Spawning Periodicity of the Asiatic Clam Corbicula fluminea in the New River, Virginia

Spawning periodicity of the Asiatic clam Corbiculafluminea in the New River, Virginia, is reported. Numbers of newly recruited larvae in the New River sediment, number and life stage of larvae naturally released and collected from adults held in the laboratory, and presence of developing veligers within the brood chambers of sacrificed adults were collected weekly from May to December 1984. Abiotic data collected consist of mean weekly water temperatures, daily total daylight hours for the western Virginia vicinity and mean daily discharge rates. Density of larvae in sediment, total numbers of larvae collected from laboratoryheld adults, and brood chamber condition are all highly similar in timing, duration and intensity of spawning effort. There were three major peaks in larval abundance-late spring, midsummer and early autumn. These observations do not coincide with previously reported patterns of spring and autumn reproductive peaks by Corbiculafluminea. INTRODUCTION Since the introduction of Corbicula fluminea to the Pacific northwest coast of North America in the 1930s, the Asiatic clam has spread S and E. Its northerly migration has been limited only by winter extremes in water temperature (McMahon, 1982). Initial reports of C. fluminea in Virginia placed this species in the James River (Diaz, 1974) and the New River (Rodgers et al., 1977). Colonization of available habitats by this organism has not been limited to natural waterways but includes the establishment of viable populations in utility and industrial cooling water systems (Morton, 1979; Cherry et al., 1980). The impact of these incursions into environmental and industrial facilities has produced both ecological and economic repercussions of significant proportions that are reviewed in detail elsewhere (Ingram, 1959; Sinclair and Isom, 1963; Fuller and Imlay, 1976; Gardner et al., 1976; Fuller and Richardson, 1977; Goss and Cain, 1977; McMahon, 1977; Boozer and Mirkes, 1979; Kraemer, 1979; Smith et al., 1979; Harvey, 1981). One attribute that apparently facilitates rapid colonization is the reproductive capacity of this species. Kraemer and Lott (1977) reported that Corbicula sp. is hermaphroditic, possessing both oogenic and spermatogenic tissue. While the male and female gametes are formed in different parts of the gonads, they seem to be shed through the same ducts. This prompted the speculation that individuals may possess the ability for internal self-fertilization. Britton and Morton (1982) suggested that cross-fertilizing individuals achieve fertilization in the maternal suprabranchial chamber after spermatozoa have entered the infrabranchial chamber through the incurrent siphon. Once fertilized, eggs are either retained and incubated within the inner demibranch (gill) or discharged as conglutinates of eggs, the mode being species-specific (Sinclair and Isom, 1963). Incubatory species retain the developing embryos through the trochophore and early veliger stages prior to being expelled through the exhalant siphon as pediveligers. This study summarizes our efforts in documenting the reproductive activities of Corbicula fluminea in the New River, Virginia, during 1984. Three different approaches 'Send correspondence to F. G. Doherty. Present address: Joint Graduate Program in Toxicology. College of Pharmacy, Busch Campus, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854.