Reproduction of a fingernail clam in contrasting habitats: life-history tactics?

Abstract

In a population of a semelparous fingernail clam (Pisidium casertanum) inhabiting an ephemeral pond, litter size was larger and generation time was shorter than that of a conspecific population in a lake. This agreed with the r–K selection model, which predicts selection for a shorter generation time and larger litter sizes in relatively unstable habitats. Cage experiments at varying densities, however, revealed that considerable phenotypic variability in litter size may be environmentally induced. These results indicate that this and other comparative studies of intraspecific variation in the life-history traits of fingernail clams cannot conclude that varying evolutionary tactics are present until simpler hypotheses of environmentally induced variation are dismissed.