Chromosome studies of three families of pelagic heteropod molluscs (Atlantidae, Carinariidae, and Pterotracheidae) from Hawaiian waters

Chromosome number and morphology were studied in gonadal tissue of 11 species of Atlantidae, 2 species of Carinariidae, and 3 species of Pterotracheidae, using an air-drying technique and Giemsa staining. In the Atlantidae the diploid chromosome number was the same in males and females and there were no heteromorphic chromosomes. The diploid chromosome number in nine species of Atlanta was 30 and the majority of chromosome pairs were metacentric and submetacentric. In Protatlanta souleyeti the diploid number was 28, and included five metacentric, six submetacentric, and three subtelocentric chromosome pairs. Oxygyrus keraudreni had a diploid chromosome number of 32, with 10 metacentric and 6 submetacentric chromosome pairs. A striking difference between the Atlantidae and the Carinariidae and Pterotracheidae was the presence of heteromorphic sex chromosomes in the latter two families. Male Pterosoma planum (2n = 32) had simple XY sex chromosomes, but males of Carinaria japonica (2n = 33), Pterotrachea scutata (2n = 33), Pterotrachea hippocampus (2n = 31), and Firoloida desmaresti (2n = 31) showed three heteromorphic chromosomes, suggesting a multiple sex-determining mechanism, X1X2Y. The locations of the female sex chromosomes in the karyotypes of female Pterotrachea hippocampus (2n = 32) and Firoloida desmaresti (2n = 32) were tentatively proposed. Cytogenetic features observed among the three families are supportive of previous interpretations of evolutionary trends in the Heteropoda based on morphology, i.e., that the Atlantidae are the most primitive family and gave rise to the Carinariidae and Pterotracheidae.