GONSALVES-JACKSON, D.C.: Developmental strategies in opisthobranch mollusks across the Isthmus of Panama

The embryonic/larval developmental period of invertebrates is a significant component of life history, influencing population distribution, recruitment, genetic variation, and extinction. A dichotomy in developmental patterns has been documented in gastropods, bivalves and bryozoans where a higher incidence of planktotrophy occurs in the Pacific Ocean, and a dominance of aplanktonic development is evident in the Atlantic Ocean. This same dichotomy in developmental strategies may also exist in opisthobranch mollusks, organisms ideally suited for investigating developmental strategies because they readily deposit egg masses in the laboratory that can be reared to hatching. This hypothesis was tested through collections of five major groups of opisthobranchs from the Pacific and Atlantic coasts of Panama. Specimens plus the egg masses they deposited in the laboratory provided data to determine developmental mode and compare proportions of developmental types in Atlantic and Pacific faunas. 68 species laid egg masses that were reared in the laboratory: 29 species collected from the Atlantic and 39 from the Pacific coast of Panama. All those collected from the Pacific coast were planktonic. In contrast, 5 species from the Atlantic coast had aplanktonic development while the remaining 24 species released planktonic larvae. These results indicate that the Panamanian opisthobranch fauna is predominantly planktonic on both coasts, lending only minor support to the dichotomy theory of development. The occurrence of benthic development in 5 species on the Atlantic coast may be related to substratum and low primary productivity. The Pacific coast of Panama experiences higher levels of primary productivity, and presumably can more reliably support planktonic larval feeding.