The main purpose of this study was to investigate the gonadal cycle of the musselLithophaga truncata (Gray, 1843). Specimens were sampled regularly twice every season at depth 1- 2 meters in the year (2015 - 2016) along the northern estuarine harbor of the Arabian Gulf - Saudi Arabia. Parts of gonads (ovaries and testes) were prepared for histological and statistical studies. The histological observations of the gonad indicated that there was only one gonadal cycle in the year. The ovaries produced oocytes which undergone vitellogenesis to change these oocytes into comparatively ripe ones condensed with yolk. With slide micrometer a fixed length and width of the ovary was determined in serial histological sections (30000 µm). The different stages of oocyte development were counted and statistically analyzed. During November - February the gonadal follicles appeared small in size and in a quiescent state. Follicles were few in number and scattered throughout the vesicular connective tissue. In males the follicles were small and scattered throughout the connective tissue in the same way as females. Follicles contained undifferentiated residual cells, spermatogonia and negligible number of primary spermatocytes. The oogonia were the most dominant type. Few number of previtellogenic oocytes were observed. The ooplasm of these stages, oogonia and previtellogenic oocytes, was homogeneous and with comparatively large nucleus. In previtellogenic oocytes small granules of the same nature were scattered randomly in the ooplasm. These granules consisted of proteins (peptide linkage). Carbohydrates and lipids were entirely absent.During April - July previtellogenic oocytes began to undergo growth phase and extending towards the center of the follicles. In males spermatogenesis became more apparent and proceeded at a faster rate. In some mussels spermiogenesis was formed in great number. The gonadal follicles ramified rapidly, and the development of gametes of both sexes was very fast. In females the vitellogenic oocytes showed prominent development. Atretic oocytes and their debris were negligible. Very few number of oogonia were observed. Yolk bodies (glycoproteins) increased rapidly and replaced the vacuoles in the ooplasm. Other large granules gained a dark blue color when Aqueuos bromophenol blue was applied. These granules are basic proteins. Other large-sized granules began to appear at the periphery of the ooplasm. These granules gained positive reaction with Alcian blue, PAS and Alcian blue- PAS reactions. The vitelline envelope became conspicuous under the follicle cells. Mucosubstances were positively intense at the periphery of the oocyte while proteins surrounded the nucleus. Gradually fat deposition increased. The size range of these oocytes was 130 - 180µm with the ratio of the nucleus to the ooplasm 0.5:2. During August - October both sexes had ripe gametes. In males, the central portion of the follicles was empty because of the recent discharge of large quantities of ripe gametes. At the end of this step resorption of the gonads proceeded very rapidly and emptied follicles began to acquire an elongated form. At this step the oocyte attains its maximal size 180 - 200µm. The mean of the total number of oocyte developmental stages in all seasons showed that the ovary was active in all seasons of the year but different labors were present. It concluded that the oogonia , the postvitellogenic oocyte development and atretic oocytes were conspicuous during the first step of oogenesis ; the vitellogenic oocytes were dominant during the second step of oogenesis whereas the postvitellogenic and atretic oocytes were dominant during the third step of oogenesis. All data obtained were subjected to analysis of variance (ANOVA) means significant difference P>0.05 or P 0.05.
gonadal cycle –oogonia - previtellogenic oocytes - vitellogenic oocytes - postvitellogenic oocytes - atretic oocytes – spermatogenesis – spermiogenesis - analysis of variance (ANOVA) - Tukey post hoc test