Bouncing Snail-y Clams

For the most part, bivalves are a fairly conservative bunch. They seem to have worked out what they are good at early on in their history and most of them stick to it. There are, however, notable exceptions and perhaps few groups of bivalves are as exceptional as the Galeommatidae.

Waldo paucitentaculatus, from Valentich-Scott et al. (2013).

Galeommatids are small bivalves, less than a centimetre in length, with more or less thin shells. The hinge teeth are generally weak or absent. The valves of the shell are more or less gaping and in life are at least partially covered by the large, reflected mantle. The outer surface of the mantle is warty and bears several slender tentacles, the exact arrangement of tentacles varying by species. The foot is large and extends well outwards from the central body of the animal. In the most extreme cases, you might be forgiven for thinking you were looking at some sort of snail rather than a clam.

Many galeommatids have been found living as symbionts with other invertebrates such as in the burrows of annelids and crustaceans, or crawling on the surface of echinoderms. So far as is known, these relationships are commensal only, the clams using their hosts as a source of shelter and possibly excess food scraps, but species may be very exclusive in their choice of hosts. For instance, Mikkelsen & Bieler (1989) found the species Divariscintilla yoyo and D. troglodytes only in burrows of the mantis shrimp Lysiosquilla scabricauda, never in burrows of other potential hosts in the same area. It seems likely that this commensalism has allowed galeommatids to diversify in soft-bottom habitats, their larger hosts being able to dig into sediments in which the smaller clams would be quickly smothered (Valentich-Scott et al. 2013).

Unidentified galeommatid, copyright Ria Tan.

Many galeommatids possess a distinctive 'hanging-foot' morphology with the foot divided into two sections, a muscular anterior portion adapted for snail-like crawling, and an elastic posterior section (Bieler & Mikkelsen 1992). The primary byssus gland is located in the anterior section and is connected by a ciliated ventral groove to a terminal adhesive gland in the posterior section. Mikkelsen & Bieler (1989) found that Divariscintilla individuals kept in an aquarium spent most of their time hanging suspended via the posterior part of the foot. Threads produced by the byssus gland were transferred to the terminal adhesor and used to attach to a surface such as the glass of the aquarium (presumably, the clams would normally hang in this manner on the interior wall of the host burrow). When disturbed, hanging clams would rapidly bounce themselves up and down from their attachment point (hence one species being dubbed 'Divariscintilla yoyo'). If the clams wished to change their location, they would crawl on the muscular section of the foot, breaking the byssus threads behind them. The elastic part of the foot was not functional in crawling.

The majority of galeommatid clams are hermaphrodites, either protandrous (beginning life as males before maturing into females) or simultaneous. Eggs are not released into the water column but brooded within the ctenidia until larvae are released at a relatively advanced stage of development (whether the parent is able to feed while its gills are so occupied, I don't know). In a number of species, dwarf males are also present that do not live independently but reside within the mantle cavity of a female (I have seen these males referred to as 'parasitic' but I do not know if they are directly so). In this position, they are able to fertilise the female directly. Such behaviour may be seen as a further adaptation to the clam's commensal lifestyle, contained within the burrow of its host and potentially secluded from more conventional mates. Hidden away in the darkness, they make matryoshkas of themselves.

REFERENCES

Bieler, R., & P. M. Mikkelsen. 1992. Preliminary phylogenetic analysis of the bivalve family Galeommatidae. American Malacological Bulletin 9 (2): 157–164.

Mikkelsen, P. M., & R. Bieler. 1989. Biology and comparative anatomy of Divariscintilla yoyo and D. troglodytes, two new species of Galeommatidae (Bivalvia) from stomatopod burrows in eastern Florida. Malacologia 31 (1): 175–195.

Valentich-Scott, P., D. Ó. Foighil & J. Li. 2013. Where's Waldo? A new commensal species, Waldo arthuri (Mollusca, Bivalvia, Galeommatidae) from the northeastern Pacific Ocean. ZooKeys 316: 67–80.