OBLIGATE AND FACULTATIVE SUSPENSION FEEDING IN ANURAN LARVAE: FEEDING REGULATION IN XENOPUS AND RANA

¹ Center for Great Lakes Studies, University of Wisconsin—Milwaukee, Milwaukee, Wisconsin 53201

The obligate suspension-feeding Xenopus laevis larva can survive solely on particles over a wider concentration range, but the facultative suspension-feeding Rana pipiens complex larva has a greater behavioral plasticity in its feeding. Regulatory mechanisms for adjusting filtering and ingestion rates reflect the morphological differences between these tadpoles. Xenopus larvae regulated feeding primarily by altering buccal pumping rate and Rana by adjusting buccal volume. The feeding data for both species fitted a rectangular hyperbola, with a lower concentration "threshold," and were consistent with most predictions of "energy-optimization" models. The significance of a feeding "threshold" is discussed. Regulation by satiation may be a fundamental universal regulatory mechanism for feeding in phylogetically-diverse suspension feeders. Negative allometry for buccal volume vs snout-vent length was noted for Rana and Xenopus larvae. Over all size classes, buccal volumes of Rana were smaller than those for Xenopus, confirming conclusions from earlier morphological studies. Over evolutionary time, the energetic consequences of different rates of supply of particles in temperate (Rana) vs tropical (Xenopus) waters, among other factors, may have selected for the observed differences in these tadpoles' feeding abilities. A number of behavioral adjustments, such as migration, metamorphosis, threshold feeding, and prey switching, may serve to minimize overexploitation of the prey of vertebrate suspension feeders, while insuring adequate food to supply energetic needs.