ADAPTIVE PLASTICITY IN AMPHIBIAN METAMORPHOSIS - RESPONSE OF SCAPHIOPUS-HAMMONDII TADPOLES TO HABITAT DESICCATION

Amphibians exhibit extreme plasticity in the timing of metamorphosis, and several species have been shown to respond to water availability, accelerating metamorphosis when their ponds dry. In this study we anal yzed the plasticity of the developmental response to water volume redu ction in the western spadefoot toad, Scaphiopus hammondii. Also, we at tempted to identify the environmental cue(s) that may signal a desicca ting larval habitat. We spawned adults in the laboratory and raised ta dpoles in aquaria in a controlled environmental chamber. Water levels of aquaria were gradually reduced by removing water at the rate of 0.5 -1 L/d; water in control aquaria was similarly disturbed but not remov ed. Tadpoles subjected to water volume reduction showed significant ac celeration of metamorphosis. The developmental acceleration depended o n the rate of reduction of the water level; i.e., tadpoles exhibited a continuum of response. This developmental response did not result fro m thermal differences between treatments. Furthermore, the response wa s reversible in that refilling of the aquaria to the starting water le vel at various times following the onset of volume reduction resulted in restoration of body mass and a tendency to decelerate metamorphosis . Several lines of evidence suggest that the developmental response is due neither to the concentration of compounds in the water nor to che mical or physical interactions among conspecifics. Rather, the respons e appears to be related to the reduced swimming volume and perhaps the proximity to the water surface. When the water level is reduced, tadp oles reduce foraging, and food restriction of prometamorphic tadpoles maintained in a constant high water environment accelerated metamorpho sis. Spadefoot toad tadpoles are a valuable model system for explainin g both the proximate mechanisms (environmental cues and physiological responses) and the ultimate causes for adaptive phenotypic plasticity in amphibian metamorphosis.