Massive polymorphism and natural selection in Donacilla cornea (Poli, 1791) (Bivalvia: Mesodesmatidae)

Abstract

We describe a massive polymorphism for shell colour and pattern inDonacilla cornea, a shallow-burrowing bivalve which inhabits themid-littoral of sandy and stony beaches in southern and western Europe.Of the four sites studied, the one where the substrate was a mixture ofsmall stones and sand contained the highest densities of Donacilla. Atthis site the densities of stones and animals and the frequencies ofshell and stone colour phenotypes are both highly correlated. Althoughthe four populations sampled comprise a similar range of phenotypes, thefrequencies of most morphs and population diversity indices differsignificantly between sites. At two sites the similar range of colourand pattern variation between shells and substrate suggests selectionfor crypsis and masquerade. On the other hand, at two other sites, oneor two common phenotypes (and a number of rare ones at all sites) failto mimic any aspect of the background. We therefore suggest that iffrequency-dependent selection by predators is responsible for themaintenance of polymorphism in Donacilla, then the mechanisms involvedinclude crypsis and masquerade in some cases and selective advantagesfrom being different in others. (C) 1997 The Linnean Society of London.