Diel and seasonal changes in gut contents of omnivorous-carnivorous macroinvertebrates in the Yura River, Japan

Diel and seasonal changes in the gut contents of omnivorous-carnivorous macroinvertebrates were examined in a Japanese mountain stream based on direct observation of gut contents. Carnivorous-omnivorous (predator) macroinvertebrates were sampled at four time periods (02:00-03:00, 08:00-09:00, 14:00-15:00, and 20:00-21:00) on 3 days in each of March, June, September, and December. A total of 1391 individual macroinvertebrates belonging to 71 taxa were dissected: identifiable gut contents were observed in 928 individuals and prey identified to the family level were observed from the guts of 564 individuals. Using nonmetric multidimensional scaling, interspecific variation in foraging patterns was summarized into two axes that approximately correspond to body size and habitat preferences of prey taxa. Although seasonal patterns in the number of prey individuals in a gut varied among taxa, in March and December, several predators notably foraged on taxa such as Simuliidae, which inhabits stone surfaces in river sections with a rapid current. This pattern may be explained by season-specific prey availability and predation risks from vertebrates. Diel changing patterns in the number of prey individuals per gut varied among predator taxa, and most predators did not show obvious proportional changes in gut contents. These results indicate that subcompartments of interaction networks and numerous indirect effects via intraguild predation potentially exist within the "guild " of macroinvertebrates, which are often summarized as mesopredators of stream food webs.