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.

Digging for Tellina

When I was a child, a large part of my extended family would gather over the Christmas period to park their tents and caravans alongside the estuary downhill from my great-grandparents' house (in the usual way of these things, my memory has these summer camping periods lasting for ages, but I don't think they could have been longer than a week or so). While we were there, I would spend a fair chunk of the day looking for the wildlife that inhabited the slightly muddy estuary beach. Among these were various bivalves whose shells could be found littering the shoreline, or which might be found by digging in the sand at low tide. Close to the surface were New Zealand cockles Austrovenus stutchburyi (not actually a direct relative of the English cockle but a member of the Veneridae family that has adopted a similar body form). A little deeper were pipis and tuatuas. And a little deeper again were the flat, slender shells of Tellina.

Thin tellin Tellina tenuis, copyright S. Rae.

I should note that Tellina species are not really deep burrowers in the grand scheme of things, generally only embedding themselves about one to three centimetres below the surface, but again I must ask that you make allowances for childhood memories. Their low profile and weakly inflated shells also make them fast diggers so they were probably able to elude most casual explorations. Like most subsurface bivalves, Tellina species are sediment feeders. Their usual aspect is lying horizontally beneath the sediment, extending their long, unfused siphons to the surface to gather detritus (Ujino & Matuskuma 2010; the shells of Tellina are usually twisted slightly to one side at the end to facilitate the siphons' passage). Even if you've not seen the Tellina animals themselves, you may have seen the radiating trails made by the siphons as they extend along the top of the sediment.

Sunrise tellin Tellina radiata, copyright James St. John.

Tellina is an extremely diverse genus, with species found worldwide and recognised through the entirety of the Mesozoic (Moore 1969). These species vary greatly in appearance, with shells varying from almost completely smooth to strongly ornamented, and from subcircular to quite elongate. It should therefore come as little surprise that numerous attempts have been made to divide Tellina between various subgenera and genera but issues such as homoeomorphy in Tellina's evolution (where distinct lineages have converged on similar body forms) have lead to disagreement over the best system to adopt. In 1934, the malacologist A. E. Salisbury complained that, "The number of genera, subgenera, and sections into which the Tellinidae has been cut up is getting somewhat appalling; the list of names is still increasing every year, and, if every variation of form is magnified, it is quite possible to go on until at last each species becomes the representative of a different genus and each variety that of a subgenus" (of course, as seems almost inevitable when one encounters complaints of this kind, Salisbury himself then proceeds to add to the tally of generic names in that same paper). Though I suspect most modern malacologists would probably disagree with the extremely broad concept of Tellina advocated by Salisbury, the question of how best to handle the genus taxonomically remains an open one.

REFERENCES

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Salisbury, A. E. 1934. On the nomenclature of Tellinidae, with descriptions of new species and some remarks on distribution. Proceedings of the Malacological Society of London 21: 74–91.

Ujino, S., & A. Matsukuma. 2010. Inverse life positions of three species in the genus Cadella (Bivalvia: Tellinidae). Molluscan Research 30 (1): 25–28.