Spermatogenesis and ultrastructure of the spermatozoa at three ploidy levels in the bivalve species, Corbicula fluminea

A histological and ultrastructural examination of spermatogenesis and spermatozoa in diploid, triploid and tetraploid Corbicula fluminea was conducted. In the Yangtze River estuary, the spermatozoa of C. flumiena exhibited a primitive uniflagellate structure, whereas a modified biflagellate morphology was observed in inland freshwater habitats. This study presents the initial observation of sperm morula in spermatogenesis in C. fluminea. The mature spermatozoa were released as single cells and as spermatozeugmata. It was observed that uniflagellate spermatozoa undergo two meiotic divisions during it formation, whereas biflagellate spermatozoa undergo only one. The uniflagellate and biflagellate spermatozoa were observed to comprise of three distinct parts: the head, middle piece and tail. The longest and shortest spermatozoa (69.24 μm ± 1.70 vs 54.11 μm ± 1.26) were both biflagellate spermatozoa, originating from tetraploid and diploid individuals, respectively. The number of mitochondria was observed to increase in accordance with ploidy. In the uniflagellate spermatozoa, the two centrioles were observed to be oriented perpendicular to each other. In contrast, the two centrioles were oriented in parallel with each other in biflagellate spermatozoa, with each centriole attaching a flagellum. The undulating membrane of the tail contains a substantial number of glycogen granules. The differences in spermiogenesis, sperm morphology and structure between C. flumiena from estuarine (amphogenesis and in vitro development) and inland freshwater habitats (androgenesis and brooding larvae in the inner demibranch) result in better adaptations to the respective reproduction modes.