Ontogenetic and Temporal Shifts in the Diet of the Amphipod Gammarus fasciatus, in the Ohio River

Abstract

-A field study was conducted to determine if ontogenetic or temporal shifts occur in the diet of the amphipod Gammarus fasciatus Say in the Ohio River. Amphipods were collected monthly from cobble and snag habitats in the Ohio River for I yr. Gut contents of collected amphipods were analyzed microscopically for the presence of detritus, filamentous algae, diatoms and animal matter. Gammarusfasciatus consumed each food type in different amounts, depending on amphipod size and month collected. Detritus was the most common food item found in amphipod guts (100% of microscope fields in guts examined), followed in order by filamentous algae, diatoms and animal matter (the ranges of each food type in the guts of G. fasciatus were: 0.036-0.287, 0.061-0.281, 0.002-0.072, respectively). Food use shifted ontogenetically: small G. fasciatus were limited to a diet consisting mainly of detritus, whereas larger animals consumed significant amounts of filamentous algae, diatoms and animal matter. There were also monthly differences in foraging, presumably due to differences in seasonal abundances of food types and other environmental factors. We suggest that the abundance of, and ability to use filamentous algae, diatoms and animal matter allow populations of G. fasciatus to persist and maintain significant year-round populations in the Ohio River.