Metamorphosis of Summer Flounder (Paralichthys dentatus): Thyroid Status and the Timing of Gastric Gland Formation

We investigated the timing of stomach formation in metamorphosing summer floun- der (Paralichthys dentatus) and the role of thyroid status in gastric gland appearance and pepsin- ogen 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). Thy- roxine 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 differ- entiation of gastric glands could be dissociated from metamorphosis. This artificially induced heterochrony has potential application in aquaculture. J. Exp. Zool. 280:413n420, 1998. © 1998 Wiley-Liss, Inc. Differentiation of the gastrointestinal tract of summer flounder (Paralichthys dentatus) during metamorphosis is of interest for both basic and practical reasons. The digestive system of fishes transitioning between habitats and diets undergoes dynamic remodeling (Dabrowski, '86; Govoni et al., '86). Chemicals signals of the endocrine system co- ordinate much of the transformation both of the organism and of the gastrointestinal tract, and it has been proposed that thyroid hormones in par- ticular prepare the digestive system for shifts in habitat (Specker, '88). This topic has received re- cent review for vertebrates generally (Collie, '95) and for the period of early development in mam- malian neonates (Koldovsky, '94) and metamor- phosing amphibians (Shi and Ishizuya-Oka, '96). Summer flounder, like most marine fishes (Tanaka, '73), are altricial at hatch, and their gas- trointestinal tract is fairly undifferentiated (Bisbal and Bengtson, '95). In the growing practice of cul- turing flatfish, this simple gut has necessitated feeding larval flounder live organisms such as ro- tifers and brine shrimp. Not until metamorphosis— the period of transformation from the larvae to the juvenile (see Youson, '88)—can they be fed cheaper formulated diets and, even during metamorphosis, survival on formulated diets is lower than survival on live food (Bengtson et al., '98). As various flat-