Reference : Dial. Bull.. 155 : 235-245. (August, 1978) CHROMOSOME KARYOTYPES OF THREE BIVALVES: THE OYSTERS, ISOGNOAION ALATUS AND PIN CT AD A IMBRICATA, AND THE BAY SCALLOP, ARGOPECTEN IRRADIANS IRRADIANS l KATSUHIKO WAD A Xatiotnil Pearl Research Laboratory, Kasliikojiina, Mic, Japan Since Menzel and Menzel (1965) reported on the chromosomes of two species of clams, several workers have observed the chromosomes of bivalves (pelecypod molluscs) using squash or air-drying techniques (Ahmed and Sparks, 1967, 1970: Longwell, Stiles and Smith, 1967; Menzel, 1968; Patterson, 1970; leyama and Inaba, 1974; leyama, 1977). However, there are still only a few species of bivalves for which chromosome numbers have been established, as compared with the greater numbers of gastropod molluscs with known chromosome karyotypes. This paper presents the chromosome number and gross morphology of three species of Bivalvia not previously reported: Isognoinon alatits, the flat tree oyster; Pinctada imbricata, the Atlantic pearl oyster; and Argopcctcn irradians irradians, the commercial bay scallop. MATERIALS AND METHODS Specimens of Isognomon alatus were collected in June, 1976, from Biscayne Bay, Miami, Florida. Specimens of Pinctada imbricata \vere collected in February, 1977, near Pompano Beach, Florida. Cultured specimens of Argopectcn irradians irradians were obtained from a commercial hatchery on Long Island, New York. Eggs and sperm were obtained by stripping the gonads of /. alatns (10 animals). Spawning was induced thermally in P. imbricata (14 animals) and in A. i. irradians (25 animals). A. i. irradians is hermaphroditic and usually spawns both eggs and sperm almost simultaneously. "When unfertilized eggs were needed, these animals were induced to spawn in sea water containing 0.02% EDTA (eth-ylene-diaminetetraacetic acid) which, by chelating calcium out of the sea water, must prevent the acrosome reaction of the sperm essential for fertilization. Self-fertilization was effectively prevented by the EDTA. EDTA-exposed eggs were washed with fresh sea water just after spawning, and no chromosome damage was observed in them. Following fertilization, eggs and embryos were fixed at intervals in Carnoy fixative (3:1). Chromosome preparations were made by squashing the eggs or embryos in 1 to 2% aceto-orcein. To obtain colchicine metaphase in some cleaving eggs, fertilized eggs were held in sea water containing 0.02% colchicine for 15 to 30 minutes. P. imbricata eggs were not treated with colchicine. 1 Contribution No. 245 from the National Pearl Research Laboratory, Kashikojima, Alie, Japan. 235
BioStor
