Energetics of food selection by the herbivorous parrotfish Sparisoma radians: roles of assimilation efficiency, gut evacuation rate, and algal secondary metabolites

Abstract

Experiments were conducted to investigate the effect of plant energy content and the presence of secondary metabolites on the assirmlation efficiency. ahmentary tract evacuation rate, and feeding rate of the bucktooth parrotfish Sparisoma radians. Alimentary tract evacuation rate was significantly greater (ca 2 . 5 ~ ) when the fish fed on the relatively energy-poor calcareous green alga Halimeda incrassata than on the seagrass Thalassia testudinum. The greater evacuation rate of H. incrassata provided the fish the capacity to increase consumption rate to compensate for low food energy content. However, due to unpalatable secondary chemical metabolites present in H. incrassata. actual biomass consumed per unit time was less than for T testudinum. Experiments using T testudinum blades coated with H. incrassata organic crude extract showed that the secondary metabolites had no effect on the fish's assimilation efficiency or gut evacuation rate. The increased rate of H. incrassata evacuation is more than sufficient to allow compensation for lower gross energy content. S. radians can increase evacuation rate sufficiently to allow compensation for differences in available energy over the normal range for most plants consumed in nature. Studies of herbivorous fish foraging and feeding energetics must consider differences in alimentary tract evacuat~on rate and feeding rate, and the effects of secondary metabolites, in addition to the assimilation efficiency, structural defense and relative plant abundance factors usually considered.