Seasonal thyroid activity in the bonnethead shark, Sphyrna tiburo

Abstract

Despite its importance to the physiology of vertebrates, thyroid function has not been characterized in elasmobranchs. The bonnethead shark, Sphyrna tiburo, has been the focus of recent physiological studies, including reproduction, feeding and migration. The reproductive steroids of this shark have been correlated to the reported cycles of gonadal maturation, gestation and mating. Bonnetheads collected near Tampa Bay have a high incidence of infertility and developmental problems, which may be caused by organochlorine contamination. This study characterizes thyroid function in two populations of the bonnethead from south Florida, correlates it to the reported seasonal physiological cycles and investigates the possibility of thyroid disruption. Sharks were collected from Anclote Key near Tampa Bay, and within Florida Bay near Long Key. Thyroid glands and serum were sampled once a month throughout the reproductive cycle. The thyroid gland of this species was found within the mandibular muscles, near the ventral aorta. Histology of the gland showed that it was encapsulated and flat, with follicles arranged in a sheet that was a few follicles thick. The individual follicles were similar to those observed in other vertebrates. Seasonal histology indicated that thyroid follicles had significantly taller epithelial cells and larger lumen areas in the spring and fall, suggesting greater activity (p=0.05). There were few abnormal follicles found in the thyroids sampled from these two populations, and no incidence of goiter. Radioimmunoassay (RIA) was used to measure circulating thyroid hormones. The RIA was validated for bonnethead serum and showed that the sharks sampled for this study had very low circulating levels of triiodothyronine (T₃; approximately 0 to 2 ng/ml) and higher levels of L-thyroxine (T₄; approximately 2 to 15 ng/ml). Seasonal serum samples indicated a bimodal cycle of T₄ with circulating levels significantly elevated during the spring and fall. Mature females showed an additional significant elevation in T₄ during the summer with peak levels during implantation. The results of this study indicate that the thyroid system of these bonnethead populations is dynamically regulated on a seasonal basis, perhaps in conjunction with reproduction, feeding and migration, and that the thyroid was not disrupted.