Laboratory observations on the feeding behavior of four species of thaidid gastropods (Muricacea), when fed on the intertidal barnacle Balanus glandula (Darwin), revealed two interesting patterns. First, large inividuals of Thais (or Nucella) emarginata (Deshayes) (> 15 mm shell length) exhibited remarkably little variation in the locations at which they drilled barnacles, either among individual snails, among populations along a wave exposure gradient (≈ 5 km), or among populations along the Pacific coast of North America (≈ 3000 km). The results of laboratory crosses suggested that the small differences which did exist between populations from southeast Alaska (U.S.A.) and Vancouver Island, British Columbia (Canada) were genetically determined.

A second pattern of interest was an ontogenetic shift in the preferred location of drilling in all four species of Thais examined (T. canaliculata (Duclos), T. emarginata T. lamellosa (Gmelin), and T. lima (Gmelin)): larger snails drilled much more frequently in the opercular region and, concomitantly, more frequently at the occludent margins of the scutal plates. The ontogenetic shift in these snails appeared to be primarily genetically predetermined rather than learned, since individuals of T. emarginata grown from one size class to the next on mussels (Mytilus edulis L.) did not differ greatly in their selection of drilling locations on Balanus glandula from those grown similarly on barnacles.