Histological structure of gonads in the freshwater pearl mussel, Hyriopsis (Hyriopsis) bialatus Simpson, 1900

Abstract

Summary The macroscopic and microscopic structures of male and female gonads of Hyriopsis (Hyriopsis) bialatus are described. The sex of individuals was determined from the presence or absence of sperm and eggs in the gonadal fluid under light microscopy analysis. The gonads of both sexes are composed of numerous acini. Male acini contain germ cells at various developmental stages of typical spermatogenesis, i.e., spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and spermatozoa. A striking feature is the presence of morulae with atypical spermatogenesis, interspersed among typical germ cells. Early stage morulae comprise a few large spermatogonia, those at middle stage contain spermatocytes and those at late stage consist of spermatids and spermatozoa. Female acini include oogonia and five different stages of developing oocytes. Stage 1 oocytes are attached to the acinar wall without yolk vesicles and stalk. Stage 2 oocytes have a stalk connected to the acinar wall. Stage 3 oocytes are larger in size, having yolk vesicles accumulated both at the base of the stalk and at the periphery of nucleus. Stage 4 oocytes have yolk vesicles dispersed throughout the cytoplasm. During this period, the basophilia of cytoplasm is replaced by eosinophilic yolk. Stage 5 oocytes contain both mature and developing oocytes. Immature oocytes are surrounded by a vitelline layer and attached to the acinar wall by a narrow stalk. Mature oocytes, however, are completely detached from the acinar wall, being released into the acinus lumen. The male and female germinal ducts are lined by a simple ciliated columnar epithelium. Several acini are connected to a single germinal duct. Numerous mature germ cells are found in the lumen of the germinal duct. No change was detected in the histological structure of male and female gonads collected at different times of the year. Female gonads collected at different stages of marsupial development are also morphologically similar. These observations strongly suggest that H. (H.) bialatus has no distinct reproductive season and may be suitable for commercial culture.