THE ROLE OF LARVAL DIET IN ANURAN METAMORPHOSIS

Focus on tadpole diet and foraging behavior offers potential for integ rating ecological and endocrinological approaches to understanding anu ran metamorphosis, Natural larval diets vary widely in relative amount s of protein, carbohydrate and lipid, factors known to influence thyro id hormone function, which in turn is essential for metamorphosis to o ccur, Previous work has shown that tadpoles fed high protein diets gro w and develop quickly, This pattern is consistent with findings from o ther classes of vertebrates that some aspects of thyroid function, e.g ., activity of 5'D monodeiodinase, an enzyme that convects thyroxine ( T-4) to the more potent metamorphosis-inducer 3,5,3'-triiodothyronine (T-3), are proportional to availability of dietary protein, In field e xperiments at a northern California river, I found that nutritional va riation among algal taxa routinely consumed by tadpoles (chlorophytes, diatoms, cyanobacteria, etc.) influenced metamorphosis several ways, Growth rate was positively correlated with percent protein content of a food type, Tadpoles fed filamentous green algae with epiphytic diato ms developed more quickly and metamorphosed at larger sizes than tadpo les raised on other diets, The dominant diatoms in this system are hig h in protein because they host nitrogen fixing cyanobacterial endosymb ionts. Resource quality also mediated the effects of competition and p redation, Changes in abundance of high quality algae caused by invadin g bullfrog (R. catesbeiana) tadpoles explained most of their competiti ve effects on metamorphosis of native tadpoles (Hyla regilla and (Rana boylii). In choice experiments, tadpoles foraged selectively on the a lgal foods that promoted most rapid growth and development, In the pre sence of garter snake predators, however, tadpoles avoided such patche s thus decreasing growth. Direct examination of thyroid hormone produc tion in tadpoles consuming different diets may reveal a proximate mech anism linking diet quality to size at and time to metamorphosis.