Also present are the exclusively deep-sea Limopsidae ( Fig. 5.19 ) , along with deep-sea species of the mytilid genus Dacrydium ( Fig. 10.2 ) and species of the families Pectinidae and Arcidae .sx All are epifaunal , probably suspension-feeding forms .sx One curious feature of deep-sea suspension feeders seems to be that , although the soft parts may be much reduced , the shell is proportionately much less reduced in size ( Fig. 5.20 ) , possibly to enhance protection from predation at minimal metabolic cost .sx As a result they include some of the largest known , non-vent bivalves of the abyss ( Knudsen , 1979) .sx The superfamily Galeommatacea in shallow water are known to include many small bivalves that live commensally with a variety of larger infaunal invertebrates , particularly with echinoderms .sx At least two unrelated species of this small-bodied group are known from the deep sea :sx Montacuta ( Axinodon ) symmetros lives with a byssal attachment to the spines of the echinoid Pourtalesia ( Bouchet & War e n , 1979 a ) while Galatheavalve holothuriae , collected from cavities in the skin of the elasipod holothurian Psychropotes , has a completely internal shell ( Knudsen , 1970) .sx There are few bivalves in the deep sea with truly world-wide distributions ; most are restricted to one or more adjacent basins with morphological differences in the populations suggesting a degree of ongoing speciation through isolation , whilst , even in the cosmopolitan species Malletia cuneata ( Fig. 5.21 ) , subtle changes in shell shape can be discerned ( Sanders & Allen , 1985) .sx Although vertically segregated populations can be recognized amongst the bivalves , overall , the degree of restriction in vertical range , like the vertical range of prochaetodermatid Aplacophora ( see below ) , seems to be positively correlated to the horizontal range displayed by species ( Scheltema , 1985) .sx figures&captions .sx GASTROPODA The class Gastropoda encompasses a much greater variety in form and habit than bivalves .sx Most snail-like species with coiled shells are thought to be motile carnivores , probably feeding on polychaete worms and bivalves , or are ectoparasites , often on echinoderms ( Fig. ) .sx However , the limpet-like shape also occurs mainly as opportunistic scavengers .sx As in shallow water , gastropods are probably at least as diverse as any other molluscan group .sx From the Gay Head-Bermuda Transect , Rex ( 1976 ) identified 93 species of shelled marine snails ( subclass Prosobranchia ) and 30 species of the more slug-like subclass Opisthobranchia which have a reduced shell , or lack it completely as in sea slugs ( order Nudibranchia ) ; all with a wide range in feeding type .sx Deposit feeders included 20 species amongst the more primitive prosobranch families in the order Archaeogastropoda , 15 species of more primitive families of the very diverse order Mesogastropoda ( which show particularly impressive radiation in the tropics ) and five species of the more primitive order of opisthobranchs , the Cephalaspidea which retain a reduced thin shell and move semi-burrowed through the sediment .sx At least one species , the deposit-feeding archaeogastropod Bathybembix aeola is known to be selective in terms of particle size , suggesting adoption of an energetically more efficient strategy of selection of finer particles with a relatively greater microbe-coated surface area ( Hickman , 1981) .sx Bivalve and polychaete predators include one whelk-like mesogastropod ; 46 neogastropods , including a variety of small poison-secreting cone shells and larger whelk-like scavenger/predators with a typical roving tentaculate nostril ; 17 cephalaspids belonging to the more primitive families Acteonidae ( which eat polychaetes ) , Ringiculidae and Retusidae ( both of which seem to eat only foraminifers ) and three nudibranchs which , at least in shallow water , may be highly specific in diet on colonial coelenterates .sx Ectoparasites of echinoderms and anthozoans include 11 and 8 mesogastropod species in the families Eulimidae and Epitonidae , respectively , the former of which have lost the jaws and radula and pump out the body fluids of their sedentary invertebrate hosts through a proboscis inserted into the body cavity ( Fig. 5.22 ) , while the latter rasp off tissue fragments with their radula ; and five pyramidellids which , although with normal-looking small , spired shells , possess elaborate mouthparts that are modified for piercing and sucking , with a diet of liquid protein that is pumped out of their living mollusc or polychaete prey .sx Although these two groups are conspicuous , no one group can be said to predominate as amongst bivalves .sx figure&caption .sx The neogastropod family , Turridae , are toxoglossan ( poison-toothed ) cone shells that have emerged as the single most diverse group of carnivorous gastropods in the deep sea , with a species richness , peaking in the bathyal , rivalling that of the cone shells of tropical sand flats ( Bouchet & War e n , 1980) .sx Turrids are highly specialized predators on polychaetes , killing their prey with poison from salivary glands that is injected through modified radular teeth .sx They are thought to have developed complex patterns of resource partitioning amongst co - occurring species ( Hickman , 1984) .sx Eulimids are one of the most diverse families of deep-sea gastropods , possibly even richer in species than the Turridae ( War e n , 1984 ; Bouchet & War e n , 1986) .sx It seems very likely that the high degree of dietary specialization or host specificity amongst the Turridae and Eulimidae has led to the spectacular radiation of these families in the deep sea .sx Other neogastropods in the deep sea include an assemblage of mostly large-sized species , some of which have a wide - spread distribution ( Bouchet & War e n , 1985) .sx Although most of the different functional types amongst gastropods seem to correspond to taxonomic groupings , the cocculiniform limpets appear to have converged from at least 15 families ( Hickman , 1983) .sx These are epifaunal scavengers on rather unusual substrates such as wood , squid beaks ( which can form highly localized and dense accumulations , see Belyaev , 1966 ) , empty Hyalinoecia tubes , shark or skate egg cases , and fish and whale bones .sx Most of these have been recorded from eutrophic trenches where woody plant detritus is common ( George & Higgins , 1979 ; Wolff , 1979) .sx Although only some of these species , mostly belonging to the family Cocculinidae , are believed to ingest wood , the remainder graze the cover of microbes .sx Other cocculinid limpets have been found on waterlogged wood from oligotrophic abyssal bottoms far from land where their opportunistic , wood-ingesting lifestyle parallels that of the deep-sea xylophaginous bivalves ( Wolff , 1979) .sx Although diverse in the abyss , gastropod diversity seems to peak at mid-slope depths ( Rex , 1973 , 1976 ; Bouchet & War e n , 1980) .sx The distribution of gastropods also appear to be much more sharply zoned with depth than amongst bivalves ( Grassle , Sanders & W. Smith , 1979 ) , with a particularly marked compression in vertical range amongst predatory species compared to deposit-feeding species on the slope ( Rex , 1977) .sx Despite this , certain species , such as the unusually open-coiled epitonid Eccliseogyra nitida ( Fig. 5.23 ) , are known to be widespread , with distributions throughout the Atlantic ( Rex & Boss , 1973) .sx Although it is becoming apparent that many deep-sea gastropods have an early development in the plankton ( see Chapter 13 ) , many of them , certainly most of those living at bathyal depths , lay egg capsules of characteristic form attached to hard substrata ( Fig. 5.24 ) from which the young hatch directly .sx SCAPHOPODS The other major group of molluscs in the deep sea , the Scaphopoda , are far less well known .sx This might be because they constitute less than 1% of the mollusc fauna in shallow water .sx But in the deep sea this proportion is much increased ( Clarke , 1962 ) , and , although species richness may be less than that of other molluscan groups , they achieve population densities next only to those of bivalves .sx Modern species of this group have an ancient lineage going back at least to the Devonian .sx They live in cylindrical , often tapering , tube-like shells ( Fig. 5.25( a) ) and in shallow water are known to be expert burrowers in soft sediments using an elongated plug - like foot with a complex musculature .sx This is thrust down into the sediment , while the body and shell are pulled down to it by muscular contraction in a manner analogous to the burrowing movement , using the foot , of protobranch bivalves .sx The head hears curious bunches of ciliated tentacles terminating in a secretory bulb , the captaculum , which reach out to collect particles such as foraminifers , to which they show highly developed selectivity ( see Davies , 1987 , for references) .sx These prey are crushed using a radula structure similar to that of gastropods .sx They are best known from species large enough to be collected by trawls :sx Dentalium megathyrus being known from 0.45-4.1 km depth in the E. Pacific measures up to 70 mm in length ( Wolff , 1961) .sx This , with other mainly larger species , is put into the order Dentalioida .sx These large species , at least , may be active burrowers in the deep sea leaving a conspicuous track as they move through the superficial sediment ( Fig. 5.25( b)) .sx The other subdivision , the order Siphonodentalioida may contain the majority of , mainly small-sized , species .sx On the basis of shell and radula morphology and the anatomy of soft parts , Scarabino ( 1979 ) has separated 59 mostly small-sized species from epibenthic sled samples taken from 0.3-6 km depth all over the Atlantic .sx figure&caption .sx APLACOPHORA Although still poorly known , the worm-like Aplacophora are ubiquitous in the deep sea , occurring down to hadal depths .sx They include two divisions , the Neomeniomorpha and Chaetodermomorpha .sx Like the Scaphopoda , these forms are relatively abundant in the deep-sea macro - fauna , yet are still very poorly known .sx Species of the chaetodermomorph family Prochaetodermatidae are found world-wide in the deep sea , with six species described from the best-known area , the N. Atlantic ( Scheltema , 1985 a) .sx They are often numerous in deep-sea samples , being one of the numerically dominant macrofaunal animals in quantitative samples in the N.W. Atlantic slope and the Aleutian Trench ( Jumars & Hessler , 1976 ; Scheltema , 1981 ; Maciolek et al. , 1987 a , b ) , a density of 247 m -2 for the species Prochaetoderma yongei ( Fig. 5.26 ) being recorded at 3.64 km depth off New England ( Grassle & Morse-Proteous , 1987) .sx Both divisions have a skin covered with protective calcareous spicules and possess simplified guts and a radula in the buccal cavity .sx The Chaetodermomorpha probably make shallow burrows in the sediment with the posterior end bearing the gills at the sediment surface ( Fig. ) .sx The Neomeniomorpha have the simplest gut , and most species feed suctorially on the tissues of gorgonians and hydroids ; while most species of Chaetodermomorpha feed on small organisms such as foraminifers and small macro - fauna .sx Scheltema ( 1981 ) argued that large population densities of Prochaetoderma may be attributable in part to their gastropod-like rasping mouth - parts allowing a wide range of particle sizes to be available to them .sx CHITONS AND MONOPLACOPHORANS .sx Of the Polyplacophora , or chitons ( coat-of-mail shells ) , only a few species have been collected from the deep sea , these being restricted to manganese nodules and pieces of wood , and belong to the primitive genus Lepiodopleurus ( Paul , 1976 ; Wolff , 1979) .sx Discovery of living examples from 3.57 km depth in the Pacific of an archaic molluscan group of Monoplacophora , which are related to Cambrian and Silurian fossils and retain partial segmentation of body organs ( Fig. 5.27( a ) , ( b) ) , was one of the great discoveries of the 'Galathea' expedition ( Lemche , 1957 ; Lemche & Wingstrand , 1959) .sx However , a related species dredged in 1869 has only recently been recognized ( War e n , 1988) .sx Although having a limpet-like form , Neopilina has been photographed ploughing through soft sediments , the elongated gut ( Fig. 5.27( c) ) suggesting that it subsists on the sparse and refractory organic matter .sx Fragments of the xenophyophore Stannophylum ( see Fig. 5.37 ) have been found among the gut contents .sx This , together with radula scraping marks on living specimens of the xenophyophores , from the same locality , suggests that N. galatheae preys on these giant protozoans ( Tendal , 1985 b) .sx Six genera of recent monoplacophorans have now been described , the shell of the smallest , Micropilina from about 0.9 km depth S. and W. of Iceland being only about 1 mm in diameter ( War e n , 1989) .sx The taxonomy , morphology and relationships of the group as a whole have been recently treated by Wingstrand ( 1985) .sx figure&caption .sx MEIOFAUNAL TAXA .sx This category is much less well known than macrofauna .sx It consists of both multicelled ( metazoan ) animals traditionally regarded as 'meiofaunal' and larger single-celled protozoans , such as Foraminifera .sx THE MULTICELLED TAXA :sx NEMATODA .sx Although little is known of the taxonomic composition or mode of life of the traditionally meiofaunal group , the Nematoda ( thread worms ) , they are probably by far the most numerous metazoans in all marine soft bottoms .sx There are few data from the deep sea ; but there can be little doubt that this group is as important and numerous in deep-ocean sediments as they are in shallow water ( see review by Heip , Vincx & Vranken , 1985) .sx