A taxonomic and distributional survey of marine benthonic Ostracoda off Kerguelen and Heard Islands, South Indian Ocean

From an examination of 34 grab and dredge samples ranging from 110 m to 3584 m water depth, collected during Eltanin cruise 47 across the Kerguelen Plateau, 26 shallow-water and 35 deep-sea benthonic ostracod species have been identified. Systematic notes and illustrations of the common and some of the rare species are presented. Two new species are described: Philoneptunus cassidyi n. sp. and Taracythere abyssora n. sp. Comparisons made with the Atlantic and SW Pacific Oceans and circum-Antarctic regions indicate that the fauna comprises dominantly cosmopolitan deep-sea species while most of the other species have close affinities with the SW Pacific. In the Kerguelen material, seven distinct depth assemblages appear to correspond well with differing watermasses and there is evidence that the relatively shallow position of Antarctic Intermediate Water permits elevation of the upper depth limits of some deep-sea species. Some species have developed ornament of fine reticulation, features not previously seen in those species outside the Kerguelen region.


INTRODUCTION
The present study documents benthonic ostracod crustaceans obtained from surface sediment samples on the Kerguelen Plateau and adjacent abyssal regions of the south Indian Ocean, a locality that, with respect to the Ostracoda, has remained largely unstudied since the Challenger Cruise report of Brady (1880). The Kerguelen Plateau is a large topographic high situated in the south-central Indian Ocean, north of Antarctica. The NW-SE trending northern sector of the Plateau extends from approximately 46(S north of Kerguelen Island to approximately 56(S south of the much smaller Heard Island (Fig. 1). The heavily glaciated Kerguelen Island is 160 km in length and 5820 km 2 in area, with peaks of up to 1965 m, and lies on an aseismic ridge constructed by volcanism from hotspot activity on the Ninetyeast Ridge. Recent investigations as part of the Ocean Drilling Program during ODP leg 183 (Coffin et al., 2000) further confirmed the geological history of this plateau, which indicates it as an uplifted remnant of a Cretaceous ocean plateau that existed to the west of Australia following the separation of India from Western Australia and Antarctica.
The Plateau is an important locality for investigating deep-sea ostracod distribution because of its high latitude and also because conditions that are characteristic of the deep sea, such as low temperatures and high nutrient levels, occur near the surface around Kerguelen. In addition, several watermasses are encountered at the various depths from which the collection originates, thus allowing one to distinguish the relationship of ostracods to depth and watermass characteristics. Deep-sea ostracods from the Southern Ocean are incompletely known because of a lack of sampling and study. Furthermore, previous sampling has indicated generally low ostracod abundance and poor preservation. This study is the first to report species from a wide depth range of material, often well preserved, in the Southern Ocean. Changes seen in the ostracod assemblages with depth, between 110 m and 3584 m, and their relationship to environmental parameters are the focus of this contribution.
The bathymetric distribution of selected ostracod species from the Kerguelen Plateau found in the present material was reported previously (Ayress et al., 1997). That paper indicated that there may be a correlation of the ostracod distributions & Gordon (1996) who concurred that the northern portion of the Polar Front is located near Kerguelen. Nevertheless, surface waters around Kerguelen Island display much seasonal variation in salinity, temperature and nutrient levels (Fig. 3). The Plateau itself is also located within the region of iceberg drifts, and erosion due to glacial activity on the island must contribute to the coarse sediment (sands and gravels) that is deposited on the sea floor down to 500 m. In addition, turbidity flows are a likely feature of this tectonically active portion of an ancient continental crust that has witnessed much volcanic activity. Ostracod shells displaced downslope are usually readily recognized as such by their preservation, morphology (e.g. presence of eyespot) or unusual depth of occurrence (Passlow, 1997).
The waters surrounding Kerguelen Island only represent a small fraction of the vast Kerguelen Plateau that is immersed by Southern Ocean waters. It is of no surprise, therefore, to find that the water characteristics down to 2000 m in the northern end of the Plateau (e.g. 74(E 47(S, called here Station A) differ greatly from those waters east or south of the Plateau (e.g. 76(E 51(S, called here Station C, and those near 70(E and 53-54(S, herewith labelled Stations D-F) as shown in the physicochemical profiles illustrated in Figure 2.
Several local watermasses are recognized, see Table 2 and Figures 2-3, and will not be described as they belong to well-known watermasses characteristic of the Southern Ocean (Tomczak & Godfrey, 1994).

Bathymetric distribution of the Ostracoda
It may be seen from Figure 4 that the ostracod fauna recorded from the Kerguelen Plateau includes shallow-marine to deep-sea taxa, together with a few (e.g. Bradleya normani) showing a wide bathymetric distribution. As indicated by peaks in species' upper and lower depth limits and, to some extent, peaks in diversity (see Fig. 5), a depth of around 500 m marks a distinct change in the nature of the ostracod fauna. As in other basins of the world's oceans, a dramatic increase in the deep-sea dominant Krithe (Fig. 6) is seen from around 500 m. With increasing depth beyond 500 m the upper limits of various deep-sea (psychrospheric) species are gradually encountered. A dendrogram of similarity analyses (see Fig. 7) based on species' presence/ absence data presented in Figure 4, indicates clearly five depth assemblages (labelled 1-5), with the lower two further divisible into four distinct assemblages. Comparing the distribution of these assemblages against watermass (Figs 4,8), one can suggest that the depth ranges of ostracod species are governed to a high degree by differing hydrographic properties of the various watermasses. The observed ostracod assemblage composition occurring within each watermass appears to be significantly distinct.
In the Subantarctic Upper Water (SAUW), the ostracod fauna is mainly composed of typical shallow-marine circum-Antarctic species, notably the hemicytherids Patagonacythere wyvillethomsoni and Hemicythere kerguelenensis, together with Neonesidea labiata and Xestoleberis setigera. The shallowestmarine sample analysed at 110 m within Upper SAUW, contains the dominant species Patagonacythere wyvillethomsoni and Loxoreticulatum fallax, an assemblage (Cluster assemblage 1) significantly different from deeper samples (215-280 m) taken from within Lower SAUW that consists of the Cluster 2 assemblage, characterized most notably by the addition of species Bradleya normani and Henryhowella sp.
In the samples of upper bathyal depth (280-457 m) a transition from a characteristically shallow to deep-sea fauna is seen. This faunal change is possibly the most pronounced of the entire ostracod depth zonation recorded here, and clearly influenced by the position of the Antarctic Intermediate Water (AAIW). The AAIW core, at around 500 m, appears to be a significant factor controlling both the upper depth limit of Assemblage 3 species (e.g. Krithe cf. capensis and Philoneptunus cassidyi), as well as the lower depth limits of almost all species occurring in SAUW.  also found the AAIW core, a salinity minimum zone, to be a significant factor in controlling faunal depth distribution. For additional details on watermasses in the Indian Ocean sector of the austral oceans, refer to the recent work of Müller & De Deckker (2003) which shows that the AAIW is definitely a low salinity watermass characteristic of low temperatures (2-4(C). Here, around Kerguelen Plateau, it is somewhat shallower and that explains the 'shallower' depth distribution for some ostracod species. Rare specimens of the characteristic deep-sea species Legitimocythere acanthoderma are also noted close to the upper boundary of AAIW at and below 425 m. Compared to elsewhere in the world's oceans, the relatively shallow position of AAIW over the Kerguelen Plateau has allowed some species, particularly L. acanthoderma and P. cassidyi, to reach unusually shallow limits.
In the deep-water globigerinid and diatom oozes sampled at and below 600 m within the AAIW and Circumpolar Deep Water (CDW) deep-water species dominate the assemblages. Relatively rare, allochthonous shallow-marine taxa identified by eye tubercles (e.g. Cativella bensoni) are locally present down to 1587 m, but are extremely rare below this depth. Eye tubercles in Bradleya normani are reduced in size down to 613 m and absent at, and below, 747 m.
The upper depth limits (UDL) of four species (Krithe reversa, Trachyleberis bathymarina, Henryhowella cf. dasyderma and Poseidonamicus aff. ocularis) coincide with the upper boundary of the CDW watermass and characterize Assemblage Cluster 4a of CDW Upper watermass. From 1200 m, where temperatures are less than 2(C, Lower CDW (I) Assemblage Cluster 4b species are seen, notably Krithe cf. dolichodeira and Pelecocythere trinidadensis. Further decrease in temperature with depth appears to be an important factor for Cluster Assemblage 5, comprising additional 'psychrospheric' species within watermass CDW Lower (II). The assemblage is similar to that of CDW Lower (I) but consists of additional species from 1902 m (e.g. Bathycythere audax and Poseidonamicus major). The cluster analysis separates a further grouping in the deepest samples from 3277 m to 3584 m. This is mainly because several species have their lower depth limits in the sample above, at 3150 m, but also additional species occur at and below 3150 m (e.g. Pterygocythere mucronalata and Taracythere abyssora) and below 3584 m (Dutoitella suhmi and Vitjasiella belyaevi), where they inhabit the coldest temperatures <1(C, recorded in this study.

Faunal comparison and affinities
A total of 35 deep-sea species (those occurring >500 m) recovered from the Kerguelen Plateau are listed in Table 3, Table 4, together with their recorded Quaternary and Recent occurrences in five selected deep-water study areas (in which one can be confident that species taxonomy does not vary significantly from that of the present study) in order to show the geographical affinities of the Kerguelen deep-water ostracod fauna. The areas comprise the North Atlantic Ocean (as defined by Whatley & Coles, 1991), the Southeast Atlantic off southwest Africa studied by , the Southwest Atlantic area including the Strait of Magellan and Scotia Sea (Whatley et al., 1996(Whatley et al., , 1998 and the Northeastern Indian and Southwest Pacific Oceans (Whatley & Ayress, 1988;Coles et al., 1990). Additional data on the distribution of Krithe species are taken from Coles et al. (1994) and Ayress et al. (1999).
From Table 4 it can be seen that the majority of deep-water species recorded on the Kerguelen Plateau also occur in one or more of the other deep-sea areas, with 14 taxa previously known from three or more of the regions. This is not unexpected since the Southern Ocean is in direct communication with all the other   (20), with fewer (13) from the Indian Ocean and off southwest Africa (9). Fewest (6) species are shared with the Southwest Atlantic, probably due to the lack of research on bathyal and abyssal faunas from this area, despite the occurrence of typically psychrospheric species at shallow shelf depths due to the cold water temperatures and strong upwelling in the southern Strait of Magellan . Further research, especially on the relatively poorly known Indian Ocean faunas will undoubtedly reveal more cosmopolitan species, as will further study on small taxa, particularly the Cytheruridae, and on genera such as Krithe where species-level taxonomy has been problematic. Nevertheless, the Kerguelen fauna does contain elements of exclusively Pacific affinity. Philoneptunus, represented by abundant P. cassidyi in the present study, is the first record of this genus away from the Australian/New Zealand region (Whatley et al., 1992). Also new are the occurrences of Bisulcocythere, Clinocythereis and Trachyleberis bathymarina, taxa previously known only from the Australasian region (Ayress & Swanson, 1991;Ayress, 1993); these taxa are clearly more widely distributed in the Southern Hemisphere than previously reported. Other species of 'eastern' affinity include Cytheropteron aff. C. planalatum and Taracythere abyssora, although these species have not, as yet, been definitely recorded from the Pacific. Few taxa seem to be of Atlantic origin; Buntonia, a genus common in the Atlantic and rare in the western Indian Ocean is absent from the Kerguelen material. Echinocythereis echinata has a similar distribution to Buntonia and is found in this study, albeit very rarely.

Observations on preservation state and unusual carapace calcification
The physical appearance of the ostracod valves found in this study varies greatly. Pristine valves that are transparent, often with soft-parts intact, were found to predominate at shallow stations down to 457 m. White opaque and corroded valves become increasingly numerous from 600 m onwards. Two samples, at 1098 m and 1885 m, are represented only by a few severely corroded specimens, whereas other deep samples   contain abundant specimens showing a range of glassy and opaque states, the latter more frequent with increasing depth. This trend in deteriorating preservation with depth mainly reflects the effects of the lysocline, where seawater progressively approaches carbonate undersaturation and becomes increasingly so with depth. Nevertheless, transparent valves were sometimes found at considerable depth and these, presumably, were sampled live or soon after their death. During life, a thin chitinous covering provides protection from corrosion. After death, the coating decomposes or is attacked by bacteria, thus, exposing the calcitic shell to chemical corrosion. A number of factors, such as rapidity of burial within the sediment (Kaesler et al., 1993) and shell morphology (Swanson, 1995) retard shell corrosion.
Corrosion of calcite within the lysocline is a global phenomenon but certain morphological aspects of the Kerguelen material are apparently unique to the region and, therefore, deserve comment. Fine reticulation occurs on the surface of usually smooth-shelled cosmopolitan species, such as Pterygocythere mucronalata. Similarly, Legitimocythere praesequenta has a fine lace-like reticulum (see Pl. 1, fig. 16) and, perhaps, this phenomenon is best seen in Philoneptunus cassidyi where the secondary reticulum is increasingly apparent with depth. Clearly, microreticulation development has a genetic basis in some species (Neil, 2002), however, observation leads to the conclusion that for the taxa noted above, there appears to be a direct relationship between microreticulation development and calcite content within waters of the lysocline.

CONCLUSIONS
Over 60 benthonic ostracod species have been found in grab and bottom dredge samples ranging from 110 m to 3584 m water depth across the Kerguelen Plateau, a region hitherto lacking study of its Ostracoda for over a century. Two new species are described: Philoneptunus cassidyi n. sp. and Taracythere abyssora n. sp. The ostracod fauna comprises dominantly cosmopolitan deep-sea species, while most of the other species have close affinities with the SW Pacific. In the Kerguelen material, at least seven distinct depth assemblages appear to correspond well with differing watermasses in contact with the seafloor at water depths ranging from 110 m to 3584 m. The relatively shallow position of the AAIW has allowed some species to extend to an unusually shallow limit. This study confirms previous suggestions that ostracod species could provide useful proxies for various watermasses of the deep sea.

ACKNOWLEDGEMENTS
This study is dedicated to Richard Benson who died in 2003. He very generously allowed access to the picked slides housed at National Museum of Natural History, Washington, DC upon which the present study is based. The original samples were collected during Eltanin cruise 47. This study developed from a collaboration established during discussions at an international workshop of deep-sea ostracod researchers held in Reston, Virginia in 1993, organized by Tom Cronin and sponsored by the United States Geological Survey. Rosalie Maddocks of the University of Houston is gratefully thanked for her extensive and helpful comments on an early version of the manuscript. Alicia Moguilevsky helped with comparison of unpublished    Description. Carapace large, well-calcified without eye tubercle; right valve outline sub-rectangular, left valve outline subtriangular, males more elongate than females. Anterior margin weakly convex above mid-height, strongly convex below with approximately 10 prominent marginal spines. Posterior margin asymmetrically convex with ventral apex bearing about five prominent marginal spines. Dorsal margin steeply inclined to a high anterior hinge ear; ventral margin straight. Subcentral tubercle weak. Short but prominent, arcuate 'ocular' ridge links with a long, continuous anterior and ventro-lateral ridge at the anterior hinge ear; the latter ridge has spinose posterior termination. A shorter dorsal ridge extends close to margin posteriorly where it is prominent and continues ventrally to about midheight just posterior of a short median ridge. Inter-ridge areas primarily and variably secondarily reticulate, muri of the former often weak away from ridges. Internal features typically trachyleberid with holamphidont hinge and undivided subcentral muscle scars.
Distribution. This species is recorded over a wide depth range (457-3150 m) on the Kerguelen Plateau. However, it is only common or abundant between 747 m and 2322 m, suggesting it is an outer shelf to slope species. The findings here represent the first modern record of the genus Philoneptunus outside the Australasian and SW Pacific area.
Philoneptunus cassidyi has also been recorded in the Pleistocene of DSDP Sites 206, 208 and 284 (present-day water depths 3196 m, 1545 m and 1066 m, respectively) west of New Zealand (Whatley et al., 1992); off southeast Australia, in Late Pleistocene cores at water depths 2346 m and 3552 m (seen by MA in the unpublished MSc. thesis material of Passlow, 1994) and in coretop and sediment surface grab samples on the Chatham Rise at water depths between 800 m and 1400 m (pers. comm. H. Neil, 1993). A similar species has also been recorded from Miocene ODP site 689, Maude Rise, Antarctica (Majoran & Dingle, 2002).

Remarks.
Although always at least partially secondarily reticulate, the degree of fossal subdivision and strength of primary muri of this species is variable. Through its upper depth range, between 457 m and 1110 m, the species is distinctly smaller and subdivision of the reticulation is minimal (Pl. 2, figs 1-3). At and below 1110 m a larger form occurs which may be moderately secondarily reticulate (Pl. 2, figs 7 and 8) or densely so (Pl. 2, figs 9-11). Philoneptunus paragravezea Whatley et al. (1992), described from deep water surrounding Australia and New Zealand, is very similar. That species differs from P. cassidyi, however, in having secondary reticulation confined to anterior and posterior fields and its median ridge is more robust. Description. Carapace large, sub-ovate in lateral view, with weakly rimmed and denticulate, evenly convex anterior and posterior margins; ventral margin gently convex with very slight oral incurvature. Dorsal margin straight to slightly convex with distinct hinge-ears. Inflation tumid postero-ventrally, anterior somewhat compressed. Surface covered with polygonal reticulation bearing densely crowded short mural spines. Muri are aligned sub-parallel to margins anteriorly and ventrally and the spines in these regions are usually paired. A distinct stout spine occurs close to margin postero-ventrally. Normal pores very small emergent at centre of spine clusters. Inner lamella narrow, without vestibule. Radial pore canals straight, about 21 true canals anteriorly, nine posteriorly. Hinge holamphidont. Subcentral muscle scars undivided, typically trachyleberid.

Distribution. Only presently known from the Kerguelen Plateau at abyssal depths between 3150 m and 3584 m.
Remarks. This is the first record of the genus outside the western Pacific region. The new species has the distinctively upswung posterior margin and prominent postero-ventral spine charac-teristic of Taracythere (Ayress, 1995). Among the Taracythere species described by Ayress (1995) T. abyssora is most similar to T. conjunctispinosa, but T. abyssora has a more even and densely spinose ornament than the latter species, distinct from the web-like reticulation and conjunctive spines of T. conjunctispinosa. 'Cythere' melobesoides Brady of Nohara (1987) from Okinawa-Jima also belongs to Taracythere but to a separate species which differs from T. abyssora mainly in its fewer and stronger spines. Schornikov, 1976 (Pl. 3, fig. 1) Brady (1880) may have included this species in Vitjasiella fenestrata (Brady, 1880) and originally recorded at two stations: 149 (274 m off Kerguelen Island) and 335 (2606 m in the central South Atlantic). Puri & Hulings (1976) selected a lectotype from Brady's station 149. The material here differs from the lectotype of V. fenestrata in being considerably larger, length 1.37 mm (lectotype is 0.93 mm 0.56 mm) and having a less angular dorsal margin. This species has been reported from the northwestern Pacific, between 5090 m and 5200 m (Schornikov, 1976(Schornikov, , 1981 and the authors (MA, GC) have seen it in the material of Barkham (ms, 1985) off northwest Africa between 1775 m and 3083 m. Thus, both species appear to occur on the Kerguelen Plateau.

Vitjasiella belyaevi
Cluthia sp. (Pl. 4, fig. 7) Ayress & Drapala (1996) recorded six species of Cluthia from the Late Eocene to Recent of Australasia, a genus better known from cold waters of the Northern Hemisphere. The material here is closest to Cluthia micra and Cluthia sp. 2 of Ayress & Drapala (1996), but seems to differ from all species of the genus in its continuous posterior ridge uniting dorsal and vento-lateral ridges. The present record further indicates the bihemispheric distribution of the genus.

Pterygocythere mucronalatum (Brady, 1880) (Pl. 3, fig. 3)
This cosmopolitan abyssal species usually has a smooth valve exterior surface. The specimens here are finely reticulate on the posterior half of the valve surface, but in all other respects they conform to the species. Some workers place this species within Bosquetina Keij, 1957. Its similarity with that genus seems to be rather superficial, however, since there are clear internal differences from Bosquetina in the present species, notably the accommodation groove, two frontal scars and a prominent anterior median hinge tooth. It appears to be closer to Pterygocythere, differing from that genus only in lacking an eyespot, and was assigned to Pterygocythere by Coles et al. (1990), Whatley & Coles (1991) and Cronin (1996). Stratigraphical considerations also suggest a closer relationship to Pterygocythere as the oldest occurrence of P. mucronalatum is in the Early Palaeocene of the North Atlantic (Whatley & Coles, 1991) and the genus Pterygocythere is well known from the Late Cretaceous (Coles et al., 1990), whereas Bosquetina is confined to the Oligocene to Recent in relatively shallow-marine sediments. Guernet & Moullade (1994) propose a new genus, Pseudobosquetina, for this species on the basis of hingment that, in the authors' opinion, is insufficient to support separation at the generic level.

, figs 4-6)
This species is closely related to a group of species, many of which are yet to be described (see for example Cytheropteron sp. 1 Ayress, 1994, fig. 12;and Cytheropteron sp. 1 Ayress, 1996, pl. 1, fig. 12), from the Indo-Pacific. Cytheropteron planalatum Guernet, 1985, from the Upper Eocene of DSDP Site 214, is typical of the group and the present species differs from it in having coarser punctation and in lacking dorsal ridges. It is similar to an Antarctic species erroneously referred by various authors (e.g. Neale, 1967) to Cytheropteron abyssorum Brady (1880) and well illustrated by Whatley et al. (1988, pl. 3, figs 5-7). That species is confined to shallow water and differs from the present species, and from C. abyssorum sensu stricto, in having an eyespot, and a more rhomboidal lateral outline (Passlow & Ayress, 1994).

Cytheropteron perlaria Hao, 1988 (Pl. 3, figs 7-8)
Following the review of Swanson & Ayress (1999), the material here is referred to Cytheropteron perlaria rather than the very similar species Cytheropteron testudo Sars, based on the weaker alar process and more triangular shape of C. perlaria. In the material here, C. perlaria is confined to relatively shallow-water depths between 110 m and 457 m and is only common at 110 m. The deep-sea global distribution of this species today indicates its preference for cold water (Swanson & Ayress, 1999), and its record here in Antarctic waters is consistent with this observation. It has also been found in Antarctica at 330 m in the Ross Sea (Briggs, 1978) and living at 434 m on the MacRobertson Shelf (pers. comm. A. Rathburn, 1994). Ishizaki, 1966 (Pl. 2, figs 15-19; Fig. 9A) The Antarctic species Krithe magna (Hartmann, 1986), described from female valves as Profundocythere magna, resembles K. antisawanensis in valve shape and in the relatively narrow inner lamella, but has a much larger anterior vestibule. Krithe magna (Hartmann) illustrated by Whatley et al. (1998) from 1555-3925 m water depth in the Scotia Sea resembles the material here in size and sexual dimorphism, although the specimens of Whatley et al. differ in shape, especially in the more arched dorsal margin of the male valve. Krithe dilata Ayress et al. (1999), described from the Early Pliocene to Recent of the Tasman Sea, is rather similar in valve shape, but has a much larger, distally expanded anterior vestibule. Amongst the present material, K. antisawanensis most closely resembles K. cf. K. capensis , although the posterior is more truncate in the present species, and the anterior vestibulae of the two species differ in size and shape.

Krithe antisawanensis
This species is restricted in the material here to four samples ranging from 280-915 m; it is common only at 747 m. Ayress et al. (1999) recorded Krithe antisawanensis from the Middle Miocene (NN6) to Recent of the Tasman Sea off Southern Australia, in water depths ranging from 686-3403 m. Zhou & Ikeya (1992) report this species from the Miocene to Pleistocene of Japan and from 150-600 m water depth in Suruga Bay, Japan. et al., 1990 (Pl. 4, figs 5-10; Fig.  9C) In valve size and shape, particularly the rather narrowly convex and somewhat upturned anterior margin, and in the form of the anterior vestibulum, this species is comparable to K. capensis . However, a firm identification of the current material with K. capensis is not made as the Kerguelen specimens are less prominently postero-dorsally arched than K. capensis. In addition, the material here displays strong sexual dimorphism, but Dingle et al. do not allude to this in their treatment of the species. Krithe sp. 19 of Coles et al. (1994) closely resembles the material here and is considered conspecific, although the anterior vestibule in the Kerguelen specimens is rather smaller. Krithe sp. from 274 m in Sulzberger Bay, Ross Sea (Benson, 1964) probably also belongs to this species, although the muscle scar pattern illustrated by Benson, if correct, does not match the specimens here. This species is separated from the similar K. antisawanensis in the material here by virtue of its more gently truncate posterior margin and larger mushroom-shaped anterior vestibulum.

Krithe cf. K. capensis Dingle
Krithe sp. cf. K. capensis is recorded over a wide depth range between 600 m and 3584 m on the Kerguelen Plateau, where it is locally common, most notably at 1587 m. Krithe capensis was recorded from 238-1430 m off southwestern Africa by  and was considered the dominant species on the continental shelf and uppermost slope. Coles et al. (1994) recorded this species as Krithe sp. 19 rarely in the Quaternary (NN19-21) of DSDP Hole 606A in the North Atlantic. , 1946 (Pl. 5, figs 11-16; Fig.  9D)

Krithe dolichodeira van den Bold
The Kerguelen specimens in this study and supplementary material from the Drake Passage (Eltanin Cruise 43,52( 38'S,75( 25'W,787m) conform to K. dolichodeira in all respects except size; the length of that species is usually less than 0.90 mm, according to Coles et al. (1994). The slight size difference between the Kerguelen and the North Atlantic material is not considered to warrant any specific distinction. Abate et al. (1993) demonstrate that Krithe compressa (Seguenza, 1880), previously considered by Coles et al. (1994) as a possible senior synonym of K. dolichodeira is actually a senior synonym of K. aequabilis Ciampo, 1986. Krithe dolichodeira is present at 787 m and is rare to moderately common between 1404 m and 3150 m on the Kerguelen Plateau. It is very widely distributed in the Lower Eocene (NP 10) to Recent from localities in the Atlantic, Caribbean and Meditteranean (Coles et al. 1994) and in the Late Miocene to Recent of the Tasman Sea between 686 m and 3281 m . , 1960 (Pl. 5, figs 17-23; Fig.  9E) Although the Kerguelen material is rather larger than K. morkhoveni from the North Atlantic (comparing dimension ranges given in Coles et al., 1994), the shape of the anterior vestibule, inner lamella and details of the lateral outline conform with the range of variation seen in K. morkhoveni. Relatively large size appears to be a consistent phenomenon among the Krithe species of this study compared with other regions; this may be a function of low water temperatures represented by the material here.

Krithe morkhoveni van den Bold
Krithe morkhoveni is recorded from three samples between 1045m and 1587m, but is only common at 1110m. This species is very widely recorded from the Lower Eocene (NP 10) to Recent of the Atlantic and adjacent onshore areas (Coles et al. 1994). Ayress et al. (1999) 5, figs 1-4; Fig. 9B) The carapace morphology matches very closely K. praetexta (Sars), a common northwest European shelf species recently reviewed and illustrated by Abate et al. (1993). However, the more bluntly truncate posterior margin and somewhat larger size (Northern Hemisphere forms rarely exceed 0.90 mm in length) of the specimens here suggest that the Kerguelen material may be separable.
This species was only found in two samples: it is common at 425 m but is represented by a single, possibly transported, valve at 747 m. Krithe praetexta is known from the shelf sediments in the Quaternary and Recent of northwest Europe and Mediterranean and Upper Pliocene to Quaternary of Sicily (Abate et al., 1993;G. Coles, pers. obs.).

Krithe reversa van den Bold, 1958
This cosmopolitan deep-water Krithe species is easily identified by its reversed (RV > LV) overlap, large size, strong sexual dimorphism and anterior radial pore canal pattern. In China and Japan this species has usually been recorded as Krithe sawanensis Hanai, 1959. In the material here Krithe reversa was sporadically recorded in the deeper waters of the Kerguelen Plateau between 1902 m (where it is abundant) and 3584 m (the deepest analysed sample). Two valves are also noted at 915 m. The species is cosmopolitan in the deep sea from the Middle Miocene (NN 6) to Recent and most typical of waters of 1000 m or deeper (Coles et al., 1994). Zhou & Whatley (1997) record K. reversa between 640 m and 2054 m in the East China Sea, while Ayress et al. (1999) record it from the Lower Pliocene (NN 13) to Recent of the East Tasman Sea.
The species has a considerable depth range on the Kerguelen Plateau, occurring from 215 m to 3584 m and is present in most (67%) of the samples. In the material here, eye tubercles are large in this species down to a depth of 457 m, reduced in size down to 613 m and absent at and below 747 m. Internal soft-parts and pore setae are occasionally seen intact (Pl. 2, fig. 13). It is superabundant at 280 m and common between 425 m and 613 m. However, in the deeper-water samples examined between 747 m and 3584 m it is, except for the sample at 1587 m, a minor component of the ostracod assemblage. Given the abundance of sighted Bradleya spp. occurring today at shallow depths, it seems likely that the closest ancester of B. normani was also sighted and, in this instance, blindness in deeper occurring individuals does not necessarily indicate deep-water ancestry, as has been suggested for some deeper living taxa (Dingle, 2002(Dingle, , 2003. Benson, 1972 (Pl. 3, figs 17-19, 23) The greater emphasis of vertical muri of the mid-posterior reticulum and a slightly more angular posterior margin distinguish P. major from P. aff. P. ocularis of this study. Juveniles were also distinguished by virtue of their secondarily punctate anterior field (Pl. 3, fig. 23).

Poseidonamicus major
Poseidonamicus major is confined to the deeper analysed samples on the Kerguelen Plateau, recorded between 1902 m and 3277 m. Whatley (1985) records only juveniles from the Neogene of Coral Sea and Tasman Sea DSDP sites.  discuss the stratigraphical and geographical occurrence of P. major and record the species off southwestern Africa at 2070 m and abundantly at 2916 m.

Poseidonamicus aff. P. ocularis Whatley et al., 1986 (Pl. 3, figs 11-16)
In the material here there is variation between a uniformly developed post median reticular field and some tendency for vertical mural alignment and, in this latter respect, the material here shows affinity to P. ocularis Whatley et al., 1986. The truncate anterior and posterior margins reported to be diagnostic of P. ocularis were sometimes also observed in the material here. In adults an ocular sinus is usually absent but, in juveniles, an ocular sinus is usually present and, externally, a small eye tubercle is sometimes visible. Specimens below a depth of some 2700 m show a tendency towards subdivision of the anterior punctate field (Pl. 3, fig. 15). Because of the observed differences, especially in the pattern of the reticulation, the material here has been separated from P. ocularis s.s., although the authors prefer not to formalize this distinction at present. For comparative purposes, specimens from the East Tasman Plateau (water depth 1900 m, lat. 46( 15.08'S, long. 146( 32.49'E), which also have affinities with P. ocularis are illustrated (Pl. 3, figs 20-22). Particularly notable are the smaller size and relatively truncate anterior compared to the Kerguelen material. The relationship between these forms and Poseidonamicus major s.s. is unclear at present and awaits a better understanding of the range of variation of each species. This species is present over a wide depth range (915-3584 m) on the Kerguelen Plateau and is most commonly recorded between 1404 m and 1902 m.

Bathycythere audax (Brady & Norman, 1889) (Pl. 1, figs 1-5)
The material is assigned to Bathycythere audax (Brady & Norman) based on comparison with the illustrations provided by Brady & Norman (1889). The degree of spine development in juveniles (Pl. 1, figs 4 and 5) is very similar that seen in the type species, B. vanstraateni Sissingh, 1971 recorded from the Quaternary of the Mediterranean, and western Indian Ocean. Previously, most records of what one recognizes as B. audax have been assigned to B. vanstraateni, possibly because only juvenile specimens were found. It is observed that the two species can be separated on the basis of the lateral outline: that of B. vanstraateni is trapezoid with its maximum length situated ventrally, whereas that of B. audax is more ovate with its maximum length at mid-height. In Kerguelen, the reticulation and greater number of spines in adult B. audax further serve to distinguish these species. Also in the Kerguelen material, the adductor muscle scars and the ovate frontal scar are never subdivided. Only rarely in some juvenile specimens from the Tasman Sea (Ayress, pers. obs.) does subdivision of the scars approach the condition seen in B. vanstraateni, as illustrated by Sissingh (1974). Ruan (1989, pl. 24, figs 1-3) illustrates a similar form to B. audax collected from 2004 m and 2120 m in the South China Sea. However, it is slightly smaller (length around 1.30 mm) is somewhat more elongate and has a much stronger subcentral tubercle.
The current material was recorded from the deeper waters of the Kerguelen Plateau between 1902 m and 3584 m. Bathycythere audax has been documented world-wide from the Middle Miocene to Recent as B. vanstraateni by Whatley & Coles (1987) and Whatley & Ayress (1988) and as B. audax by Coles et al. (1990) and Cronin (1996). Judging from the illustrations and size range, Ruan & Hao (1988) have also found this species (recorded as Acanthocythereis cf. araneosa Howe) in their material from the Okinawa Trough.
The form illustrated by Briggs (1978) as 'Echinocythereis' sp. from the Pleistocene Taylor Formation of Ross Island, Antarctica appears to be conspecific with the material here.

Legitimocythere acanthoderma (Brady, 1880) (Pl. 1, figs 6, 7)
This species can be confused with Bathycythere audax but is easily separated, however, by virtue of its strongly spinose and well-developed subcentral tubercle present in all juvenile stages, its smaller size, denser spine distribution and reticulation which joins all spines. Legitimocythere Coles & Whatley differs from Bathycythere Sissingh in muscle scar pattern: Legitimocythere has a 'V'-shaped frontal scar compared with the ovate frontal scar of Bathycythere. In addition, the subcentral tubercle is much more prominently developed in Legitimocythere compared with that of Bathycythere.
Legitimocythere acanthoderma is consistently present over a wide depth range between 425 m and 3584 m on the Kerguelen Plateau, although it is only common at and below 600 m. This is a common world-wide deep-sea species, originally described by Brady (1880) from 1375 fm. (2627 m) near Prince Edward Island (46( 46'S;45( 31'E). It has been recorded over a considerable depth range from as shallow as 318 m in Prydz Bay, Antarctica (Ayress, own data) to greater than 5000 m in the North Atlantic (Coles & Whatley, 1989;. In addition to the occurrences cited in Coles & Whatley (1989), L. acanthoderma has also been recorded at 2916 m and 4736 m off southwestern Africa , between 386 m and 456 m in the southern Strait of Magellan (Whatley et al., 1996) and at 2370 m in the Scotia Sea (Whatley et al., 1998).

14-16)
The material here displays fine secondary reticulation, comprising a dense lace-like network on the valve surface, a feature not recorded previously in this species, and is considerably larger than the Eocene specimens illustrated by Coles & Whatley (1989).
Legitimocythere presequenta is confined to the deepest analysed samples in the material here, between 2970 m and 3584 m. This is a widely distributed abyssal species, although it is typically a rare component of any particular assemblage. Coles et al. (1990) record it from the Lower Eocene to Recent of the North Atlantic and the Pliocene to Quaternary of the Pacific; Ruan & Hao (1988) record it from the Okinawa Trough and Ruan (1989) from the Xisha Trench, South China Sea. Coles & Whatley (1989) detail other known occurrences. It is likely that this species has been previously under-reported, perhaps due to confusion with L. acanthoderma (Brady). Ayress, 1993 (Pl. 3, fig. 10) The type material from the Tasman Sea display fine secondary reticulation. The Kerguelen material lacks this ornament detail. Ayress (1993) has discussed the relationship of this species with other similar trachyleberid genera.

Trachyleberis bathymarina
Trachyleberis bathymarina is superabundant at 915 m on the Kerguelen Plateau, and is also more rarely recorded in the material here between 1110 m and 2306 m. Ayress (1993) described this species from the Late Quaternary to Recent of the Tasman Sea between 1340 m and 2238 m water depth and from 1204 m to 3125 m water depth on the Chatham Rise.
Polycope sp. (Pl. 2, fig. 14) Ruan (1989) illustrates an identical form from around 2000 m in the South China Sea. In the Kerguelen material this species was found in two samples, commonly at 3584 m, usually as juveniles.