The adaptive significance of the tadpole stage with comments on the maintenance of complex life cycles in anurans

Abstract

Istock (1967) argued on theoretical grounds that "complex life cycles are inherently unstable over evolutionary time," yet the vast majority of frogs have distinct adult and larval forms. The maintenance of complex life cycles in anurans seems related to the unusual morphology and feeding ecology of the larval stage.An argument is presented that tadpoles are highly specialized suspension feeders adapted for utilizing rapid rises in primary production as a food source. Such resources are likely coupled to environmental fluctuations and available for only a limited amount of time during any year. In being adapted to feed on a temporary resource, tadpoles are obliged to change their feeding habits after a certain length of time. From a general consideration of matters such as predator susceptibility, feeding behavior, and growth patterns for suspension feeders, a case is made for why neotenic tadpoles have never been found and are unlikely to occur.The argument presented here links complex life cycles with short term environmental perturbation. Frog species with direct development tend to occur in relatively aseasonal environments, supporting this conclusion. The oldest known fossil frog is found in a seasonal rock formation. There has been a gradual trend toward increased seasonality since the early Mesozoic, apparently due to continental drift. This phenomenon may have some indirect and long-term bearing on the general maintenance of complex life cycles throughevolutionary time.Understanding larval ecology is fundamental to understanding the process of metamorphosis and the maintenance of complex life cycles. © 1975 American Society of Zoologists.