Class APLACOPHORA

Aplacophora

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Class APLACOPHORA

Solenogasters

Compiler and date details

30 May 2016 - A. Mututantri, Australian Museum

30 August 2000 - A.H. Scheltema, Woodshole Oceanographic Institute, Massachusetts, USA

Introduction

The Aplacophora comprises two taxa, the Neomeniomorpha or neomenioids (= Solenogastres sensu nomine Salvini-Plawen) and the Chaetodermomorpha or chaetoderms (= Caudofoveata Boettger). The two taxa have been ranked as subclasses within the class Aplacophora (Scheltema 1978) or as separate classes (Salvini-Plawen 1967). Both are characterised by a vermiform body covered by a cuticle invested with numerous aragonite spicules, epidermal papillae, a tetraneural nervous system, radula with two teeth per row in its (probable) plesiomorphic state, and a reproductive system in which, uniquely among molluscs, the gonads empty into the pericardial cavity. The gametes pass through the pericardium to paired gametoducts leading to the mantle cavity. Representatives of both taxa are found primarily in the deep-sea.

Neomeniomorpha and Chaetodermomorpha are distinguished by presence (neomenioids) or absence (chaetoderms) of a small foot within a ventral groove. Chaetoderms have a cuticular oral shield, separate stomach and digestive gland, and paired ctenidia in the mantle cavity. Neomenioids lack a cuticular oral shield, have a combined stomach-digestive gland, and lack ctenidia. Chaetoderms are dioecious; neomenioids are hermaphrodites.

Sperm morphology has been described for a single chaetoderm and a single neomenioid (Buckland-Nicks & Chia 1989; Buckland-Nicks & Scheltema 1995). In Chaetoderma there is a highly derived ectaquasperm (externally fertilising), but in Epimenia australis (Thiele) there is an introsperm (internally fertilising) of a primitive type. Development has been described for a few species. In Epimenia australis and E. babai Salvini-Plawen development is direct with a short-lived larva (Baba 1938, 1940a; Scheltema & Jebb 1994). Pruvot (1890) described a single neomenioid larva in which he depicted seven rows of spicules, or 'plaques', which have been interpreted as being homologous with the chiton shell; however, this observation is still to be substantiated. The most complete published description of larval development is for Neomenia (Thompson 1960), but research on Epimenia babai is presently being carried out (Okusu, in ms). The larva develops within a ciliated cellular test, similar to a pericalymma larva. A review of existing literature on aplacophoran reproduction and development is to be found in Hadfield (1979), and Salvini-Plawen (1985).

There are two recent hypotheses relating the two aplacophoran taxa to each other and to other Mollusca. One is that the chaetoderms form the sister taxon of all other Mollusca, and that neomenioids achieved a worm shape in a separate evolutionary event (Salvini-Plawen 1985) or that neomenioids are sister taxon to all other Mollusca including the chaetoderms (Salvini-Plawen & Steiner 1996). The second hypothesis is that the Aplacophora is monophyletic and progenetic; it includes both chaetoderms and neomenioids and forms the clade Aculifera with the Polyplacophora (Scheltema 1993).

Most published information about aplacophorans is anatomical or phylogenetic (for recent works and references leading to the older literature, see Salvini-Plawen 1972, 1985; Ivanov 1990, 1996; Scheltema et al. 1994; Salvini-Plawen & Steiner 1996; Scheltema 1996) and little is known about life histories, behaviours, or physiologies. Notable exceptions for neomenioids are the sublittoral Australian species Epimenia australis, which lives at depths accessible to divers, and E. babai (Baba 1938, 1940b; Scheltema & Jebb 1994). Ivanov, alone (1979, 1981, 1984), and together with Pavlinov (Ivanov & Pavlinov 1991), has published primarily on Chaetodermomorpha, including observations on living animals. Observations on living neomenioids are listed in Scheltema & Jebb (1994). The recently discovered shallow-water species Falcidens poias Scheltema from Rottnest Island off Perth, WA, lives at densities of about 50 per square metre among sea-grass rhizomes and gives the opportunity to collect and investigate a living chaetoderm. In the deep sea, experimental recruitment boxes have provided some data on the life history of one species of Prochaetoderma (Scheltema 1987).

The aplacophoran fauna of Australia was scarcely known until recent collections made in Bass Strait and the southeastern continental slope brought numerous species to light (Scheltema 1998). Prior to this, only two species, both from off the north coast, were recorded (Thiele 1897). A rich and diverse fauna exists in the benthos of southeastern Australia. In Bass Strait alone there are 32 species in 14 or more genera, most awaiting description. At one 400 m depth locality on the continental slope, a single epibenthic sled sample contained 22 species and more than 600 specimens. Many of the genera represented in these collections are geographically widespread, but the radiation expressed by very closely related species within some of these genera suggests high endemism.

One genus, Ocheyoherpia, is unplaced to family, and treated at the end of the species records. It comprises two species with a unique radula morphology of paired, denticulate bars with fused lateral denticles (Salvini-Plawen 1978; Scheltema 1999). The spicules are upright and nonskeletal (i.e. not layered within the cuticle); barbed hooks are present. Their relationship to other neomenioids is unclear. At Macquarie Island, Ocheyoherpia trachia Scheltema occurs on rocks at shallow depths, where several individuals were collected by divers.

Acknowledgements

(A. Scheltema, 2000) I am grateful for the provision of samples from the Museum Victoria, the Tasmanian Museum and Art Gallery and the Australian Museum, and from Peter Arnold, Museum of Queensland at Townsville. I would also like to thank my editor, Alice Wells, for her extreme patience and help.

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.