Marine Biology Research Division

We report here the first chromosome numbers for sea-skaters. Meiotic metaphase figures were obtained from the testes of three littoral species: Asclepios shiranui, Halobates flaviventris and H. robustus and two pelagic species: H. germanus and H. micans. In the four species of Halobates 2n~ = 31, the highest chromosome number so far determined for the Gerridae. In a proposed ancestral form, A. shiranui, 2n6' = 23. Males of all five species have an unpaired chromosome, assumed to be the X chromosome of an XO sex chromosome system.