Pelagic Gastropods

Abstract

Here we deal with two different lines of evolutionary adaptation of holoplanktonic gastropods. Only a small proportion of gastropods spend their entire life cycle as part of the plankton; these include forms contained in two subclasses of Gastropoda: the Prosobranchia and the Opisthobranchia. The former are represented by the Superfamily Heteropoda, and opisthobranchs by the orders Thecosomata and Gymnosomata. Of these two orders, we considered only the two suborders of Thecosomata: Euthecosomata and Pseudothecosomata, both widely known under the informal name of Pteropoda. This revision updates the knowledge about the taxonomic composition and distribution of the species of pteropods and heteropods recorded in both the Pacific and Atlantic coasts of Costa Rica. In general, it was found that investigations on the Costa Rican pelagic gastropods have been quite scarce and the literature on adjacent areas is scattered. The number of heteropod species effectively recorded in the Pacific coast of Costa Rica (16 spp.) versus the worldwide known species represents about 44%; the figure is 60% with respect to the Eastern Pacific. There are only three records of heteropods in the Caribbean Sea off Costa Rica, the corresponding proportional figures are 10% worldwide, 12% Northwestern Atlantic Ocean. As for pteropods, the percentage of species effectively recorded in the Pacific coast of Costa Rica versus the worldwide known species is low: 7%. The proportion is higher (36%) with reference to the Eastern Pacific. Only nine species of pteropods have been reported in the Costa Rican Caribbean (17% of the Northwestern Atlantic; 4.7% worldwide). Comments are also provided on the relevance of the group and the methodological problems involved in the taxonomic study of these organisms.