeis Ehrenberg. In addition, we demonstrate that

M. De Stefano et al.: Ultrastructure and taxonomy of Cocconeis distans and Amphicocconeis debesi nov. comb. 439


Figures 1–7 Original drawings and LM micrographs of Cocconeis distans and related synonyms.(1) Sternum valve of C. distans from Gregory 1857. (2) Raphe sternum (a) and sternum valves (b–c) of C. granulifera from Peragalloand Peragallo 1897–1908. (3) Sternum valve of C. granulifera from Cleve-Euler 1953. (4) Sternum valve of C. distans var. sardoa fromZanon 1948. (5) Raphe sternum (a) and sternum valve (b) of C. distans from Hustedt 1933; raphe sternum (c) and sternum valve (d)of C. distans from Hustedt 1955. (6) LM micrographs of raphe sternum (a) and sternum valve (b) of C. distans from the historicalHustedt collection, slide 200/07. Scale bars10 mm. (7) LM micrographs of raphe sternum (a) and sternum valve (b) of C. distans fromthe historical Hustedt collection, slide 199/97. Scale bars10 mm.

the valve fine structure of C. debesi differs from that ofthe genus Cocconeis, whereas it agrees with that of thegenus Amphicocconeis De Stefano et Marino, includingat present only A. disculoides (Hustedt) De Stefano (DeStefano and Marino 2003). Therefore, we propose thenomenclatural transfer of C. debesi to Amphicocconeisas A. debesi (Hustedt) De Stefano nov. comb. The occur-rence of A. debesi as an epiphyte on Posidonia oceanicafrom different coastal regions of the Mediterranean Seaallows us to extend its geographical distribution.

Materials and methods

The list of type material and raw samples containing Coc-coneis and Amphicocconeis taxa is presented in Table 1along with sampling locations. Shoots of Posidonia ocea-nica were sampled in spring 2004 by SCUBA divers, fromsalt marshes located at different locations in the centraland southern Tyrrhenian Sea as well as in the easternMediterranean Sea. Leaves were cut from the rhizomesat the level of the ligulae and placed on a plexiglass

440 M. De Stefano et al.: Ultrastructure and taxonomy of Cocconeis distans and Amphicocconeis debesi nov. comb.


Figures 8–14 Drawings of Cocconeis distans and its variety bahusiensis from various authors.(8) Raphe sternum (a) and sternum valve (b) of C. distans from Grunow 1896. (9, 10) Sternum valves of C. distans from Schmidt etal. 1874–1959, pl. 193, figs 29, 32. (11) Sternum valve (a) and a complete frustule (b) of C. distans from Schmidt et al. 1874–1959,pl. 193, figs 30, 31. (12a–c) Sternum valves of C. distans from Peragallo and Peragallo 1897–1908. (13) Sternum (a) and raphesternum valve (b) of C. distans from Cleve-Euler 1953. (14a–c) Sternum valves of C. distans var. bahusiensis from Cleve-Euler 1953.

board. The epiphytic microflora was removed from theleaf surface by scraping with a scalpel and rinsed into aPetri dish with filtered seawater. The collected materialwas transferred with a pipette into a tube; volumesbetween 20 and 30 ml of concentrated scraped materialwere enough to make several observations on each sam-ple. The diatom material was cleaned with 70% nitricacid at 608C for 1 h, washed several times with distilledwater, then treated with concentrated sulphuric acid,washed again and permanent slides were prepared usingHyrax mounting medium (De Stefano et al. 2000). Slideswere examined with a Zeiss Axiophot light microscope(LM) (Carl Zeiss, Oberkochen, Germany). Cleaned mate-rial was also mounted on stubs and coated with platinumfor scanning electron microscopy (SEM) (Philips 505,Philips Electron Optics BV, Eindhoven, The Netherlands),

and mounted on formvar-coated grids for transmissionelectron microscopy (TEM) (Philips 400).

Terminology follows recommendations by Anonymous(1975), Ross et al. (1979), Holmes et al. (1982) and Roundet al. (1990), while the classification follows Round et al.(1990).

Results

Order: Achnanthales Silva; Family: CocconeidaceaeKutzing; Genus: Cocconeis Ehrenberg

Cocconeis distans Gregory (Figures 1–7, 15–32)

Basionym Cocconeis distans Gregory 1855, p. 39, pl.4, fig. 9.



Table 1 List of examined historical collections and current material collected from Posidonia oceanica, with collecting locations.

Taxa Material Locality1 Prepared by Preparation Source2

date

Cocconeis granulifera Slide M2/50 Posillipo, Gulf of Naples, Italy F. Hustedt 01.01.1927 BRMGreville Cleaned material E 95Cocconeis distans Gregory Slide 199/97a Nordasot, Norway F. Hustedt 01.01.1933 BRM

Slide 199/94Cleaned material E5308Slide 200/07 Skjoldafjord, Norway F. Hustedt 01.01.1933 BRM

Amphicocconeis debesi Slide 199/94 Nordasot, Norway F. Hustedt 01.01.1933 BRM(Hustedt) De Stefano Cleaned material E5308Cocconeis distans Gregory Cleaned material from Mallorca (ES), Genova (IT), M. De Stefano 06.05.2004 SZN,

Posidonia oceanica leaves Western Sardinia (IT), SUNIschia and Tavolara islands (IT),Gulf of Saronico (GR),Patmos Island, Gulf of Kusadasi(TR) and Edremit (TR)

Amphicocconeis debesi Cleaned material from Ventotene (IT) and Ischia island M. De Stefano 06.05.2004 SZN,(Hustedt) De Stefano Posidonia oceanica leaves (IT), Gulf of Saronico (GR), SUN

Patmos Island, Gulf of Kusadasi(TR) and Edremit (TR)

1 ES: Spain; IT: Italy; GR: Greece; TR: Turkey.2 BRM: F. Hustedt Diatom Collection in Bremerhaven; SZN: Stazione Zoologica ‘‘A. Dohrn’’ of Naples; SUN: Second University ofNaples.

Synonyms Cocconeis granulifera Greville 1861, p. 73,pl. 8, fig. 19; Schmidt et al. 1874–1959, pl. 193, fig. 34;Cleve 1895, p. 168, pl. 2, fig. 36; Peragallo and Peragallo1897–1908, pl. 3, figs 16–18 (Figure 2a–c); Cleve-Euler1953, p. 8, fig. 491 (Figure 3). Cocconeis villosa H. Pera-gallo 1888, pl. 4, fig. 35.

References Gregory 1857, p. 490, pl. 9, fig. 23 (Figure1); Hustedt 1933, p. 343, fig. 797 (Figure 5a,b); Hustedt1955, pl. 5, fig. 4 (Figure 5c,d); Foged 1979, p. 33, pl. 13,fig. 24; Foged 1984, p. 29, pl. 31, figs 7, 8; Foged 1986,p. 20, pl. 13, fig. 4; Riaux-Gobin 1991a, p. 128, pl. 3, figs3 (4?); Witkowski 1993, p. 104, pl. 14, fig. 6; Witkowskiet al. 2000, p. 106, pl. 38, figs 12, 13.

Valve outline elliptical, sometimes rhombic (Figures 15,16, 25, 26). Apical axis (AA): 18.3–27 mm. Transapicalaxis (TA): 12–17.3 mm.

Sternum valve (SV) Valve surface flat or slightly con-cave (Figures 16, 20), separated from the mantle almostperpendicularly to the valvar plane (Figures 16, 20) by ahyaline submarginal area reinforced by a thickened rim(Figure 17, arrow). The axial area is thin, slightly depress-ed on the valve surface (Figure 20) and sometimes cov-ered with fine scattered granules (Figure 16). The unise-riate striae, 9–11 in 10 mm, are radiate (Figures 15, 16,21, 24) and protrude from the valve surface (Figures 19,20). Striae consist of regularly spaced protuberancesopening externally by transapically elongated poroids(0.6–0.8 mm in size) near the margins and by morerounded poroids on the remaining part of the valve sur-face (Figures 17, 19, 20, 22). Poroids are internally oc-cluded by imperforated silicified hymenes, which appearto be finely granular (Figure 23). The external poroidopenings are surrounded by a crown of fine granules(Figures 16, 17, 19, 22). The mantle displays a single rowof pervalvarly elongated poroids (Figure 20, arrow), which

are more numerous (approximately three for each valvestria), but smaller and structurally simpler (Figure 17).

Internally, the valve is almost flat, lacking any thick-ening on the sternum and interstriae (Figure 24). The sizesimilarity between poroids and intra-poroidal spacesconfer a peculiar appearance to the valve ‘‘so that italmost loses the aspect of striation’’ (Gregory 1855). Thevalvocopula is fimbriate. The tongue-shaped fimbriae, allof the same length, are connected with the distal endingsof interstriae (Figure 18).

Raphe-sternum valve (RSV) Externally, the valve isslightly convex, more delicate than the SV and slightlydepressed close to the central nodule (Figures 26, 32). Anarrow, hyaline sub-marginal area separates the well-defined mantle from the valve face (Figures 26, 32). Thethin, straight raphe-sternum lacks a central area andextends within the perimeter of the sub-marginal area(Figure 26). The uniseriate striae, 18–19 to 21–23 in10 mm, are radiate and consist of circular poroids inter-nally occluded by non-perforated hymenes (Figures 28,30). Internally, the axial and sub-marginal areas are thick-ened and prominent (Figure 29). The external distal rapheends are simple and terminate at the sub-marginal area,while the proximal raphe ends are co-axial (Figures 27,31). Internally, the proximal raphe ends are slightly dis-placed in opposite directions (Figure 28) and the distalends terminate in slightly raised helictoglossae (Figure30). The structure of the valvocopula is unknown.

Geographical distribution Cocconeis distans occur-red at all sampling sites throughout the MediterraneanSea. It was always present in the eastern basin, whereasit showed a more discontinuous distribution in the west-ern side and in the Adriatic Sea (De Stefano 2001). Hus-tedt (1933) also recorded this taxon from theMediterranean Sea and Norway.



Figures 15–24 Sternum valve of Cocconeis distans.Figure 15, LM; Figures 16–20, 23, 24, SEM; Figures 21, 22, TEM. Scale barss10 mm (Figure 15); 5 mm (Figures 16, 18, 20, 21, 24);1 mm (Figures 17, 19); 0.5 mm (Figures 22, 23).(15) Complete valve. (16) External view of the valve. (17) Detail of the hyaline submarginal area reinforced by a thickened rim (arrow).(18) Internal view of the valve showing the fimbriate valvocopula. (19) Cross section of the valve in 608 tilted view exhibiting poroidssurrounded by a crown of granules. (20) 608 tilted image of the external valve surface. The pervalvarly elongated mantle poroids arearrowed. (21) Complete valve. (22) View of a poroid showing its crown of hyaline granules. (23) Internal view of the imperforated,heavyly silicified poroidal hymenes. (24) Internal view of the valve.

Genus: Amphicocconeis De Stefano et Marino

Amphicocconeis debesi (Hustedt) De Stefano comb.nov. (Figures 8–13, 33–53)

Basionym Cocconeis debesi Hustedt 1933, p. 344,fig. 798.

Synonyms Cocconeis distans sensu Schmidt 1874,pl. 3, figs 22, 23; Grunow in Van Heurck 1896 p. 287, pl.29, fig. 817 (Figure 8a,b); Grunow in Schmidt et al.1874–1959, pl. 193, figs 29–32 (Figures 9–11a,b); Weiss-flog in Schmidt et al. 1874–1959, pl. 193, figs 36, 37;Peragallo and Peragallo 1897–1908, p. 14, pl. 3, figs14–16 (Figure 12a–c); Cleve-Euler 1953, p. 8, fig. 491a

(Figure 13a,b). Cocconeis distans var. minima Frenguelli1921, p. 110, pl. 9, fig. 21; Frenguelli 1930, p. 271, pl. 2,fig. 7.

Emended diagnosis

Valvae ellipticae, 22–31 mm longae, 12–15 mm latae.Areovalva leviter concava in area axiali et convexa in areamarginali. Sternum lineare aut leviter lanceolatum. Striaeleviter radiatae et mono-seriatae (8–10 in 10 mm) sunt etconstant ex areolis oblongis in crateribus positae (7–10in 10 mm). Areolis hymenes convexi sunt, qui superficiemaequalem habent foraminibus dispersis praediti. Valvo-copula caret fimbriis internae faciei valvae tengens. Duaecopulae ligula praeditae sunt.



Figures 25–32 Raphe-sternum valve of Cocconeis distans.Figure 25, LM; Figures 26–32, SEM. Scale barss10 mm (Figure 25); 5 mm (Figures 26, 29, 32); 2 mm (Figures 27, 31); 1 mm (Figures28, 30).(25) Complete valve. (26) External view of the valve. (27) Detail of the simple and co-axial external proximal raphe endings. (28) Detailof the central raphe endings in internal view converging on to central nodule. (29) Internal view of the valve. (30) Detail of a terminalraphe ending with its straight helictoglossa. (31) Detail of a distal raphe ending in external view. (32) 608 tilted view of the externalvalve surface. Note the hyaline sub-marginal area separating the mantle from the valve face.

Raphovalva complanata habet raphem rectam cumarea centrali orbiculata, plus minusve lata. Extremaepartes externae raphes habent rimas extremas, mediaepartes internae in recta linea collocatae sunt, extremaepartes internae parvas helictoglossas habent. Raphoval-vae striae (22 in 10 mm) radiatae et mono-seriatae sunt.Magnorum exemplarium striae constant longis poris (;15in 10 mm) qui hymenes similes areovalvae habent. Par-vorum exemplarium striae constant singulo poro qui araphe ad marginem pervenit.

Valves elliptical-lanceolate to lanceolate (Figures 33–36, 43, 44). AA: 22–31 mm; TA: 12–15 mm.

Sternum valve (SV) Externally, the valve face is most-ly convex, with a depression along the sternum (Figures34, 40). The mantle is shallow, undifferentiated, and hasa narrow marginal hyaline area (Figures 38, 40). A straight

or sometimes slightly lanceolate axial area extends as faras the valve apices (Figures 34, 36). Striae are mostlyparallel, 7–10 in 10 mm, becoming radiate toward theapices (Figures 34, 36). Striae are separated by thickinterstriae which give a coarse appearance to the distri-bution pattern of poroids, which form almost ordered lon-gitudinal lines (Figure 36). Each stria consists of a row oftransapically elongated poroids, 7–9 in 10 mm (Figure34), located at the bottom of crater-shaped openingsshowing a granular surface (Figure 39). Poroids are inter-nally occluded by hymenes having a regular surface withrounded, more or less linearly arranged perforations (Fig-ure 37). Along the mantle, the poroids of each stria arelarger and they have a canyon-shape appearance withsmaller intercalated ones (Figures 38, 40).

The internal and external striations are similar. The axialarea is thickened (Figures 35, 41). The opened valvoco-pula (Figure 41) is characterised by small, digitate fim-



Figures 33–42 Sternum valve of Amphicocconeis debesi.Figure 33, LM; Figures 34, 35, 38–42, SEM; Figures 36, 37, TEM. Scale barss10 mm (Figure 33); 5 mm (Figures 34–36, 40, 41);2 mm (Figures 38, 39, 42); 0.3 mm (Figure 37).(33) Complete valve. (34) External view of the valve. (35) Internal view of the valve. (36) Complete valve. (37) Detail of a poroidalhymen showing a regular surface with rounded, more or less linearly arranged perforations. (38) 608 tilted view of the external valvemantle. (39) Detail of the external valve face showing transapically elongated poroids placed at the bottom of crater-shaped openings.(40) 608 tilted view of the valve. The concave area facing the sternum is evident. (41) 608 tilted, internal view of the valve showingthe valvocopula. (42) Detail of the internal valve surface. Arrow indicates the finger-like fimbriae.

briae which fuse with the interstriae at the valve margin(Figure 42, arrow).

Raphe-sternum valve (RSV) Externally, the valve sur-face is very slightly convex to almost flat (Figures 43, 44,50, 51). The mantle is thin and undifferentiated (Figures44, 48, 51). The raphe is straight (Figures 44, 50) with co-axial, proximal endings (Figure 45) and distal endings ter-minating in polar fissures (Figures 46, 48), deflected

toward the same side (Figures 44, 51). Raphe sternumstraight and narrow with a sub-orbicular to sub-rhom-boidal central area (Figure 49). Striae are radiate and con-tinuous or segmented, 22 in 10 mm; in this latter casethe pattern appears more regularly organized along theraphe sternum and the valve margin. Striae consist oftransapically elongated poroids, externally occluded byhymenes, which show very minute and rounded perfor-ations (Figure 47). In small specimens, each stria consists



Figures 43–53 Raphe sternum valve of Amphicocconeis debesi.Figures 43–46, 48–53, SEM; Figure 47, TEM. Scale barss5 mm (Figures 43, 44, 50–52); 2 mm (Figures 48, 49); 1 mm (Figures 45,46, 53); 0.5 mm (Figure 47).(43) Valve of a small specimen in external view. (44) Valve of a large specimen in external view. The transapical elongate poroids arespaced by transverse thickenings of the interstriae. (45) External view of the valve central area with the proximal raphe endings. (46)Detail of a valve apex showing the thickenings of the interstriae and a distal raphe ending with its extended curved terminal fissure.(47) Fine structure of valve poroidal hymenes. (48) 608 tilted view of a frustule apex showing the extension of the raphe terminalfissure and two valvocopulae. (49) External, central area of the valve in a large specimen. Note the rhomboidal hyaline area delimitedby poroids facing the raphe-sternum. (50) 608 tilted view of a small specimen in external view. The slight convex area facing thesternum is evident. (51) 608 tilted view of a large specimen in external view. (52) Valve of a large specimen in internal view. (53) Detailof the previous image showing the distal raphe ending with its reduced helictoglossa.

of a single poroid that extends from the raphe sternumto the margin (Figures 43, 50), while in larger specimens,the poroids forming the striae are irregularly interruptedby thin siliceous thickenings (Figures 44, 51), but areclearly visible close to the margin and the raphe sternum(Figures 44, 46, 51). This arrangement gives a marked

aspect to the proximal and distal series of poroids (Fig-ures 44, 51).

In internal view, the raphe lies on a slightly thickenedrim. The proximal endings, arranged on an elliptical cen-tral nodule, are thin and co-axial (Figure 52) whereas thedistal endings terminate in reduced helictoglossae that



Table 2 Main morphometric data for Cocconeis distans.

Morphometric data Gregory Cleve (1895)1 Peragallo Hustedt Hendey Sar et al.2 This study(1857) and Peragallo1 (1933) (1964) (2003)

(1897–1908)

Apical axis (mm) 14–26 28–57 28–60 28–60 35 20–31 18.3–27Transapical axis (mm) 10–20 18–38 18–40 18–40 26 10–18 12–17.3SV striae (in 10 mm) 10 4–5 4–5 4–5 5 (4) 7–9 9–11SV areolae (in 10 mm) 4–5 5 – 2–5 2–6 5–8 4–5RSV striae (in 10 mm) – 12–13 12–13 12–14 – 20–23 18–23RSV areolae (in 10 mm) – – – 18 18–19 25 –1 As Cocconeis granulifera. 2 As Cocconeis guttata.

partially merge with small subtriangular terminal areas(Figure 53).

Geographical distribution Amphicocconeis debesi isdistributed mainly in the eastern sector of the Mediter-ranean Sea, especially in Greece and Turkey. In the west-ern Mediterranean Sea, this species has been identifiedwith certainty in samples from some Italian Tyrrhenianislands (Ventotene, Ischia, Genova) (De Stefano 2001).

Discussion

Cocconeis distans

In the original description, Gregory (1855) reported asdiagnostic character for this species the presence of avalve surface ‘‘characterised by the equal size of the dotsor granules, and their great distance from each other, sothat it almost loses the aspect of striation’’. In 1857,Gregory clarified the valve ornamentation describing‘‘distant lines w«x, about 10 in 0.0010 w«x consisting ofwhite hyaline faint bars, on which are set small and dis-tant granules. w«x They are, as nearly as possible, ofequal size, and from their distance, give to the valve aspotted rather than a striated aspect’’. SEM analysis ofSV shows a peculiar kind of striae protruding on the valvesurface and consisting of repeated protuberances open-ing on the top with transapically elongated poroids. Thecomplex morphology of the striae explains the presenceof ‘‘out of focus areas’’ surrounding each poroid, whichhave been drawn and/or described by several authors(e.g., Gregory 1857, Hendey 1964). The lack of thicken-ing in the interstriae, which are of a size similar to theporoids, confers a regular arrangement to the SV poroidalpattern. The SV poroids, internally occluded by a mas-sive hymen lacking perforations, appear to be almostunique in the genus Cocconeis. The same can be saidfor the fine structure of the RSV poroids, as the typi-cal hymenes with radially arranged slits occurring in allCocconeis species so far studied, are lacking here.

The morphometric data of the specimens examinedfrom both the Hustedt collection and leaves of the sea-grass Posidonia oceanica fall within the size range pre-viously recorded for Cocconeis distans (Gregory 1857,Hustedt 1933, 1955, Hendey 1964, Witkowski et al. 2000)and C. granulifera (Greville 1861, Cleve 1895, Peragalloand Peragallo 1897–1908), except for the number of stri-ae (see Table 2). This parameter, in the SV of our samples(9–11 striae in 10 mm), confirms the number of striaereported by Gregory (1857), whereas it is almost twice

the values reported by Cleve (1895), Peragallo and Pera-gallo (1897–1908), Hustedt (1933) and Hendey (1964)(Table 2). The number of striae on the RSV ranges from18 to 23 in 10 mm in our specimens, whereas it wasabout 13 in 10 mm in specimens described by Cleve(1895), Peragallo and Peragallo (1897–1908) and Hustedt(1933) (Table 2). Sar et al. (2003) described some speci-mens as Cocconeis guttata (figs 16–19, 21), whose grossmorphology should be referred to C. distans. The numberof striae on the SV (7–9 in 10 mm), considered by Sar etal. (2003) as the main reason for assigning their speci-mens to C. guttata, matches with values measured forspecimens of C. distans analyzed in this study. In con-trast, the RSV morphology of the specimen reported bySar et al. (2003) in fig. 20 seems not to be related to C.distans, lacking its characteristic sub-marginal hyalinearea.

The only two validly described varieties of Cocconeisdistans (VanLandingham 1968, 1979), C. distans var. sar-doa Zanon (1948) and C. distans var. bahusiensis Cleve-Euler (1953), were introduced exclusively on the basis ofa drawing of the SV. The overall morphology of C. distansvar. sardoa, with its lyrate sternum (Figure 4), suggeststhe possibility that Zanon observed a specimen belong-ing to Naviculaceae. On the other hand, the valve of C.distans var. bahusiensis (Figures 14a–c), truncate inshape and with large rectangular poroids, could beassigned to Raphoneis Ehrenberg or Achnanthes Bory.The lack of detailed information on both the RSV and SVprevents us from verifying the taxonomic position ofthese two varieties.

Amphicocconeis debesi

Our SEM/TEM observations on historical material fromthe Hustedt Diatom Collection and on epiphytes of Posi-donia oceanica leaves demonstrate that the overall mor-phology and ultrastructure of Cocconeis debesi are nottypical of the genus Cocconeis, whereas they fit those ofAmphicocconeis. The following diagnostic characters ofAmphicocconeis are clearly visible: i) an external raphewith terminal fissures, ii) an internal raphe with co-axialcentral endings, iii) transapically elongated poroidsoccluded by internal, unornamented hymenes bearingirregularly distributed circular perforations, iv) fimbriatevalvocopulae, or with fimbriae merged in a continuouscrenulated edge (Table 3).

The presence of these ultrastructural characters clearlyshows that Cocconeis debesi can be transferred toAmphicocconeis. Following the rules of the ICBN (Article



Table 3 Comparative morphology of Psammococconeis brasiliensis, P. disculoides sensu Garcia, Amphicocconeis disculoides andA. debesi.

Taxa Psammococconeis Psammococconeis Amphicocconeis Amphicocconeisbrasiliensis disculoides sensu

Garciadisculoides debesi

References 1 1 2, 3 4Valve outline Elliptical Elliptical Elliptical, sometimes Elliptical-

ovoid lanceolate to lanceolateApical axis (mm) 16–23 23–32 15–35 22–31Transapical axis (mm) 10–15 14–22 9–26 12–15

Sternum valveCurvature Convex Convex Mostly convex, Mostly convex,

slightly concave in concave in axialaxial area area

External sternum Linear Linear Linear LinearInternal sternum – – Slightly lanceolate Linear to slightly

lanceolateStriae (in 10 mm) 14 7–8 8–10 7–10Striae development Uniseriate Uniseriate Uniseriate UniseriateStriae arrangement Radiate Radiate Parallel, radiate near Parallel, radiate

apices near apicesAreolae (in 10 mm) – – 5–6 7–9External areolae Transapically Transapically Transapically Crater-shaped

elongated elongated elongated with granularsurface

Internal areolae – – Transapically elongated Transapically elongatedMantle areolae Triangular and Triangular and Transapically elongated Transapically elongated

transapically transapicallyelongated elongated

Areolae occlusions Internal hymenes Internal hymenes Internal hymenes Internal hymeneswith fimbriae

Structure of areolar perforations – – Poriform PoriformType of valvocopula Opened – Opened OpenedPars interior: extension Extended – Extended ReducedPars interior: morphology Flat surface with – With partially fused With small,

crenulated edge fimbriae or flat digitiform andand cluster surface with partially fusedof pores crenulated edge and fimbriae

cluster of poresRaphe-sternum valveCurvature Almost flat Almost flat Almost flat Flat to slightly

convexStriae (in 10 mm) 16–23 18–22 24–26 22Striae development Uniseriate Uniseriate Uniseriate UniseriateStriae arrangement Strongly radiate Radiate Parallel, radiate near apices RadiateAreolae in the area Transapically Transapically Irregularly elongated Transapicallyadjacent to sternum elongated poroids elongated poroids poroids elongated

poroidsAreolae in rest of valve Transapically Transapically Transapically Transapically

elongated poroids elongated poroids elongated poroids elongatedporoids

Areolae (in 10 mm) – – About 15 About 20External, central raphe Thin, co-axial Thin, co-axial Broadened, co-axial Thin, co-axialendingsInternal, central raphe Thin, co-axial Thin, co-axial Thin, co-axial Thin, co-axialendingsExternal, polar raphe Thin, straight – Thin, straight Thin, straightendingsInternal, polar raphe Reduced – Reduced Reducedendings helictoglossae helictoglossae helictoglossaeTerminal fissures Absent Absent Deflected to same Deflected to

direction same directionType of valvocopula Opened Closed Opened –Pars interior: extension Extended over 1/3 Extended over of1/2 Extended –

of the valve the valvePars interior: morphology Flat surface with Flat surface with Flat surface with –

sinuate edge and smooth margin and scattered poreselongated pores rows of pores

1 Garcia 2001. 2 De Stefano and Marino 2003. 3 Sar et al. 2003. 4 Present study. –: unknown data.



7), the type material of C. debesi is ‘‘permanentlyattached to the name’’ debesi (Greuter et al. 2000).Therefore, the slide BRM 199/94 and the cleaned mate-rial E5308, putatively the first quoted by Hustedt (1933)(Table 1), should be considered as the lectotype and orig-inal material, respectively of the new Amphicocconeisspecies, which maintains the specific name debesi withCocconeis debesi as the basionym.

The genus Amphicocconeis has up to now includedonly one species, A. disculoides. In addition, to the diag-nostic characters of the genus, A. disculoides exhibits anelliptical, sometimes ovoid outline, SV poroids rectan-gular in shape, and a submarginal hyaline area that sep-arates the mantle from the SV valve face (Table 3).Amphicocconeis debesi differs from A. disculoides inthese last morphological features. It is elliptical-lanceo-late or lanceolate in shape and lacks a clear separationbetween the valve face and the mantle. Its transapicallyelongated poroids are placed in crater-shaped openingshaving a granular matrix. Furthermore, the SV valvoco-pula is fimbriate, but in all specimens observed, the fim-briae are only partially fused (Figures 41, 42). In A.disculoides, unfused or partially fused fimbriate valvo-copulae are present in only small specimens, whereas inlarge specimens fimbriae merge in a continuous crenu-late edge, covering most of the valve surface and areprovided with transapically elongated perforations (Table3). Morphometric parameters of Mediterranean speci-mens match those reported in the literature, with theexception of the higher number of striae (22 rather than16 in 10 mm) on the raphe valve.

In light microscopy the gross morphology of Amphi-cocconeis species could be confused with those ofPsammococconeis, a diatom genus introduced by Garcia(2001). However, the lack of terminal fissures in externalpolar raphe endings in Psammococconeis and their pres-ence in Amphicocconeis easily differentiate the two gen-era. Table 3 reports the comparative morphology ofAmphicocconeis species and those belonging to Psam-mococconeis, which actually are P. brasiliensis Garciaand P. disculoides sensu Garcia, a species that shouldbe renamed after its invalidation (see De Stefano andMarino 2003).

Acknowledgements

The authors are indebted to Dr. R.M. Crawford and F. Hinz fortheir friendly cooperation and for giving the opportunity toobserve types and samples from the Hustedt Diatom Collectionin Bremerhaven, Germany, and to F.A.S. Sterrenburg for con-structive comments, nomenclatural and linguistic assistance onthe final version of the manuscript.

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Received 9 March, 2006; accepted 15 September, 2006

cocconeis distans gregory and amphicocconeis debesi

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