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.