Developmental mode in benthic opisthobranch molluscs from the northeast Pacific Ocean: feeding in a sea of plenty

Abstract

Mode of development was determined for 130 of the nearly 250 species of shallow-water, benthic opisthobranchs known from the northeast Pacific Ocean. Excluding four introduced or cryptogenic species, 91% of the species have planktotrophic development, 5% have lecithotrophic development, and 5% have direct development. Of the 12 native species with non-feeding (i.e., lecithotrophic or direct) modes of development, 5 occur largely or entirely south of Point Conception, California, where surface waters are warmer, lower in nutrients, and less productive than those to the north; 4 are known from habitats, mainly estuaries, that are small and sparsely distributed along the Pacific coast of North America; and 1 is Arctic and circumboreal in distribution. The nudibranchs Doto amyra Marcus, 1961 and Phidiana hiltoni (O'Donoghue, 1927) were the only species with non-feeding development that were widespread along the outer coast. This pattern of distribution of developmental mode is consistent with the prediction that planktotrophy should be maintained at high prevalence in regions safe for larval feeding and growth and should tend to be selected against where the risks of larval mortality (from low- or poor-quality food, predation, and transport away from favorable adult habitat) are higher. However, direct development, which includes the most derived mode of non-feeding development, was also correlated with small adult size, reflecting an association common in marine invertebrates. Planktotrophic development also predominates in decapod Crustacea from the northeast Pacific, but is less common in echinoderms and prosobranch gastropods from this region owing to the presence of lineages constrained by phylogeny to non-feeding modes of development.