The Sperm Ultrastructure and Some Reproductive Characteristics of the Chemosymbiotic Bivalve Calyptogena pacifica Dall, 1891 (Vesicomyidae: Pliocardiinae)

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Pliocardiines (Bivalvia: Vesicomyidae: Pliocardiinae) are a chemosymbiotrophic group of bivalve mollusks that are obligate for reducing environments. These mollusks house endosymbiotic thioautotrophic bacteria in their gills, which provide nutrition for the host. The ultrastructure of spermatozoa and the state of the gonads in the pliocardiine bivalve Calyptogena pacifica in June 2016 were studied. Material was collected in the Bering Sea on the slopes of the Piip’s Volcano at a depth of 466 m. The condition of the gonads indicated a pre-spawning state. Active processes of spermatogenesis and oogenesis were noted in the gonads. The mature spermatozoon has an elongated bullet-shaped head with an average length of 4 ± 0.2 μm from the tip of the acrosome to the base of the mid-piece. The mid-piece was formed by a complex of four spherical mitochondria with a diameter of approximately 0.7 μm. An electron dense material of a lipid nature was observed in the distal region of the mid-piece of the sperm. C.pacifica mature eggs are approximately 200 μm in diameter. The results are discussed in the context of the available data on the morphology of pliocardiine gametes.