Seasonal changes in the activity of the thyroid follicles in relation to testicular activities in mullet Liza parsia (Hamilton)

Abstract

The thyroid gland of Liza parsia was unencapsulated and was in the form of groups of follicles which were scattered mainly around the ventral aorta in the pharyngeal region. Each follicle possessed a central lumen either fully or partially filled up with the agranular eosinophilic colloid materials and the lumen was encircled by a single layer of epithelial cells. The activity of the follicles fluctuated seasonally in harmony with the reproductive cycle of L. parsia. The low active condition of the thyroid follicles during the growth phase i.e. August-September was well in coincidence with the increase of spermatogonial and spermatocyte cells. The most active condition of the thyroid follicles were noted during spawning phase i.e. January and February as revealed by the maximum follicular cell height and also correlated with the highest frequency percentage of spermatozoa. Thereafter, the thyroid activity remained gradually on a low level up to July and revealed an abrupt increase in November at the start commence of active maturation. It was concluded that the peaks of thyroidal and testicular activities correlate well and run parallel. The possible involvement of salinity may have a primary significance in provoking changes of the thyroid as well as reproductive activity of this mullet.