THE EFFECTS OF REDUCED OXYGEN-CONTENT ON PREDATION AND SIPHON CROPPING BY THE BROWN SHRIMP, CRANGON-CRANGON

Aquarium experiments were performed to test for critical oxygen levels in relation to the predation efficiency of C. crangon. Short-term (60 h) experiments where done in normoxia (> 95%), 50, 40, 30, and 20% ox ygen saturation with the amphipid Bathyporeia pilosa as prey. A signif icantly reduced predation rate was detected at 30% oxygen. Sublethal e ffects of C. crangon on adult Macoma balthica (mainly by siphon croppi ng) were studied by measuring the condition of the clams (morphometric , somatic, and biochemical). To test for these effects experiments wer e conducted under normoxia and moderate hypoxia (40% oxygen). Conditio n and siphon indices revealed no change in condition after a 3-week ex posure to moderate hypoxia, while a significant reduction in the condi tion of adult clams was found in both normoxia and hypoxia when subjec ted to siphon cropping by C. crangon. No enhanced siphon cropping coul d be detected due to hypoxia. The results indicate that C. crangon is able to maintain its predatory role until a level of 30% oxygen in sho rt-term exposure. After a 20% oxygen level, however, the abiotic stres s alters the functional role of C. crangon as a predator, indicating a threshold of 20-30% oxygen for the normal functioning of the predator -prey food webs.