Marginal Limpets

Shells of Emarginula solidula, photographed by Jan Delsing.

For today's subject taxon, I've drawn the Emarginulini. This is a tribe within the gastropod family Fissurellidae, members of which are commonly known as keyhole or slit limpets. They get these names because of openings in their shells: keyhole limpets have a distinct hole at the apex of their shell, while slit limpets have a longitudinal slit running back from the front of their shell. In both groups, the slit or 'keyhole' functions in excretion. Gastropods undergo a process early in development known as torsion: the viscera of the embryo twists around so that it reverses its original direction (you can see a basic diagram of the process here). The reasons why this happens remain somewhat uncertain (one early suggestion was that it is what allowed the larval gastropod to retract into its shell and close the shell opening with an operculum) but a potentially negative side-effect of the process is that the anus comes to open directly above the mouth. Unless you want to go through your life with a bit of a funny taste in your mouth, this is not ideal. Therefore, many torted gastropods develop some sort of sinus or recess in their shell so the anal opening can be moved rearwards, away from the mouth.

Live individual of Tugali parmophoroidea, from here. Tugali has an expanded mantle, but is still able to retract it body underneath the shell for protection.

The members of the Emarginulini as recognised by Bouchet et al. (2005) are slit limpets rather than keyhole limpets, though in some species the slit has become much reduced and may only be visible on the underside of the shell (i.e there is a ventral groove rather than a full slit). Bouchet et al. (2005) divided the Fissurellidae between the Fissurellinae and Emarginulinae, a classification based on the structure of the radula and shell muscles (Aktipis et al. 2011). The Fissurellinae are all keyhole limpets, but Bouchet et al.'s Emarginulinae included (in addition to the Emarginulini) the tribe Diodorini, whose members are keyhole limpets like the Fissurellinae but have emarginuline internal anatomy. Other authors have recognised this group as a third intermediate subfamily. Two further tribes of Emarginulinae, the Scutini and Fissurellideini, include species with expanded mantles and reduced shells that may be entirely concealed within the soft body of the slug-like animal.

An elephant snail or shield limpet Scutus sp., from here. This genus has a greatly enlarged mantle, which usually folds over to conceal the reduced shell.

However, this classification of the fissurellids was challenged by the molecular analysis of Aktipis et al. (2011). The results of these authors indicated that the Emarginulini of Bouchet et al. (2005) is para- or polyphyletic. A clade of Fissurellinae with Diodorini indicates a single origin of keyhole limpets, with the emarginuline radula and muscle structure being ancestral for fissurellids as a whole. This result is also consistent with the fossil record: 'emarginulines' are known as early as the Triassic, but fissurellines and diodorines have not been found earlier than the Caenozoic. Aktipis et al. (2011) therefore recognised a more restricted monophyletic Emarginulinae containing the genera Emarginula, Montfortula, Tugali and Scutus, while more basal forms (sister to all other fissurellids) were separated as the Hemitominae (Aktipis et al. did not analyse the position of the Fissurellideini). Of Aktipis et al.'s Emarginulinae proper, only Emarginula has a well-developed slit (the others have ventral shell grooves; Scutus has a quite reduced shell), but this genus was also not monophyletic. Instead of aligning by morphology, the species analysed formed clusters corresponding more to their biogeography: a Mediterranean Emarginula clade, a Pacific clade of Emarginula and Montfortula species, and an Australian clade of Scutus and Tugali. The relationships between these three clades varied by analysis method. The Australian Scutus clade was not necessarily sister to the remaining Emarginulinae, so it may not be worthwhile at this point in time distinguishing the tribes Emarginulini and Scutini.

REFERENCES

Aktipis, S. W., E. Boehm & G. Giribet. 2010. Another step towards understanding the slit-limpets (Fissurellidae, Fissurelloidea, Vetigastropoda, Gastropoda): a combined five-gene molecular phylogeny. Zoologica Scripta 40 (3): 238-259.

Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.