Functional response of suspension feeding anuran larvae to different particle sizes at low concentrations (Amphibia)

Abstract

The influence of particle size, initial particle concentration and larval stage on the ingestion rate, ‘retention efficiency’, and filtering rate of anuran larvae with varying filter apparatus anatomy and different life histories was investigated for four species. Larvae of premetamorphic Stages 28 and 32 and prometamorphic Stage 40 were selected for filtering experiments on the basis of their different growth rates. Three different sizes of silica gel particles were offered as mock food. Particle concentration was measured photometrically. The Michaelis-Menten model was used to describe the dependency of ingestion rate, filtering rate, and ‘retention efficiency’ upon initial particle concentration, and to calculate maximum ingestion rate, threshold concentration, and the half-saturation constant. (1) The highest ingestion rates, filtering rates and ‘retention efficiencies’ were achieved by Xenopus laevis larvae, followed by Bufo calamita larvae. Bufo bufo larvae lay at the opposite end of the scale. Rana temporaria larvae were placed between B. calamita and B. bufo larvae. This order is attributed to differences in life histories, especially the different breeding environments in which these larvae occur. (2) The larger the particle size and the older the stage, the greater the tendency toward saturation of the ingestion rate, filtering rate and ‘retention efficiency’. These filtration parameters are graded according to particle size. The ingestion rate (number of particles), filtration rate and ‘retention efficiency’ are greatest for PS3. Ingestion volume is greatest for PS 1. The difference between PS3 and PS2 on the one hand, and PS1 on the other, is often great; for Stage 28 X. laevis it is very great. This shows that larvae ingest large particles more effectively, and that the most effective ingestion takes place at Stages 28 and 32, owing to the growth function of these stages. The ability of larvae to ingest large particles effectively is possibly a very basic phylogenetic characteristic. (3) The threshold concentration is lowest when the particles are at their largest. In accordance with conclusions drawn by other authors, threshold feeding is attributed to regulation by buccal pumping and mucus production. Considerable importance is attributed to threshold feeding with respect to larval adaptation to oligotrophic environments.