Metamorphosis in the summer flounder, Paralichthys dentatus: Developmental and osmoregulatory responses to altered thyroid status
These studies using summer flounder (Paralichthys dentatus) were carried out to determine the effects of altered thyroid status on larval development, growth, and osmoregulatory ontogeny.^ Whole-animal thyroxine (T4) concentrations from summer flounder larvae increased during metamorphic climax (MC). Thyroid hormone (TH) status was elevated by treatment with exogenous T4, or decreased with thiourea (TU). TU-treatment induced a developmental stasis at the start of MC. T4-treatment accelerated the start of MC, but did not affect the timing of development to the juvenile stage. Thyroid status did not affect growth. However, starting at an age when most untreated and T4-treated larvae had developed to the end of metamorphosis, they began to grow longer than TU-treated fish in developmental stasis. Thus, T4 is necessary and sufficient for metamorphosis, but not growth.^ Tolerance to low and high salinity was high prior to metamorphosis, lowest during metamorphosis, and high at the end of metamorphosis. T4-treatment reduced tolerance to low salinity during metamorphosis, but increased tolerance to high salinity. These findings suggest that THs are more important for the development of tolerance to high salinity than low salinity.^ Whole-animal water content was highest in premetamorphic larvae, decreased to the lowest levels at the start of MC, became elevated during mid- and late MC, and then returned to the lowest levels at the end of metamorphosis. TU-treatment resulted in abnormally low whole-animal water content by the end of the experiments. These and other findings suggest that TH are necessary for the regulation of whole-animal water content.^ Metamorphosis was accompanied by gill chloride cell development, which was influenced by THs. Chloride cell development from the start of MC to the juvenile stage was characterized by increased Na$\sp+$,K$\sp+$-ATPase immunoreactivity, reactivity of intracellular membranes to an osmium-based cytological stain, and greater development of the mitochondria. TU-treatment induced a developmental stasis in the start of MC characterized by less developed chloride cells. The concurrent addition of T4 with TU rescued fish from TU-induced developmental stasis, and can accelerate chloride cell intracellular membrane development. These findings suggest that during metamorphosis TH, or TH-mediated developmental progression, are necessary for chloride cell development. ^
Biology, Animal Physiology|Biology, Zoology
Alexander M Schreiber,
"Metamorphosis in the summer flounder, Paralichthys dentatus: Developmental and osmoregulatory responses to altered thyroid status"
Dissertations and Master's Theses (Campus Access).