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

Ming-Chung Tu, Chia-Wei Chu and Kuang-Yang Lue (1999) Specific gravity and mechanisms for its control in tadpoles of three anuran species from different water strata. Zoological Studies 38(1): 76-81. Buoyancy regulation is crucial for the survival of aquatic animals because it can save appreciable energy. Although anuran larvae are generally benthic, a few species live either at the surface or in mid-water. We have predicted that the specific gravity (sp. gr.) of tadpoles in surface layers should be 1.0 or slightly less, and that of mid-level tadpoles should be 1.0, whereas 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 tadpoles, 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. (x̄ = 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-layer or surface tadpoles was supported by our experiments. Lungs seem to play an important role in buoyancy regulation for anuran larvae. 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 lungs in buoyancy regulation in both M. heymonsi and M. ornata tadpoles. The larvae of M. heymonsi recovered from the deprivation of surface air treatment much better than did those of M. ornata. This phenomenon might be correlated with their different habitats.