Effect of triiodothyronine on stomach formation and pigmentation in larval striped bass (Morone saxatilis)

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We previously reported that ambient T3 was taken up by larval striped bass (Morone saxatilis) in a dose-dependent manner, and, at 100 ppb T3-salt in 5 ppt seawater, ambient T3 retarded the growth of the fish. Here we examined the effects of T3 on the histogenesis of the stomach and on pigmentation of larval striped bass. Striped bass larvae were raised in 0, 1, 10, or 100 ppb T3-Na salt in 5 ppt seawater beginning at 16 days after hatching (dah) and sampled after 1 or 2 weeks of treatment. Between ages 23 dah and 30 dah, the muscular layer of the stomach thickened in all groups, and, at both times, the stomachs of fish treated with the highest dose of T3 had a significantly thicker muscular layer than stomachs of untreated fish. The length of the fundic stomach doubled between sampling times in the control fish; this developmental change was promoted by the lowest dose after 1 week exposure, but after 2 weeks this and the development of the gastric blind-sac were significantly inhibited in fish treated with the medium and high concentrations. The fundic region of the stomach of all fish examined at each sampling time (n = 48) contained gastric glands in which pepsinogen was immunolocalized; however, because of the smaller size of the fundic region and the absence of the gastric blind-sac, many fewer gastric glands were detected in fish treated with the two higher concentrations. Somatic pigmentation increased between 23 dah and 30 dah and was also stimulated by exogenous T3 in a dose-dependent manner within 1 week. After 2 weeks of treatment, only fish exposed to 100 ppb T3-Na salt were more pigmented than the control fish. Although development of pigmentation and the muscular wall of the stomach were accelerated by T3, effects of treatment on differentiation and possibly functioning of the fundic region were more complex and possibly detrimental.

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Journal of Experimental Zoology