PRYOR, Gregory S. *; BJORNDAL, Karen A.; Univ.of Florida, Gainesville; Univ. of Florida, Gainesville: Symbiotic nematodes accelerate metamorphosis in bullfrog tadpoles

Herbivorous bullfrog larvae (Rana catesbeiana) exhibit a comparably complex gastrointestinal microbial community as other herbivores, harboring bacteria, ciliates, and nematodes within their hindguts. Tadpole-specific gastrointestinal nematodes, Gyrinicola batrachiensis, seem associated with an herbivorous host dietary regime because they are lost at metamorphosis and do not occur in adult, carnivorous bullfrogs. Curiously, the fermentative gut regions of many other ectothermic herbivores are also densely populated with nematodes, yet no pathology has been reported in these hosts. The hypothesis that such nematodes may be mutualistic symbionts, providing improved digestion or nutritional gains to their hosts, has not been tested. We examined the effects of nematode infection on bullfrog tadpole development, gut morphology, and hindgut fermentation. Nematode-infected R. catesbeiana tadpoles developed faster and metamorphosed sooner than uninfected tadpoles. However, at metamorphosis, infected and uninfected bullfrogs were not different in body size or condition. Infected tadpoles also had greater colon width, more colon contents, higher concentrations of fermentation byproducts, and greater in vitro fermentation rates. Energetic contributions of fermentation were twice as high in infected than uninfected tadpoles. Such accelerated development and improved energetic gains potentially improve the fitness of infected tadpoles. In these regards, G. batrachiensis can be considered a mutualistic symbiont inhabiting R. catesbeiana tadpoles. Such a fermentative mutualism between nematodes and an herbivorous host has not been previously described and contributes towards a better understanding of the diversity and function of symbionts in animal nutrition and digestion.