Does trophic position of the omnivorous signal crayfish (Pacifastacus leniusculus) in a stream food web vary with life history stage or density?

Abstract

Food mixing behavior of omnivorous consumers can be difficult to predict. We undertook an enclosure experiment to explore the effects of ontogenetic stage and density on food choices of the omnivorous signal crayfish, Pacifastacus leniusculus. Juvenile or adult crayfish were placed in in-stream enclosures for 6 weeks at three different densities. Gut and stable isotope analyses were used to determine the food sources utilized by these organisms. In addition, we analyzed the guts of both adult and juvenile crayfish from outside the enclosures to account for experimental effects. We found few differences in the gut contents between juvenile and adult crayfish from either inside or outside the enclosures, as the majority of food consumed by both ontogenetic stages and all densities was allochthonous detritus. Stable isotope results indicate that crayfish of both ontogenetic stages were relying primarily on detrital biofilms for nutrition, despite an additional laboratory experiment showing that growth would be far more rapid on invertebrates. The dietary choices made by P. leniusculus in its native environment seem to be primarily affected by factors other than the nutritional value of food sources, contrary to expectations that food mixing behavior of omnivores should be based on this factor alone.