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Journal of Oceanography, Vol. 57, pp. 377 to 384, 2001

Keywords:⋅ Agglutinatedforaminifera,

⋅ allogromiids,⋅ saccamminids,⋅ psammosphaerids,⋅ species diversity,⋅ meiofauna,⋅ komokiacea.

* Corresponding author. E-mail: sehkita@ipc.shizuoka.ac.jp

Copyright © The Oceanographic Society of Japan.

Monothalamous Soft-Shelled Foraminifera at an AbyssalSite in the North Pacific: A Preliminary Report

ANDREW J. GOODAY1, HIROSHI KITAZATO2*, SAORI HORI2 and TAKASHI TOYOFUKU2

1Southampton Oceanography Centre, Empress Dock, European Way, Southampton, SO14 3ZH, U.K.2Department of Life and Earth Sciences, Shizuoka University, Oya 836, Shizuoka 422-8529, Japan

(Received 10 May 2000; in revised form 30 October 2000; accepted 8 November 2000)

Soft-shelled monothalamous foraminifera, including species belonging to the subor-ders Allogromiida and Astrorhizida (families Saccamminidae and Psammosphaeridae),are an abundant and diverse component of the meiofauna in the deep NE Atlantic buthave never been systematically documented in the Pacific Ocean. We examined the32–63 µm and >63 µm fractions of a sample (0–1 cm layer, surface area 52.8 cm2)from an abyssal plain in the subarctic North Pacific, close to the Aleutian Trench(48°05.43′ N, 176°55.06′ E; 5289 m water depth). The residues yielded an estimated2876 stained foraminifera (=545 per 10 cm2) of which >75% occurred in the upper 0.5cm layer and almost half in the 32–63 µm fraction. Rather less than a third (30.5%)of individuals, and about half of the morphospecies (56 out of 121), were soft-shelledmonothalamous forms. Many of these, particularly the saccamminids, were tiny, <120µm in maximum dimension. Based on our analysis of this sample, and previous re-sults in the North Atlantic and NW Indian Oceans, we suggest that these poorly knowntaxa are a consistently important component of the abyssal meiofauna in well-oxy-genated areas.

Most deep-sea records of monothalamous soft-shelled foraminifera are from the North Atlantic Ocean.The purpose of this paper is to document the occurrenceof these poorly-known taxa at an abyssal site in the Pa-cific Ocean. We do not consider the superfamilyKomokiacea, an important group of macrofaunal-sized,soft-shelled agglutinated foraminifera which were firstdescribed largely on the basis of samples from the cen-tral North Pacific (Tendal and Hessler, 1977).Komokiaceans are difficult organisms to evaluate. Theyusually occur as fragments and, because the protoplasmis sparsely developed, it is often impossible to accuratelydistinguish dead specimens from those which were liveat the time of collection.

2. Materials and Methods

2.1 Study siteThe study site (Station 6b) is situated on an abyssal

plain, close to the Aleutian trench (48°05.43 ′ N,176°55.06′ E, water depth 5289 m) and below the car-bonate compensation depth. The sediment is a siliceousooze. At 4800 m depth in the overlying water column,the oxygen concentration was 3.3 ml.l–1, the temperature1.5°C and the salinity 34.68‰ (Hasumoto and Miura,1998).

1. IntroductionForaminifera are a major component of the deep-sea

biota, often constituting 50% or more of total abundanceand biomass in the meiofaunal and macrofaunal size frac-tions (e.g. Snider et al., 1984; Gooday et al., 1992). Theirtests encompass an enormous range of sizes andmorphologies (Loeblich and Tappan, 1987). Agglutinatedtaxa are particularly diverse in the deep sea; commonmorphotypes include tiny spheres a few tens of micronsin diameter, tubes which reach a length of 10 cm, com-plex systems of branched tubules, and a wide variety ofmultichambered forms. Studies conducted in recent yearshave demonstrated that small, monothalamous (single-chambered) foraminifera with soft, flexible test wallsconstitute an important component of the deep-seameiofauna (Gooday, 1986a, 1986b, 1990, 1996). Theseforms include the allogromiids, in which the test wall iscomposed of organic (proteinaceous) material, and spheri-cal or flask-shaped agglutinated forms, which are assignedprovisionally to the families Saccamminidae andPsammosphaeridae.

378 A. J. Gooday et al.

Our site is fairly close to the boundary between thewestern and eastern parts of the Pacific Subarctic GyreProvince, one of pelagic provinces defined by Longhurst(1995, 1998) on the basis of factors such as seasonal pat-terns of solar radiation, thermal stratification of the up-per ocean, and nutrient availability. Ocean surface chlo-rophyll concentrations are fairly low compared to valuesin the NW Pacific (Furuya and Hayashi, 1998) and pri-mary productivity values are in the range 60–100gC.m–2.yr (Berger, 1992). In contrast to similar latitudesin the North Atlantic, there is no spring or early summerbloom (i.e. an accumulation of chlorophyll in the upperwater column) in the subarctic Pacific (see Longhurst,1998; Banse and English, 1999, for recent reviews).

2.2 Sampling methodsSamples were collected using a multiple corer dur-

ing cruise KH97-2 of the R/V Hakuho Maru (Fig. 1). Onecore (diameter 82 mm, surface area 52.8 cm2) was se-lected for foraminiferal research. This was sliced into 0.5cm-thick layers from the sediment surface to a depth of 3cm, and then every 1 cm from 3 cm to 15 cm depth. Thecore sections were fixed immediately in a 4% solution offormaldehyde in seawater (i.e. 10% formalin) bufferedwith hexamethylene tetramine and with added rose Ben-gal (concentration 0.5 g per litre seawater). Only the up-per two layers (0–0.5, 0.5–1.0 cm) were examined forthis study.

2.3 Sample processingIn the laboratory, the 0–0.5 cm and 0.5–1.0 cm lay-

ers of the fixed sample were washed separately throughtwo sieves, mesh sizes 63 µm and 32 µm, to yield >63µm and 32–63 µm residues. These were stored in 140 mlglass bottles in a 20% ethylene glycol-tapwater solution.The residues were split into 1/4 (>63 µm) and 1/2 (32–63µm) fractions using a Folsom plankton splitter. In eachcase, one fraction was sorted in water, under a binocular

microscope, for all stained foraminifera, both hard- andsoft-shelled. Specimens were removed using either anIrwin loop or a fine pipette, placed in glycerol in a glasscavity slide and measured under the binocular microscopeusing an ocular micrometer (accuracy 10 µm). All speci-mens were examined in detail using a compound micro-scope (Nikon TMD cultivation microscope system).

3. Results

3.1 General characteristics of stained foraminiferal as-semblages

3.1.1 Abundance and diversityThe number of foraminifera in the upper 1 cm of

sediment was estimated to be 2876 per core (=545 per 10cm2) of which 2256 (78%) occurred in the upper layer(0–0.5 cm) and just under half (46.7%) occurred in the32–63 µm fraction (Table 1). A total of 121 morphospecieswas recognised in the sample. Of these, 75 species wereconfined to the upper 0.5 cm layer and 12 to the 0.5–1.0cm layer. Nineteen species were found only in the 32–63µm fraction.3.1.2 Gross taxonomic composition

The assemblages included a wide range of major taxa(Table 2). In terms of numbers of individuals, soft-shelledsaccamminids, allogromiids, hormosinaceans,trochamminaceans, other multichambered agglutinatedforaminifera (MAF), and rotaliids were particularly im-portant. The most specious groups were the saccamminidsand allogromiids followed by hormosinaceans,psammosphaerids and the genus Lagenammina .Komokiaceans, particularly “mudballs” resemblingEdgertonia, were also common in the sample. Most speci-mens, however, appeared to be fragments. Since it wasalso difficult in most cases to determine whether they werealive or dead, they were not considered further.

3.2 Monothalmous, soft-shelled foraminiferaThe monothalamous, soft-shelled foraminifera from

Station 6b comprised three groups: 1) allogromiids withan organic test, 2) saccamminids which have a very fine-grained, delicate, and more or less flexible agglutinatedtest with one or two apertures, and 3) psammosphaeridsin which the test has a wall structure similar to that of thesaccamminids but is devoid of obvious apertures. In allthree groups, the test contains either a dense mass of pro-toplasm or stercomata and sparse protoplasm.3.2.1 Suborder Allogromiina

The allogromiids present in the Station 6b samplecomprised forms with (i) thin, transparent organic wallsand (ii) thicker walls, often with a brownish tinge. Thefirst type include a variety of simple, more or less ovalforms with one terminal aperture as well as occasionalelongate morphotypes (Figs. 2A–C). In two species, ap-

Fig. 1. Map showing the area of study. Asterisk indicates Sta-tion 6b where the multiple core sample for this study wascollected.

Soft-Shelled Pacific Foraminifera 379

ertures are present at both ends of the test (Fig. 2E). Thesecond type includes the genera Resigella andPlacopsilinella, each represented by one species, and anumber of elongate tubular forms which may be relatedto the genus Nodellum. These include at least three spe-cies, one of which is illustrated here (Fig. 2D).

3.2.2 Suborder Astrorhiziidae, family SaccamminidaeWhen viewed in water, the agglutinated wall varies

from being very thin and translucent to thicker and al-most opaque. It is sometimes composed of flat-lying,plate-like grains, which impart a silvery reflective appear-ance to the surface. In other forms, the wall consists of

Individuals Species

per split per core (52.8 cm2) per 10 cm2

0–0.5 cm layer>63 µm (1/4 split) 308 1208 229 87

>32–63 µm (1/2 split) 524 1048 198 59

Total 832 2256 427 104

0.5–1.0 cm layer>63 µm (1/4 split) 81 324 61.3 38

>32–63 µm (1/4 split) 75 296 56.0 18

Total 156 620 117 47

0–1.0 cm layer>63 µm 389 1532 290 95

>32–63 µm 599 1344 255 69

Total 988 2876 545 121

Table 1. The numbers of “live” (rose Bengal stained) foraminifera in the upper 1 cm of the multiple corer sample from Station 6b.

Individuals Species

Numbers Percent Numbers Percent

Al l o g ro mi i na: 1 1 0 1 1 . 1 2 3 1 9 . 0Saccamminidae:

So ft -s he l l ed 1 2 8 1 3 . 0 2 3 1 9 . 0Lagenammina 50 5.06 10 8.26

Psammosphaeridae:So ft -s he l l ed 6 3 6 . 3 8 1 0 8 . 2 6Rigid spheres 2 0.22 2 1.65

Tubes 12 1.22 8 6.61Hyperamminacea 19 1.92 3 2.48Ammodiscacea 28 2.83 3 2.48Hormosinacea 134 13.6 12 9.92Trochamminacea 165 16.7 6 4.96Other MAF 127 12.9 9 7.44Miliolina 14 1.42 2 1.65Rotaliina 136 13.8 8 6.61Other taxa 2 0.22 2 1.65

Total specimens 9 8 8 1 2 1

Table 2. Gross taxonomic composition of rose Bengal stained foraminiferal assemblages in the core sample from Station 6b (0–1 cm layer; 32–63 µm plus >63 µm fractions). Specimen numbers exclude fragments but species numbers include fragmentedspecies. Soft-shelled taxa are indicated in bold. MAF = multichambered agglutinated taxa.

380 A. J. Gooday et al.

more rounded grains, resulting in a surface which is dullor has a faint reflective sheen when viewed in water.

Two basic test morphologies occur at Station 6b.(i) Flask-shaped forms with one aperture (Figs. 3A, B, G,H). These are usually oval but occasionally are more elon-gate. In one distinctive species, the aperture is located atthe end of a short neck (Fig. 3H). The more or less flex-ible nature of the test wall distinguishes these foraminiferafrom species of the genus Lagenammina, in which thewall is rigid and more coarsely grained. (ii) Elongate ovalforms with apertures at both ends (Figs. 3C–F). Theseare placed for convenience within the Saccamminidae butmay eventually require the establishment of a distinct

family. The most common single species (Figs. 3C andD) exhibits considerable variability in the thickness ofthe test wall, the development of apertures, and the ap-pearance of the test contents. In another species, thereare two test layers, the outer one forming tubular struc-tures located on opposite sides of the test (Fig. 3F).

Fig. 2. Soft-shelled foraminifera from Station 6b: A–E,allogromiids; F, psammospherid. A, Species withstercomata, protoplasm and inturned apertural structure;0.0–0.5 cm layer, >63 µm fraction, length = 159 µm.B, Species with stercomata and protruding aperture; 0.0–0.5 cm layer, >63 µm fraction, length = 142 µm. C, Specieswith protoplasm and clearly-developed aperture; 0.0–0.5 cmlayer, 32–63 µm fraction, length = 75 µm. D, Elongate formresembling Nodellum with stercomata in proximal part oftest and protoplasm in distal part; 0.0–0.5 cm layer, 32–63µm fraction, length = 134 µm. E, Species with two aper-tures and protoplasm without stercomata; 0.5–1.0 cm layer,>63 µm fraction, length = 134 µm. F, Psammosphaerid spe-cies with thin agglutinated wall; 0.5–1.0 cm layer, >63 µmfraction, length = 92 µm.

Fig. 3. Soft-shelled saccamminids from Station 6b. All speci-mens are from the 0.0–0.5 cm layer. A, Species with thick,almost opaque wall; the most common soft-shelled speciesin this sample; >63 µm fraction, length = 109 µm. B, Samespecies, smaller specimen from the 32–63 µm fraction,length = 100 µm. C, Species with two terminal aperturesand stercomata; 32–63 µm fraction, length = 117 µm.D, Specimen of same species with thicker agglutinated wall;32–63 µm fraction, length = 125 µm. E, Another specieswith two apertures but without stercomata; 32–63 µm frac-tion, length = 100 µm. F, Species with two test layers, outerlayer drawn out into tubular extensions; >63 µm fraction,length = 150 µm. G, Saccamminid with thick wall com-posed of plate-like mineral grains which impart a silveryreflection to test surface when viewed in water; >63 µmfraction, length = 134 µm. H, Species with well-developedneck; 32–63 µm fraction, length = 100 µm.

Soft-Shelled Pacific Foraminifera 381

3.2.3 Suborder Astrorhiziidae, family PsammosphaeridaeMost of the soft-walled members of this group have

more or less spherical tests (Fig. 2F) but we also includehere one elongated morphotype. The thickness of the testwall varies considerably. As in the case of thesaccamminids, it may be very thin, translucent and flex-ible, but other forms have thick or very thick walls com-posed of soft, loosely agglutinated sediment. We distin-guish between soft-shelled psammosphaerids and morecoarsely agglutinated spheres and domes in which the wallis more rigid. Occasional individuals of the latter typeoccur in the Station 6A sample.

3.3 Size distributionIgnoring tubular fragments, the maximum dimen-

sions of individual foraminifera in the Station 6b sampleranged from around 30 µm to 1600 µm (Fig. 4). Mostwere <250 µm long and the majority of specimens rangedfrom 40 to 160 µm with a peak between 80 and 120 µm.We were able to observe the concentration of specimensat the small end of the size spectrum because we exam-ined the 32–63 µm fraction which is usually ignored inforaminiferal investigations. The soft-shelled species con-formed closely to the overall size structure of theforaminiferal population. Allogromiids andpsammosphaerids occurred in equal numbers in the 32–63 µm and >63 µm fractions, but saccamminids, as agroup, were more than three times as abundant in the 32–63 µm residues than in the coarser fraction. Twenty twoindividuals of Saccamminid sp. 1 and 28 of Saccamminidsp. 4 occurred in the fine fraction compared to only 4 and6 specimens respectively in the coarser fraction.

4. Discussion

4.1 Monothalamous soft-shelled foraminifera in the deepseaOur knowledge of deep-sea foraminiferal faunas is

based, to a large extent, on the analysis of dried residues.The process of drying destroys most delicate species(Brodniewicz, 1965) and strongly biases faunal results infavour of the more robust, calcareous and agglutinatedtaxa. This is usually not a serious problem in studies witha geological focus since delicate foraminifera have verylittle fossilisation potential. Some more recent investiga-tors have examined wet sample residues, allowing themto document some fragile agglutinated genera such asReophax, Hormosina and Lagenammina (e.g. Mackensenet al., 1990, 1993). In addition, biological studies haveestablished that large, very delicate, soft-bodiedforaminifera belonging to the superfamily Komokiaceaand similar taxa are a very important component of themacrofaunal size fraction of deep-sea samples (Tendal andHessler, 1977; Gooday and Cook, 1984; Schröder et al.,1989; Kamenskaya, 1993; Kuhnt and Collins, 1995).

The soft-walled allogromiids, saccamminids andpsammosphaerids described in this paper represent an-other important group, which rarely enters the fossilrecord. These fragile organisms are moderately wellknown in shallow-water settings where they are oftenfairly common (e.g. Nyholm, 1957, 1974 and earlier pa-pers; Ellison, 1984). To a large extent, however, they haveeluded recognition in the deep sea. Recent studies in theNorth Atlantic have established that soft-shelledmonothalamous foraminifera are a consistently important

Fig. 4. Maximum test dimensions of all “live” (rose Bengal stained) foraminifera from the >32 µm fraction (i.e. 32–63 µm plus>63 µm fractions) of the sample (0.0–1.0 cm layer) from Station 6b. The black parts of the bars indicate the abundance of soft-shelled foraminifera. Note that the very large (>500 µm) soft-shelled individuals belong to tubular species of “Nodellum”.

382 A. J. Gooday et al.

component of foraminiferal assemblages at abyssal andbathyal sites (Gooday, 1986a, 1986b, 1996; Gooday etal., 1998). They also occur in the central Arctic Ocean(Schewe and Soltwedel, 1998) and at an oxic site belowthe Oxygen Minimum Zone in the NW Arabian Sea butare less abundant in low-oxygen settings (Gooday et al.,2000).

Information on these taxa in the Pacific is verysparse. Resigella moniliforme (Resig) occurs in the SEPacific (Resig, 1982; as Nodellum moniliforme) and thewell-known species Nodellum membranacea (Brady) isreported from the Pacific (Saidova, 1975). These specieshave thicker, more robust tests than other allogromiidsand may remain recognisable in dried residues. Jumarsand Hessler (1976) mention that allogromiid foraminiferawere abundant in a box core raised from 7298 m waterdepth in the Aleutian Trench. A delicate allogromiid fromthe Clipperton Fracture Zone (4960–5154 m water depth)is illustrated by Renaud-Mornant and Gourbault (1990,figures 3A and B therein). Snider et al. (1984) documentthe infrequent occurrence of tiny (<42 µm) allogromiid-like forms among the nanobiota from box cores obtainedat 5800 m depth in the central North Pacific. The presentpaper, and earlier reports on foraminifer in the equatorialPacific (Kitazato and Okamoto, 1997; Okamoto, 1998),establishes that soft-shelled monothalamous species areas abundant in the abyssal Pacific as they are in the At-lantic and Indian Oceans. We speculate that these organ-isms are an important and diverse component of bathyaland abyssal benthic communities in all well-oxygenatedareas of the deep ocean. The abundance of these and otherdelicate taxa (e.g. the Komokiacea) implies that only asmall proportion of deep-sea foraminiferal assemblagesin well-oxygenated areas are represented in the fossilrecord.

4.2 Comparison with Atlantic speciesSome of the morphospecies recognised at Station 6b

are also present in the NE Atlantic. In particular, theallogromiids species with brownish-coloured tests(Placopsilinella auriculata, three forms of “Nodellum”and Resigella), and a distinctive saccamminid species(Fig. 3F), are common to the two areas (see Gooday,1986a, 1996; Gooday et al., 1995). We are currently un-dertaking a more detailed comparison of soft-shelled as-semblages in these two regions in order to establish howmany species occur in both areas. Such comparisons arenecessary in order to establish biogeographic patterns, anessential step towards understanding the relation betweenspecies diversity at local and larger scales (Grassle andMaciolek, 1992).

4.3 Size distributionA remarkable feature of the soft-shelled foraminifera

at Station 6b is the small size of most individuals. This

applies particularly the saccamminids. Gooday (1986a)noted that many saccamminids in samples from thebathyal NE Atlantic (1345 m water depth; >45 µm frac-tion) were <200 µm and often much smaller. In one corefrom the Porcupine Abyssal Plain (4850 m depth), thenumbers of monothalamous soft-shelled foraminifera(undifferentiated) increased progressively in absoluteabundance from the 106–125 µm to the 63–106, 45–63,and 28–45 µm fractions (Gooday et al., 1995, figure 1therein).

4.4 Role in deep-sea ecosystemsThe ecological characteristics of soft-shelled

monothalamous foraminifera in the deep sea are ratherobscure but they appear to be somewhat different fromthose of calcareous taxa. In general, these foraminifera(and perhaps other agglutinated taxa) seem to be less op-portunistic than most calcareous forms, although thereare possible exceptions, for example, Tinogullmiariemanni (Gooday, 1990; Gooday and Turley, 1990). Thisimpression is consistent with colonisation experimentsconducted in the central Pacific, 100 miles south of theHawaiian Islands, by Bertram and Cowan (1999). Theyfound that calcareous species exhibited strong fluctua-tions in abundance in response to pulsed inputs of labileorganic matter whereas the agglutinated species main-tained a much steadier population density.

The mechanisms, which underlie these ecologicalcontrasts, are not clear. Indeed, the apparently greaterdegree of opportunism among calcareous species is rathersurprising in view of the fact that they need to secrete acarbonate shell. However, it may be connected, at leastin part, with trophic mechanisms. Unlike many calcare-ous species, there is usually no evidence thatmonothalamous foraminifera feed directly on labile or-ganic matter such as phytodetritus (but see Gooday 1986a,figure 10A therein). Some monothalamous species maybe deposit feeders which consume bacteria (Gooday,1990; Gooday and Turley, 1990; Turley et al., 1993), whilethose which retain large numbers of stercomata withinthe test presumably ingest sediment particles. An abilityto deposit feed on sparse food resources (e.g. bacteria anddegraded organic matter) may be one reason why somesoft-shelled forms are successful in oligotrophic deep-sea settings.

In addition to being generally less opportunistic, soft-shelled foraminifera tend to be less tolerant of hypoxicconditions than calcareous species in both natural andexperimental settings (Moodley et al., 1997; Gooday etal., 2000). The ability to tolerate low-oxygen conditionsmay be associated with physiological and ultrastructuraladaptations developed within particular phylogenetic lin-eages which happen to secrete a carbonate test (Goodayet al., 2000). Presumably, most soft-shelled foraminiferalspecies do not possess these adaptations.

Soft-Shelled Pacific Foraminifera 383

AcknowledgementsWe are indebted to Professors K. Kawaguchi and I.

Koike of the Ocean Research Institute of the Universityof Tokyo for planning and leading as directors of R/VHakuho Maru KH97-2 cruise to the North Pacific. Wethank A. E. Rathburn and an anonymous reviewer forcomments that improved the manuscript. This research ispartly supported by Grant-in-Aid from the Ministry ofEducation, Science, Sports and Culture of Japan (Funda-mental Research A, No. 08304032 and Fundamental Re-search B, No. 11440154) and scientific grant from theJapan National Oil Corporation. The first author (A.J.G.)visited Shizuoka under the FY1999 JSPS Invitation Fel-lowship Program for Research in Japan (No. S-99271).

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