More mangrove molluscs (without the mangroves)

Wouldn't you know it? All my texts on European molluscs are at work and it's a public holiday here in Oz so I don't have access to them. Such is life.

Potamides conicus is a small, rather homely snail that lives on intertidal flats and salt lakes around the Mediterranean Sea and western Indian Ocean. I say 'homely' because it belongs to the family Potamididae — the mud whelks — which are the epitome of 'homely'. Telescopium, anyone?

Potamides conicus is the only living member of the family to occur in the Med. Most of the 29 living species are clustered in the Indo – West Pacific (IWP), a few live in the eastern Pacific and at least one occurs in West Africa. Tropical and subtropical species are associated with mangroves. Elsewhere, potamidids live in the temperate equivalent — saltmarsh and mudflats.

For a long time, malacologists have treated P. conicus as special. Well, all snails are special. But this species is of particular specialness. Not only does it have a unique geographical range among Potamididae but it is also the last living member of a now extinct genus.

Or is it?

It's certainly true that it's the only mud whelk in the village Mediterranean. Two hypotheses have been offered to explain this distribution: it originated in the western Indian Ocean and spread northwards or originated in the Mediterranean and spread southwards. In both models, its tolerance to high salinities and its direct development (from egg to juvenile, without an intervening planktonic stage) would have allowed it to disperse via salt lakes. Or possibly via birds. (Which, I still maintain, is the way in which wallabies arrived in Hawaii.)

So which one is it?

The answer isn't immediately obvious. For a start, Potamides conicus isn't a Potamides at all. According to a recent study by David Reid and colleagues, it belongs to the IWP genus Cerithideopsilla. Its closest living relative is C. cingulata, which is the most widespread of the lot. Or would be, were it not a muddle of two — or maybe more — species. But never mind about that. The important bit is that our Med mollusc is related to something from the IWP.

So this indicates that the snail formerly known as P. conicus is a recent invader from the western Indian Ocean to the Mediterranean, doesn't it?

Weeeeeelllll ... it might. But the fossil evidence suggests something else. Numerous potamidid species have been dug up from the rocks in southern Europe. Of these, C. graeca appears to be most closely related to C. conica. It ranges from Late Miocene to Pliocene, post-dating the time when land movement pinched off the Mediterranean from the Indian Ocean. So Cerithideopsilla graeca wasn't marooned in the Mediterranean on closure of the Tethys Ocean. It probably spread there after the event.

Reid et al. make the case for C. conica arising from that Mediterranean stock and dispersing to the Indian Ocean during more recent times. Its salt tolerance and developmental strategy certainly make this model plausible. Birds may also have been involved. Either on their way to or from Hawaii.

Sounds good. But the authors add a note of caution to this part of their study. Their samples of C. conica all came from the Indian Ocean not from the Mediterranean Sea. What if the species is actually a complex?

Dramatic chord

We'll have to wait for the sequel.

Reference
Reid, D.G., Dyal, P., Lozouet, P., Glaubrecht, M. & Williams, ST. (2008). Mud whelks and mangroves: the evolutionary history of an ecological association (Gastropoda: Potamididae) Molecular Phylogenetics and Evolution 47: 680 – 699