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Family  Semelidae

Semelids

The family Semelidae consists of about 150 species (Huber, 2010) occurring mainly in warmer seas, of which about 30 occur in northern Australia, with only four  extending as far south as NSW. The family is spread over about 15 genera of fairly large, whitish and occasionally brightly coloured shells but only three genera occur in NSW, and these are somewhat atypical of the family. The genera found in NSW are Theora which are thin, fragile shells that live buried in mud; then Ervilia that is represented by only one species up to 7 mm in length; and lastly Rochefortina that is also represented by only one species, which is very small, with a maximum length of 4 mm.

Semelids are closely related to the Tellinidae, and live in similar habitats, buried in sand or muddy sand. The two Theora species found in NSW, T. lubrica and T. lata, are infaunal mud dwellers, burrowing in mud in the middle and upper reaches of estuaries, including Sydney Harbour and Port Hacking. Theora lubrica is native to the northwest Pacific Ocean and has been introduced to Australia and other localities worldwide and is regarded as an invasive species capable of establishing in suitable habitats. It is probably distributed in ship’s bilge water.

Family Reference

The Australian species were described in 1992, but have been updated by more recent work:

Lamprell, K. and Whitehead, T. 1992. Bivalves of Australia vol. 1. Crawford House Press, Bathurst, Australia.

Worldwide, the family was catalogued by:

Huber, M. 2010. Compendium of Bivalves.  ConchBooks, Harxheim, Germany.

Coverage

All the NSW species are detailed here.

Identification Notes

Shell Form

As mentioned above, typical semelids are similar in shell form to Tellinidae, but the posterior flexure is slight. The NSW species are atypical of the Semelidae in general, being small and fragile in the case of Theora and very small (up to 4 mm) for Rochefortina.

Hinge and Ligament

The ligament is in two parts; an external part is attached to nymphs, while the internal part is sunken in a pit on the spoon-shaped chondrophore that projects into the shell cavity (Fig. 1). The hinge teeth consist of one or two cardinals and two laterals in each valve (Fig. 2).

Shell Interior

The shell interior shows the scars of the anterior and posterior muscles, and a very large pallial sinus, extending past the shell midline. The pallial line and sinus are often difficult to discern.

Fig. 1:  Hinge of Theora lata left valve showing chondrophore and cardinal tooth in end view. (C.378034).

Fig.2:   Hinge of Theora lata right valve showing cardinal tooth in end view, two lateral teeth and chondrophore (C.378034).