Chromosomes of Some Opisthobranchiate Mollusks from Eniwetok Atoll, Western Pacific

Abstract

Chromosome numbers are reported for nine species of opisthobranchiate mollusks from Eniwetok Atoll, Marshall Islands, western Pacific. In the Nudibranchia, both Dendrodoris nigra (Dendrodorididae) and Herviella mietta (Favorinida e) were found to have 13 bivalents during male meiosis. In the Anaspidea, Dolabrifera dolabrifera and Stylocheilus longicauda (Aplysiidae) both had 17 bivalents during male meiosis. In the Cephalaspidea, Haminoea linda and H. musetta (Atyidae) each had 17 pairs of chromosomes during male meiosis and Lathophthalmus smaragdinus and Smaragdinella calyculata (Smaragdinellidae) had 18 pairs. In the Soleolifera, Onchidella evelinae had 18 bivalents during male meiosis. The extreme conservativeness of chromosome numbers in opisthobranchiate mollusks is demonstrated by that fact that all 18 nudibranchs from 10 families studied so far have the single haploid chromosome number 13, and that 18 of the 21 species of the orders Entomotaeniata, Anaspidea, Cephalaspidea, and Sacoglossa have 17 pairs of chromosomes. The haploid number 18 is here reported for the first time for nonsoleoliferan opisthobranchiate mollusks. The more advanced, mostly fresh-water, order Basommatophora, in which the haploid number 18 is the basic number, may well have been derived from a taxon within or related to this cephalaspid superfamily (Philinacea).