Spermatozoal ultrastructure of diploid and tetraploid Pacific oysters

Spermatozoal ultrastructure of tetraploid Pacific oysters, Crassostrea gigas, was examined by light and electron microscopy and compared to that of diploid Pacific oysters. Ultrastructure of sperm from tetraploids was compositionally similar to diploids except for overall size and the number of mitochondria. Instead of the four mitochondria always found in sperm from diploid oysters, 44% of sperm from tetraploid oysters had four mitochondria, 53% had five, and 3% had six. The linear dimensions of sperm components such as acrosome height and width, sperm head length and width, mitochondrial height, length of the main piece and end piece of flagellum, and total length of flagellum in tetraploids were approximately 1.25 times larger than corresponding measurements in diploids, which corresponds to a doubled volume in sperm of tetraploids compared to that of diploids. Tetraploid males produced half the number of sperm (2.7 ± 0.5 × 1010) g− 1 of gonad wet weight compared to diploid males. Despite the increased sperm sizes in tetraploids, the ratios of the sperm head length to head width, and sperm head length to flagellum length were the same for both ploidy levels for fixed and live samples. The implications of these findings on susceptibility of sperm from diploid and tetraploid oysters to damage from cryopreservation are discussed.