Rj. Denver et al., ADAPTIVE PLASTICITY IN AMPHIBIAN METAMORPHOSIS - RESPONSE OF SCAPHIOPUS-HAMMONDII TADPOLES TO HABITAT DESICCATION, Ecology, 79(6), 1998, pp. 1859-1872
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.