INFLUENCE OF FOOD TYPE ON THE GROWTH OF A RIVERINE AMPHIPOD, GAMMARUS-FASCIATUS

A laboratory experiment and natural-diet study were conducted to examine the feeding ecology and importance of food type for growth of Gammarus fasciatus from the Ohio River. To determine effects on growth, amphipods were fed four diets: (1) filamentous algae (Cladophora) and diatoms, (2) dead animal matter (chironomids), (3) coarse particulate organic matter (CPOM), and (4) fine particulate organic matter (FPOM). Organic matter was obtained by shredding preconditioned sycamore leaves (Platanus occidentalis). Growth was evaluated from weekly measurements of head length. There were no significant differences in growth among food types before week 3 of the experiment, but from week 4 to week 6, amphipods fed algae and dead animal matter were significantly larger than those maintained on CPOM and FPOM. Gut analysis of natural populations showed that G. fasciatus consumed all food types used in the laboratory study. The weighted proportions of filamentous algae and animal matter in riverine populations increased consistently from smallest to largest size-class. The availability of, and ability to use, algae and animal matter by G. fasciatus benefits natural populations in large rivers by enabling them to exploit seasonal changes in the relative abundance of different food sources.