Color pattern variation in a shallow-water species of opisthobranch mollusc

The color pattern of benthic opisthobranch sea slugs (Mollusca: Gastropoda) appears to play an important defensive role, and numerous species seem to have aposematic (warning) colorations. Color pattern is an important trait for opisthobranch identification-this conclusion is based on the assumption that most species have limited color variation. For those species in which color variation is recognized, the reasons for the variation remain unknown. In this paper we study Philinopsis pusa, a benthic putative species of opisthobranch sea slug with a broad range of color pattern. Lighter individuals appear to be camouflaged on the white sand environment in which the animals are typically found, whereas darker individuals appear conspicuously different from their background. Because of its broad color variation, P. pusa has been subdivided into different species. Animals were collected and observed in the Bahamas during a 6-year span. The color pattern of the specimens was subjectively classified into five phenotypic classes. Two mitochondrial genes (16S, CO1) were sequenced from 41 specimens. The association between color pattern, body length, burrowing escaping behavior, and the genetic structure of the population was investigated. We found two genetically distinct groups in the target population but no significant association between color pattern and genetic structure. Additionally, there was no significant association between color pattern and ontogeny or defensive behavior in these organisms. The present paper suggests that general assumptions on the biological and evolutionary role of color in opisthobranchs need to be carefully evaluated.