ONTOGENIC BUOYANCY CHANGES AND HYDROSTATIC CONTROL IN LARVAL ANURANS

Buoyancy levels are described for five species of anurans (Rana pipien s, Rana sylvatica, Pseudacris triseriata maculata, Hyla versicolor/chr ysoscelis, and Bufo americanus) by measuring forces of lift and weight during hatchling, larval, and early metamorphic phases, Hatchling sta ges were negatively buoyant, lungs were not inflated, and individuals were sessile. Lungs of B. americanus remained deflated up to early met amorphic stages, but those of the hylids and ranids were inflated earl y in the larval phase, Differences occurred among these latter species in pattern of buoyancy change during development. All but one species (R. pipiens) remained near neutral buoyancy throughout most larval st ages. Rana pipiens was negatively buoyant throughout, Buoyancy values varied with stage of development resulting from considerable changes i n lift and weight forces, Larval stages near neutral buoyancy were act ive in midwater whereas negatively buoyant stages were active or seden tary on the bottom. Metamorphic stages of all species were negatively buoyant and benthic, Larvae of R. sylvatica and Hyla sp., placed into a high density (1.008 g/ml) solution of water and Percoll, adjusted th eir buoyancy by a significant reduction of lung gas volume and a signi ficant increase in weight force, Individual R. pipiens larvae regulate d buoyancy within a narrow range over time. Variation in buoyancy was slight because lung gas volume declined slowly between air breaths. In dividual tadpoles initiated breaths at similar buoyancy levels, and bu oyancy values after an air breath were also similar. Tadpole lungs hav e a hydrostatic role in which buoyancy is regulated precisely and can be adjusted to compensate for environmental variation.