Is invertebrate shredding critical for collector invertebrates? A test of the shredder–collector facilitation hypothesis

Abstract

A laboratory experiment was conducted to test the hypothesis that leaf processing by Japanese crayfish (Cambaroides japonicus de Haan) enhances the growth and reduces the mortality of collector amphipods (Sternomoera yezoensis Ueno) through fine particulate organic matter (FPOM) production. Two treatments (crayfish shredding, no shredding control) were established using flow‐through artificial streams and permitting differential FPOM production rates. An amphipod enclosure, associated with juvenile amphipods and preconditioned leaf packs, was placed at the downstream end of each channel. During a 4‐week period, shredding by crayfish greatly increased the amount of drifting FPOM compared to that in the control channels, in which only shredder‐independent FPOM occurred. Despite the substantial contribution of crayfish to FPOM production, amphipods in both treatments grew during the 4‐week period and neither amphipod survival nor growth differed between the two treatments. Furthermore, the residual leaf mass in the amphipod enclosures did not differ between the two treatments. However, residual leaf mass in amphipod enclosures was about 20% less than that expected from the processing rates of the leaf packs in the absence of invertebrate shredding. It is likely that amphipods were directly utilizing leaf packs as their food source regardless of FPOM abundance. These results indicate that juvenile amphipods, despite their classification as collectors, were plastic in their foraging habits, and that the FPOM produced by crayfish was not critical for amphipod growth or survivorship.