Pigmentation polymorphism in the invasive amphipod Dikerogammarus villosus: some insights into its maintenance

Abstract

Dikerogammarus villosus, a freshwater invasive amphipod, exhibits conspicuous pigmentation polymorphism. This polymorphism is documented in two recently colonized areas, the Saône and Moselle rivers (north-eastern France), and some of the mechanisms by which pigmentation polymorphism can arise and be maintained are addressed. Body size, reproductive status, fecundity and mate choice are compared among morphs of D. villosus in field samples collected in summer 2001. Body size and female gonad developmental stage were comparable among the different morphs, suggesting that polymorphism is not the result of changes in pigmentation with age or moult-cycle. Fecundity and reproductive status (paired vs non-paired) were not affected by pigmentation morph either. A random combination of morphs in pre-copula pairs was observed, showing that the colour pattern does not play a major role in mate choice. A strong size-assortative pairing was found, and this pattern was similar among male morphs. Overall, our study shows that morph polymorphism is not related to different stages in the moult cycle or the life cycle, and is not maintained by morph-assortative pairing. Alternative explanations to the mate choice hypothesis for the maintenance of pigmentation polymorphism are discussed.