Study on gonad histological structure of male Hilsa shad Tenualosa ilisha (Hamilton, 1822) during reproductive migration

Hilsa shad (T. ilisha) is the only anadromous species among the fishes in south of Iran, which has reproductive migration from Persian Gulf to the rivers. In this research, the morphological properties, histological and cellular changes in testes and spermatocytes of Hilsa shad were studied in two different environments (salt water and fresh water) in Khoozestan province during its reproductive cycle. Fish samples were collected from Bahmanshir River (fresh water) and Musa creek (salt water). After biometric measurements (length and weight measured), sample pieces of testis from each fish were fixed in Bouin's fixative solution. The tissues were embedded in paraffin for preparation of section (5µm thickness) for light microscope and then stained with Hematoxylin and Eosin. According to the results obtained from male fishes, five phases of gametes were observed including spermatogonia, primary spermatocyte, secondary spermatocyte, spermatid and spermatozoa. Spermatogonia were the largest germ cells in the testis which were observed during the whole reproductive cycle. Primary spermatocytes were ovoid form and seem to be smaller and darker in comparison with spermatoginia. Secondary spermatocytes had smaller size rather than Primary types. Spermatozoa were ovoid and stained with Hematoxilin properly while their tails showed weak staining capability.