95 Nutritional Ecology of Arthropod Predator-Prey Interactions

Abstract

Abstract Arthropods can provide a valuable source of protein for many vertebrate and invertebrate consumers. Yet, not all protein in an arthropod may be equally digestible to consumers. Arthropods are defined by their exoskeleton, which is a mixture of chitin and protein that is largely indigestible to most consumers. I will discuss how exoskeleton complicates the measure of arthropod protein content for insectivores. Arthropod taxa vary widely in exoskeleton content from about 10 to 40 % of their dry mass. Exoskeleton also varies in amino acid content from about 6 to 70 % of its dry mass. When spiders feed on arthropods, they extract nearly all of the soft tissue and leave behind the exoskeleton as prey remains. This complicates the measurement of digestible protein content of arthropod prey as one of the most common methods, crude protein, does not distinguish between exoskeleton and soft tissue. Instead, our results show that some spectrophotometric methods are more accurate measures of digestible protein in arthropods, especially if solvents like NaOH are used to prepare samples. Finally, proteins are an important source of nitrogen for insectivores. Using feeding experiments, we showed that measures of digestible (i.e., non-exoskeleton) nitrogen content of arthropods are a more accurate measure of what a spider consumes than is the whole nitrogen content of an arthropod body. Hence, while exoskeleton complicates the measure of arthropod protein content, there are ways to correct for exoskeleton to get more accurate measures of the quality of arthropods as prey for insectivores.