Histological observation on gonad development of the freshwater pearl mussel, Hyriopsis (Limnoscapha) myersiana

Abstract

The microscopic structure of the gonad at different ages of the cultured mussel Hyriopsis (Limnoscapha) myersiana is described. Male gonadal acini are first observed at 6 months old; they are small and scarce, and contain only a few germ cells at the early stage of spermatogenesis. At 7 and 8 months old, the male acini increase in size and have more numerous germ cells at subsequent developmental stages: i.e., spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and spermatozoa. Male germ cell morulae are also found in the acini. At 9 months old, the male gonad becomes fully developed and occupies two-thirds of the visceral mass. Sperm are found abundantly in the acini and gonoducts, indicating spawning. The female gonad begins to develop at 8 months old with a few small previtellogenic oocytes and numerous nutritive granules in the acini. The female gonad and gonoduct are well developed at 9 months old. Several large vitellogenic oocytes are found in the acini; the cytoplasm of these oocytes contains eosinophilic yolk vesicles. At this age the marsupia are formed, confirming that the female reproductive system is fully developed and that spawning is taking place. In this study, the gonads of mussels at 10, 11, and 12 months of age were less developed than those of 9-month-old mussels, most likely due to environmental conditions. Several atretic oocytes were found in the acini of 12-month-old mussels. The present observation strongly suggests that both males and females are sexually mature at 9 months old, and that H. (L.) myersiana at this stage can be kept as brood stock for artificial culture.