Specific gravity and mechanisms for its control in tadpoles of three anuran species from different water strata

Buoyancy regulation is crucial for the survival of aquatic animals because it can save appreciable energy. Although anuran larvae are generally benthi c, a few species live either at the surface or in mid-water. We have predic ted that the specific gravity (sp. gr.) of tadpoles in surface layers shoul d be 1.0 or slightly less, and that of midlevel tadpoles should be 1.0, whe reas the sp. gr. of benthic tadpoles should be greater than 1.0. We tested this prediction in tests using larvae of Microhyla heymonsi, M. ornata, and Rana kuhlii as representatives of surface, mid-waters, and benthic tadpole s, respectively. Mean larval sp. grs. were 0.993 for M. heymonsi, 0.998 for M. ornata, and 1.041 for R. kuhlii Although the mean sp. gr. ((x) over bar = 0.998) of mid-level tadpoles partially contradicts our prediction (1.0), larval sp. grs. of the other 2 species did fit our prediction. The general trend for benthic tadpoles to have a greater sp. gr. than those of mid-lay er or surface tadpoles was supported by our experiments. Lungs seem to play an important role in buoyancy regulation for anuran larv ae. To check this prediction, we measured and compared sp. grs. of the same individuals with and without lungs. Also, observations were made on larvae of surface and mid-water species after being forcibly kept from contacting surface air. Results of these experiments confirmed the importance of lung s in buoyancy regulation in both M. heymonsi and M. ornata tadpoles. The la rvae of M, heymonsi recovered from the deprivation of surface air treatment much better than did those of M. ornata. This phenomenon might be correlat ed with their different habitats.