Colors and patterns of an intertidal snail, Thais lamellosa

Summary Shell patterns and colors and the genetic complexity of the polymorphism of the marine snail, Thais lamellosa , are similar to those of several terrestrial pulmonates. Frequent parallel evolution of such complex polymorphisms must reflect a substantial selective advantage. To form a hypothesis to account for pattern evolution, morph compositions of several populations were quantified and compared with environmental qualities. All three primary colors were found at only two of five sites. Shells with many banding patterns were found; however, the most common pattern of one site was rare or absent at most other sites. Snails were fed monotonous diets of barnacles or mussels for 7 months to evaluate the importance of diet. Colors deposited in most shells were diluted progressively during the period and, like siblings in the field, most snails had white shells at the end of the experiment. No morph characteristics could be attributed to diet. Patterns are usually prominent only on juveniles, and may be obscured for much of the juvenile period, so the colors and patterns distinguish snails from each other for only a short portion of their lives. Morph differences from site to site are correlated with environmental differences. Environmental qualities could affect densities of the principal predator, Cancer productus , and thus predation intensity, or could affect the relative visible qualities of the morphs.