We observed the thyroid glands of false killer whale (
Pseudoorca crassidens), short-finned pilot whale (
Globicephala macrorhynchus), Risso’s dolphin (
Grampus griseus), bottlenose dolphin (
Tursiops truncatus), Pacific white-sided dolphin (
Lagenorhynchus obliquidens) , finless porpoise (
Neophocaena phocaenoides), California sea lion (
Zalophus californianus), northern fur seal (
Callorhinus ursinus) and sea otter (
Enhydra lutris), that died in aquarium or were captured by fishermen. The thyroids of all cetaceans (
Pseudoorca,
Globicephala,
Grampus,
Tursiops,
Lagenorhynchus and
Neophocaena) were dark brown mass and situated on the rostral part of the trachea The thyroids of
Pseudoorca,
Globicephala,
Grampus and
Tursiops had numerous indentations on their surface, while those of
Lagenorhynchus and
Neophocaena were relatively smooth. The thyroid glands of
Zalophus and
Callorhinus consisted of two smooth lobes on the both side of the trachea without isthmus. In the sea otter, the thyroid gland consisted of two smooth lobes and isthmus that was composed of only connective tissues without thyroid parenchyma. The greater species tend to have the higher ratio of the thyroid weight to the body weight. There was no distinct differences among the nine species regarding the height of the follicular cell. We also examined the parathyroid glands of Risso’s dolphin and bottlenose dolphin macroscopically and microscopically. They had two or four parathyroid glands on the dorsal surface of the thyroids. Considering their greater body size, each parathyroid gland was rather small. This result and electron microscopic findings of the parathyroid gland of bottlenose dolphin suggest a possibility that the activity of the parathyroid gland is suppressed to adapt to a sea habitat.
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