Adult Plasticity and Rapid Larval Evolution in a Recently Isolated Barnacle Population

Balanus amphitrite, a common barnacle species, was introduced into the landlocked Salton Sea in 1943 or 1944. In 1949, Balanus amphitrite from the Salton Sea was classified as the subspecies, Balanus amphitrite saltonensis, based upon morphological differences between Salton Sea and coastal individuals. This classification was maintained following an investigation of the Balanus amphitrite complex in 1975. Such a designation implies that the morphological divergence is underlain by genetic differences. Using field and laboratory transplantations, I tested the alternative hypothesis that the observed morphological divergence in the adult stage of Balanus amphitrite was the result of phenotypic plasticity. The results show that the divergence in the examined adult characters is in fact due to environmentally induced phenotypic plasticity. There were also phenotypic differences between larvae from the Salton Sea and those from coastal habitats that only became apparent during experimentation with the adult stage. Here, however, experimental results suggest that the divergence was due to an evolutionary process, probably selection. These results also provide the basis for two slightly precautionary conclusions: (1) the observation that individuals living in typical and novel habitats differ cannot even weakly indicate a cause for the difference, and (2) a consideration of the divergence of populations is incomplete if all of the life history stages of the organism are not studied.