HORMONAL CORRELATES OF ENVIRONMENTALLY-INDUCED METAMORPHOSIS IN THE WESTERN SPADEFOOT TOAD, SCAPHIOPUS-HAMMONDII

Tadpoles of several amphibian species have been shown to accelerate me tamorphosis when their ponds dry. To understand the proximate mechanis ms that mediate the developmental response to pond drying, I analyzed changes in endocrine activity in tadpoles of the Western spadefoot toa d (Scaphiopus hammondii) exposed to experimental water volume reductio n in the laboratory. Tadpoles exposed to a declining water level accel erated metamorphosis compared with tadpoles raised in a constant high water environment. The acceleration of development was associated with the precocious elevation of whole-body contents of the hormones that control metamorphosis, the thyroid hormones thyroxine (T-4) and triido thyronine (T-3), and the interrenal steroid corticosterone (CORT). The precocious activation of the thyroid system preceded external morphol ogical change (i.e., increase in hind limb length, developmental stage ) by 3 days. To test if tadpoles are capable of responding rapidly to water volume reduction, mid-prometamorphic tadpoles (Gosner Stage 37-3 8) were raised in a constant high water environment (10 L) and then tr ansferred to either 1 or PO L. Tadpoles transferred to 1 L, exhibited significant metamorphic changes by 48 h after transfer. In addition, d ramatic elevations in whole-body T-4, T-3, and CORT contents were evid ent at this time point. Thus, the metamorphic response to pond drying is likely driven by the activation of the thyroid and interrenal axes, the hormones of which control metamorphosis. Furthermore, this respon se is rapid, occurring within 48 h after exposure to the desiccating e nvironment. (C) 1998 Academic Press.