Metamorphosis of summer flounder (Paralichthys dentatus): Thyroid status and the timing of gastric gland formation

Document Type


Date of Original Version



We investigated the timing of stomach formation in metamorphosing summer flounder (Paralichthys dentatus) and the role of thyroid status in gastric gland appearance and pepsinogen production. In the first experiment, untreated summer flounder from successive developmental stages were analyzed using histochemistry and immunocytochemistry. During prometamorphosis, the epithelial folds in the region of the incipient stomach increased. Early in metamorphosis, a few gastric glands appeared, but pepsinogen was not identified until mid-metamorphosis. The stomach of late-metamorphic summer flounder had well-developed gastric glands comparable to those in the juvenile. In the second and third experiments, premetamorphic summer flounder were maintained in seawater containing the vehicle DMSO (control) or seawater containing either 100 ppb L-thyroxine sodium salt (T4) or 30 ppm thiourea (TU, an inhibitor of T4 synthesis). Thyroxine induced metamorphosis within 1 week in the second experiment and within 3 days in the third experiment, whereas TU delayed or inhibited the transition into metamorphosis. The timing of gastric gland differentiation and pepsinogen production was accelerated by 2 weeks in T4-treated larvae and delayed or, in the case of pepsinogen production, inhibited in TU-treated larvae. Thus, T4 stimulates the formation of gastric glands and is necessary for the appearance of pepsinogen in metamorphosing summer flounder. In the third experiment, T4 induced gastric glands within 3 days and in larvae that had not entered metamorphosis, showing that differentiation of gastric glands could be dissociated from metamorphosis. This artificially induced heterochrony has potential application in aquaculture.

Publication Title, e.g., Journal

Journal of Experimental Zoology