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

A laboratory experiment and natural-diet study were conducted to exami ne the feeding ecology and importance of food type for growth of Gamma rus fasciatus from the Ohio River. To determine effects on growth, amp hipods were fed four diets: (1) filamentous algae (Cladophora) and dia toms, (2) dead animal matter (chironomids), (3) coarse particulate org anic matter (CPOM), and (4) fine particulate organic matter (FPOM). Or ganic 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 popul ations showed that G. fasciatus consumed all food types used in the la boratory study. The weighted proportions of filamentous algae and anim al 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 lar ge rivers by enabling them to exploit seasonal changes in the relative abundance of different food sources.