Thyroid Gland of Fishes

• In many teleosts, the thyroid gland is situated in the pharyngeal region in between the dorsal basibranchial cartilages and ventral sternohyoid muscle.
• The thyroid surrounds the anterior and middle parts of the first, second, and sometimes third afferent branchial arteries of the ventral aorta, as found in the Ophiocephalus species.
• It is thin-walled, sac-like, compact dark brownish, and enclosed in a thin-walled capsule of connective tissue in these fishes might be correlated with the air-breathing habit because the thyroid gland acts here as thermoregulatory to adapt the fish to a semiterrestrial environment of low thermal capacity.