Chromosome studies in pelagic opisthobranch molluscs

Chromosome number and morphology were studied in nine species of pelagic opisthobranchs ("pteropods"). Among Thecosomata, seven species were investigated. Limacina inflata has a diploid chromosome number of 2n = 20 with metacentric and submetacentric chromosome pairs. Creseis acicula has 2n = 20 (five metacentric and five submetacentric pairs). Creseis virgula has 2n = 20 (seven metacentric, one submetacentric, one subtelocentric – submetacentric, and one submetacentric – subtelocentric pair). Clio pyramidata has 2n = 22 (five metacentric, one submetacentric, two submetacentric – subtelocentric, two subtelocentric, and one telocentric pair). Cavolinia inflexa has 2n = 24 (six metacentric, one submetacentric, two subtelocentric – submetacentric, one subtelocentric – telocentric, and two subtelocentric pairs). Peraclis reticulata has 2n = 24 (six metacentric and six submetacentric or subtelocentric pairs). Cymbulia peroni has 2n = 34 (six metacentric, three submetacentric, seven subtelocentric, and one telocentric pair). Among Gymnosomata, two species were investigated. Pneumodermopsis canephora has 2n = 32 (two metacentric, three submetacentric, four submetacentric – subtelocentric, one subtelocentric – submetacentric, five subtelocentric, and one telocentric – subtelocentric pair). Pneumoderma atlanticum has 2n = 32 (three metacentric, five submetacentric, four submetacentric – subtelocentric or subtelocentric – submetacentric, and four subtelocentric pairs). For three of the nine pteropod species, material was abundant enough to permit staining for the nucleolar organizer region. This region occurred on pair 9 in Creseis acicula, on pair 6 in Cavolinia inflexa and on pair 2 in Pneumodermopsis canephora. Results on number, morphology, and size of the chromosomes of each of the studied species are discussed. The present chromosomal evidence confirms the separation, sometimes seen as controversial, of the pelagic opisthobranchs into the orders Thecosomata and Gymnosomata. The Thecosomata constitute an isolated order among opisthobranchs by their striking chromosomal diversity. In contrast, the Gymnosomata show cytogenetic features that suggest an evolutionary trend with plesiomorphic characters, as in the Anaspidea and Sacoglossa.