Class SCAPHOPODA

Scaphopoda

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Class SCAPHOPODA

Tooth Shells, Tusk Shells, Tooth Shells

Compiler and date details

2016 - A. Mututantri, Australian Museum

30 December 1999 - Kevin L. Lamprell, Queensland Museum, Brisbane, Australia, and John M. Healy, Center for Marine Studies, University of Queensland, Brisbane, Australia

Introduction

Scaphopoda, commonly known as 'tusk' or 'tooth' shells, are marine, benthic, infaunal molluscs. They are characterised by a posteriorly tapering tubular shell which is open at both ends, a well-developed foot, filamentous feeding organs termed captacula and a well-developed radula, with tooth row formula usually 1.1.1.1.1, i.e. a marginal and lateral tooth on each side of the rachidian (central) tooth, but exceptionally 1.1.1.1.1.1.1 in some species (Scarabino 1995). Eyes and gills are absent in all species.

Two orders are recognised—the Dentaliida (shells small to large, foot conical, rachidian tooth wider than high, shells often ribbed and always widest at anterior aperture) and the Gadilida (shells usually small to very small, foot vermiform often with terminal disk, rachidian tooth of radula square or higher than wide, shells usually without sculpture and often widest behind the aperture) (Emerson 1962; Palmer 1974; Boss 1982; Steiner 1992; Scarabino 1995; Lamprell & Healy 1998).

Biology

Scaphopoda inhabit a wide variety of sediments including coarse sand, fine coral rubble, clean or muddy sand, or clayey-mud (typically sand for shallow water species, mud for deep water species). Dentaliida usually orientate their shells at an angle (approximately 30–35°) to the substratum surface, with the concave (dorsal) side uppermost and the apex projecting at least periodically slightly above the substratum, for waste and gamete release (Yonge 1937; Morton 1959; Dinamani 1964; Shimek & Steiner 1997; Steiner & Palmer 1998). They inhabit only the upper few centimetres of the substratum. Gadilids, however, are known to burrow to over 30 cm in captivity (Shimek 1989). Presumably, like the Dentaliida, they must make contact with the substratum surface intermittently if only to ensure release of waste and gametes.

True ctenidia or gills of any sort are absent in all scaphopods, and respiration occurs directly across the epithelium of the inner mantle cavity, facilitated by currents generated by periodic extension and retraction of the foot (Steiner & Palmer 1998).

For feeding, a cavity is made by the foot and the epipodial lobes (Gainey 1972; Shimek & Steiner 1997; Steiner & Palmer 1998) in the sediment around the anterior extremity of the animal. Food particles are then selected from the sediment by the filamentous captacula and are transferred by captacular and oral lobe cilia (or sometimes directly by the captacular tips) to the mouth (Shimek & Steiner 1997; Steiner & Palmer 1998). The diet typically consists of foraminiferans and/or detritus, with some species showing preference for specific items (Morton 1959; Gainey 1972; Steiner 1992; Shimek & Steiner 1997; Steiner & Palmer 1998). Bivalve spat, ostracods and diatoms are also sometimes ingested (Shimek & Steiner 1997; Steiner & Palmer 1998). After the food is passed via the mouth to the proboscis, it is stored in the buccal pouch and, if necessary, crushed by the jaw and radular apparatus before reaching the stomach (Morton 1959; Steiner & Palmer 1998).

The sexes are separate and the gonad is a single unpaired organ situated dorsally. Fertilisation takes place externally after sperm or eggs are released via either the apical shell aperture (Dentaliida, some Gadilida) or the anterior shell aperture (some Gadilida) into the surrounding sea water. Spermatozoa are of the unmodified, ect-aquasperm type, with a short head and single, simple flagellum (Dufresne-Dube et al. 1983; Hou & Maxwell 1991). Development involves a pelagic larval stage (Shimek & Steiner 1997; Steiner & Palmer 1998).

Although scaphopods are a small group of molluscs, with an estimated 500–600 living species worldwide (Scarabino 1994; Lamprell & Healy 1998), they often form a significant component of the subtidal benthos. The class is well represented in Australian waters by 108 valid species, including several new species and new records added by Lamprell and Healy in their recent monograph on the Australian Scaphopoda (Lamprell & Healy 1998). Some scaphopod species in the Australian fauna have broad geographical ranges—a significant number occurring throughout the Indo-West Pacific (see also Scarabino 1995), but many appear to be very restricted in their distribution and may eventually prove to be endemic.

Identification

A variety of factors may hinder accurate identification of scaphopod shells. Many of the diagnostic shell features only develop with adulthood (for example, number, strength and growth pattern of shell ribs), thus identification of juvenile scaphopod shells to species or genus level is often difficult or impossible (Lamprell & Healy 1998). Loss of shell sculpture through erosion of the shell, both before and after death of the animal, can also create confusion. Shells of other animal groups are sometimes mistaken for those of scaphopods, including certain pteropod opisthobranch gastropods (cavoliniids-shells straight and glassy, open only one end), caecid prosobranch gastropods (shell plugged apically after shedding of coiled juvenile shell), some protistans (straight-shelled foraminiferans—shell chambered) and finally certain polychaete marine worms (curved, sometimes ribbed, calcareous tubes of certain serpulids) (see Lamprell & Healy 1998). Only the last of these are easily confused with scaphopod shells, and indeed have been on a number of occasions as discussed by Pilsbry & Sharp (1897–1898) and ten Hove & Smith (1990). Shells of the polychaete genus Ditrupa, for example, are strongly curved and narrow, usually glossy, with a nodular swelling at the aperture (swellings sometimes extending at irregular intervals along the shell length—see ten Hove & Smith (1990) and Lamprell & Healy (1998)). Shells should be inspected carefully, as empty scaphopod shells are frequently inhabited by polychaetes, sipunculids or hermit crabs.

Classification

Classification of the Scaphopoda has been re-assessed over the last two decades by various workers including Emerson (1962, 1978), Palmer (1974), Chistikov (1975, 1978, 1979, 1982a, 1982b), Scarabino (1986a, 1986b, 1995), Steiner (1991, 1992), Reynolds (1997), Lamprell & Healy (1998) and Steiner & Palmer (1998). Nevertheless, Pilsbry & Sharp's (1897–1898) basic division of the class into two broad taxa (their 'families' Dentaliidae and Siphodentaliidae) still persists (see Vanatta (1927) for publication dates of the original four parts of the Pilsbry & Sharp monograph). These two taxa now constitute the orders Dentaliida and Gadilida (see Emerson 1962, Palmer 1974, Steiner 1992, and Steiner & Palmer 1998, for discussion and further references). Continuing anatomical studies have supported the establishment of new families and genera.

In this work, we have primarily followed the preliminary classification proposed by Steiner (1992) which is based on his own work and that of many recent authors (see references above). Several new species and new records of named species have emerged from a recent review of the Australian scaphopod fauna (Lamprell & Healy 1998).

The Australian scaphopod fauna includes wide-ranging species from the Indian, Southern and Pacific Oceans as well as several possibly endemic species. Deep and shallow water benthic sampling ('Cidaris' I–III (1986–1988), NSW Fisheries Benthic Survey (1973–1975), NW Shelf Project Survey (1988), Northern Territory Pearl Bed Survey (1988–1990) and on-going 'Southern Surveyor' and 'Franklin' cruises) in particular, have yielded many new or previously unrecorded taxa (Lamprell & Healy 1998).

The majority of Australian scaphopods belong to the Order Dentaliida, notably in the families Dentaliidae (Dentalium, Fissidentalium, Tesseracme) and Laevidentaliidae (Laevidentalium). In addition, the families Omniglyptidae (Omniglypta), Gadilinidae (Episiphon) and Fustiariidae (Fustiaria) each includes at least one endemic species. The Australian Gadilida fauna consists predominantly of species from the Gadilidae (Gadila, Cadulus, Dischides, Polyschides), although, as in the Dentaliida, other minor families (Entalinidae, Pulsellidae) are also represented by a few species.

Geological record
The Scaphopoda first appeared in the Ordovician of North America (500–550 million years ago) and like the Bivalvia, probably arose from the extinct Rostroconchia (see Pojeta & Runnegar (1979) and Steiner (1992) for discussion). Most described fossil scaphopods from Australia are of Tertiary age (Western Australian, South Australian and Victorian deposits—Tate 1887, 1899; Cotton & Ludbrook 1938; Ludbrook 1956, 1959, 1960, 1984). A number of Palaeozoic species are also recorded from the Lower Carboniferous of New South Wales (Yoo 1988).

Taxonomic works

The study of living scaphopod molluscs from Australia began essentially with the descriptions of new species by Sowerby (1860). Subsequently, authors such as Tenison-Woods (1877), Brazier (1877), Watson (1879), von Martens (1881), Pilsbry & Sharp (1897–1898), Tate & May (1900), Hedley (1903), Verco (1911a, 1911b), Cotton & Godfrey (1933, 1940a), Cotton & Ludbrook (1938) and Colman (1958) added further species to the known fauna. Ponder & Stanbury (1971) clarified the type status of a number of Brazier specimens held in the Macleay Museum, Sydney (material now on permanent loan to the Australia Museum). The Scaphopoda are one of the few groups of molluscs not researched by the prodigious Australian malacologist Tom Iredale. Given the lack of work on Australian Scaphopoda since 1958, and the extensive benthic sampling programs carried out by State museums and fisheries bodies during that period, it was not surprising that Lamprell & Healy (1998) were able to more than double the known Australia scaphopod fauna. In addition to papers describing new taxa, there have been attempts to list the Scaphopoda of individual Australian states (Tasmania—May & Macpherson 1958; New South Wales—Iredale & McMichael 1962; South Australia—Cotton & Godfrey 1940b; see also Zeidler & Macphail 1978). A list of Scaphopoda recorded from Tasmanian waters is given in Groves et al. (2006).

Limital Area

Distribution data in the Directory is by political and geographic region descriptors and serves as a guide to the distribution of a taxon. For details of a taxon's distribution, the reader should consult the cited references (if any) at genus and species levels.

Australia is defined as including Lord Howe Is., Norfolk Is., Cocos (Keeling) Ils, Christmas Is., Ashmore and Cartier Ils, Macquarie Is., Australian Antarctic Territory, Heard and McDonald Ils, and the waters associated with these land areas of Australian political responsibility. Political areas include the adjacent waters.

Terrestrial geographical terms are based on the drainage systems of continental Australia, while marine terms are self explanatory except as follows: the boundary between the coastal and oceanic zones is the 200 m contour; the Arafura Sea extends from Cape York to 124 DEG E; and the boundary between the Tasman and Coral Seas is considered to be the latitude of Fraser Island, also regarded as the southern terminus of the Great Barrier Reef.

Distribution records, if any, outside of these areas are listed as extralimital. The distribution descriptors for each species are collated to genus level. Users are advised that extralimital distribution for some taxa may not be complete.