Alterations in the grazing activities of cased caddisfly larvae in response to variations in predation risk and resource level

Flexible behavioral response to avoid encountering predators has never been reported in lotic, cased caddisfly larvae with low mobility. However, an earlier laboratory experiment found the growth of such a caddisfly species, Glossosoma sp., decreased in the presence of a predatory sculpin, Cottus nozawae. We conducted laboratory experiments to test whether Glossosoma responses to sculpin varied according to resource level and/or time of day. Lower periphyton biomass resulted in increased movement at any time of day, but was followed by decreased grazing success of Glossosoma in the morning when they were sampled. Although sculpins were active in the morning, evening and night but never in the afternoon, movements of Glossosoma were suppressed by sculpin stimuli only in the morning, when food intake of Glossosoma also decreased, regardless of periphyton biomass. Glossosoma reduced the risk of predation by sculpin by flexibly controlling its grazing activities during the most risky period of the day (i.e. morning). Even in the morning in the presence of sculpin, lower resource levels resulted in longer movement distances similar to those in predator-free conditions, suggesting that lower resource availability promotes predation risk in natural streams inhabited by predatory sculpin.