Gametogenesis and sexual maturation of the surf clam Mactra veneriformis on the west coast of Korea

Gametogenesis and sexual maturation of the surf clam Mactra veneriformis Re eve, 1854, were studied by histological and cytological observations. In th e early vitellogenic oocyte, the Golgi apparatus and mitochondria present i n the perinuclear region are involved in the formation of lipid droplets an d in lipid granule formation. In the late vitellogenic oocyte, the endoplas mic reticulum, mitochondria in the cytoplasm are involved in the formation of proteid yolk granules. At this time, exogenous lipid granular substances and glycogen particles in the germinal epithelium are passed into the oopl asm of the oocyte through the microvilli of the vitelline envelope. Ripe oo cytes are about 60-70 mum (65.35 +/- 4.82 mum) in diameter. The head of the spermatozoon is approximately 2.5 mum in length, and its tail is about 58. 76 +/- 7.20 mum. The axoneme of the tail flagellum consists of nine pairs o f microtubules at the periphery and a pair at the center. The spawning peri od was from early June to September. with the main spawning occurring betwe en July and August when the water temperatures are greater than 22 degreesC . The reproductive cycle of this species can be divided into five stages: e arly active, late active, ripe, partially spawned, and spent/inactive. Percentages of first sexual maturity of female and male clams ranging in le ngth from 26.0 to 30.0 mm are over 50%, and 100% for clams over 31.0 mm in shell length. The sex ratios of females to males over 26.0 mm in shell length were not si gnificantly different from a 1:1 sex ratio. No evidence of hermaphroditism was found in histological sections of any surf clam examined.