Limpets of the North-east Atlantic

The common limpet Patella vulgata, photographed by Rokus Groeneveld.

When Linnaeus published the tenth edition of his Systema Naturae in 1758, he defined the genus Patella as having a subconical shell with a single valve and without a respiratory opening. Starting from this fairly minimal set of criteria, it is not surprising that a very broad range of limpets from all over the world ended up passing through Patella at various times. However, as time goes by the definition of Patella became further refined, and currently both morphological (Ridgway et al. 1998) and molecular (Nakano & Ozawa 2004) studies have tied the name Patella to a clade of limpets found only in coastal waters of the north-east Atlantic Ocean and the Mediterranean.

The giant limpet Patella ferruginea, up to eight centimetres in diameter, photographed by E. Volto.

This restricted sense of Patella includes nine or ten recognised species, though discussions are ongoing about whether given populations should be regarded as conspecific or not, and a large number of subspecies have been described. Limpets are fairly conservative animals morphologically, offering a fairly narrow range of characters for taxonomic study. Matters are further confused by a certain degree of environmentally-related plasticity: individuals living higher in the tidal zone tend to be larger and higher-spired than individuals living subtidally (Weber & Hawkins 2002). Patella limpets are generally believed to be protandrous, starting their lives as males and eventually metamorphosing into females; however, a study monitoring sex changes in P. vulgata identified one individual that changed from female to male (Le Quesne & Hawkins 2006). Patella limpets have been used for food by humans in many parts of their range, and collection pressure is regarded as a significant threat to the endangered western Mediterranean species P. ferruginea.

Blue-rayed limpets Patella pellucida, from here.

Within the family Patellidae, the distribution of Patella is notably unusual: this genus is largely geographically isolated from other patellids in southern Africa and the Indo-Pacific. Only a single other patellid species, the western African Cymbula safiana, has a range overlapping with Patella species (Ridgway et al. 1998). The fossil record contains little evidence how this separation came about: patellids are rarely preserved, living as they do in high-energy environments, and their morphological simplicity makes them difficult to identify though genera can be distinguished by their shell microstructure. Patella proper has not been reliably identified earlier than the Pliocene (Ridgway et al. 1998). It has been suggested that the ancestors of Patella either migrated up the western coast of Africa, or became separated from other patellids by the closure of the Tethys Sea that once connected what is now the Mediterranean with the Indian Ocean. However, molecular analyses have placed Patella as the sister taxon to all other patellids; if correct, this could push its separation back past the Upper Cretaceous as a Japanese fossil from that time has been assigned to the patellid genus Scutellastra. This would be too early for the African dispersal or Tethyan explanations, and new proposals are required.

REFERENCES

Le Quesne, W. J. F., & S. J. Hawkins. 2006. Direct observations of protandrous sex change in the patellid limpet Patella vulgata. Journal of the Marine Biological Association of the United Kingdom 86: 161-162.

Nakano, T., & T. Ozawa. 2004. Phylogeny and historical biogeography of limpets of the order Patellogastropoda based on mitochondrial DNA sequences. Journal of Molluscan Studies 70: 31-41.

Ridgway, S. A., D. G. Reid, J. D. Taylor, G. M. Branch & A. N. Hodgson. 1998. A cladistic phylogeny of the family Patellidae (Mollusca: Gastropoda). Philosophical Transactions of the Royal Society of London Series B 353: 1645-1671.

Weber, L. I., & S. J. Hawkins. 2002. Evolution of the limpet Patella candei d’Orbigny (Mollusca, Patellidae) in Atlantic archipelagos: human intervention and natural processes. Biological Journal of the Linnean Society 77: 341-353.