I am Clamp the mighty limpet,
I am solid, I am stuck.
I am welded to the rockface
with my super-human suck.
The hero of Pam Ayres' "Clamp the Mighty Limpet" is a misanthropic Napoleon, threatening would-be harassers with lost fingernails and the danger of attachment (if they would only be willing to stand in one place for two weeks). In reality, 'limpet' is a name that has been applied to a number of unrelated groups of gastropod with cap-like, more-or-less uncoiled shells. The most prominent group of limpets, however, and the one that Ayres almost certainly had in mind, is the 'true limpets' of the Patellogastropoda.
The Patellogastropoda are a mostly marine group of molluscs, though one species in south-east Asia, Potamacmaea fluviatilis, is found in rivers and brackish waters adjoining the Bay of Bengal (Lindberg 2008). The group takes its name from the north-east Atlantic genus Patella, which has been featured on this site in an earlier post. Patellogastropods are particularly prominent in the intertidal zone, where they can be found clinging to rocky surfaces, but there are also many subtidal or deep-water limpets. Some limpets are specialised for living on macro-algae or marine vegetation; one such species, the North Atlantic Lottia alveus, has previously been mentioned on this site due to its unfortunate extinction as a result of the wasting epidemic that devastated sea-grass populations in the 1930s. Deep-water patellogastropods may be found on sunken wood, or they may live in association with hydrothermal vents or cold seeps. One such genus, Serradonta, is restricted to the tubes of vestimentiferan worms (Nakano & Sasaki 2011).
Phylogenetically speaking, patellogastropods are a very interesting group indeed. Morphological studies have identified them as the sister group to all other living gastropods. They have a distinctive radula morphology, known as 'docoglossan', with each tooth-row of the radula containing a median tooth flanked by a small number of relatively simple lateral and marginal teeth (up to three pairs of each). This contrasts with the radulae of other gastropods, in which the teeth are more numerous and/or more specialised, but resembles the radula of other molluscan groups such as chitons. As such, patellogastropods may retain the plesiomorphic radula morphology of gastropods as a whole. Patellogastropods also have a shell microstructure that differs from that of other gastropods, and they have their gonads in a ventral position relative to the visceral mass instead of the dorsal position of other gastropods (Lindberg 2008). Some have even suggested that patellogastropods may represent a remnant lineage that never underwent the coiling that characterises other gastropods, but this seems unlikely. Patellogastropods do have some features, such as an asymmetrical position of the protoconch (larval shell) on the mature shell, that suggest coiled ancestors. Molecular studies, on the other hand, have been more equivocal in positioning the patellogastropods. Some have given results consistent with the morphological analyses, but a few have failed to place patellogastropods with the other gastropods at all (e.g. Giribet et al. 2006), while many have placed them in a more nested position deeper within the gastropods (Zapata et al. 2014).
At face value, the fossil record might actually appear to support a more nested position for patellogastropods. The fossil record for gastropods as a whole extends back to the Cambrian, but the oldest definite record of patellogastropods goes back no further than the late Triassic (Frýda et al. 2009). If patellogastropods were indeed the earliest surviving gastropod lineage to diverge, where were they hiding for the intervening 200 million years or so? One possibility is that they were there, but did not get preserved. Phylogenetic analysis of living limpets suggests that the immediate ancestors of the patellogastropods clung to rocks in high-energy environments, with deep-water lineages occupying more nested positions in the tree (Nakano & Sasaki 2011). Such environments are not favourable to the fossil record, as the shells of dead limpets tend to get broken up by wave action before they have the chance to be fossilised. Another, more likely, possibility is that we have failed to recognise the coiled ancestors of the Patellogastropoda for what they are. The Palaeozoic gastropod fauna included a number of groups that are not present in the modern day; it is quite possible that one of these groups gave rise to the patellogastropods. One group that has specifically been nominated as a possible relative of the patellogastropods is the euomphaloids. But for now, Clamp the Mighty Limpet is holding the secrets of his affinities well hidden, and it will not be easy prising them off him.
Frýda, J., P. R. Racheboeuf, B. Frýdová, L. Ferrová, M. Mergl & S. Berkyová. 2009. Platyceratid gastropods—stem group of patellogastropods, neritimorphs or something else? Bulletin of Geosciences 84 (1): 107-120.
Giribet, G., A. Okusu, A. R. Lindgren, S. W. Huff, M. Schrödl & M. K. Nishiguchi. 2006. Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons. Proceedings of the National Academy of Sciences of the USA 103 (20): 7723-7728.
Lindberg, D. R. 2008. Patellogastropoda, Neritimorpha, and Cocculinoidea. In: Ponder, W. F., & D. R. Lindberg (eds) Phylogeny and Evolution of the Mollusca, pp. 271–296. University of California Press.
Nakano, N., & T. Sasaki. 2011. Recent advances in molecular phylogeny, systematics and evolution of patellogastropod limpets. Journal of Molluscan Studies 77 (3): 203-217.
Zapata, F., N. G. Wilson, M. Howison, S. C. S. Andrade, K. M. Jörger, M. Schrödl, F. E. Goetz, G. Giribet & C. W. Dunn. 2014. Phylogenomic analyses of deep gastropod relationships reject Orthogastropoda. Proceedings of the Royal Society of London Series B 281: 20141739.