Substrate interstices influence the microdistribution and survival of benthic invertebrates. The benefits of interstitial refugium availability were quantified over 4 wk under baseflow conditions that are common to lowland streams in northwestern Europe. Effects on growth, feeding, and behavior of the amphipod Gammarus pulex L. were studied in 8 indoor artificial stream channels. Combinations of 2 near-bed flow velocities (3 and 9 cm/s) and 2 substrate types (medium sand and coarse gravel) were assigned to 32 experimental compartments. Growth of G. pulex was greater at 9 cm/s than at 3 cm/s, and G. pulex avoided bare sand and used gravel patches at both flow velocities. Food consumption was lower and individual growth was greater in gravel than in sand, a result that suggests that energetic costs were lower in gravel than in sand. We suggest that the interstitial refugia in gravel may have benefits beyond providing protection from flow. The presence of interstitial refugia may be of key importance to G. pulex, and the underlying mechanism of interstitial refugium use might be associated with the adaptation of organisms to seek refuge from predators.
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