REMOTE DETECTION OF ALGAE BY COPEPODS - RESPONSES TO ALGAL SIZE, ODORS AND MOTILITY

Selective feeding on large algae by copepods involves remote detection of individual particles and subsequent active capture responses. In this study we use radiotracer experiments to quantify the clearance rates of five coexisting freshwater copepods and to investigate the relative merits of the chemoreception and mechanoreception hypotheses of remote detection. Tropocyclops and three diaptomid copepods exhibited relatively high clearance rates when feeding on low concentrations of large algae, suggesting that most previous studies with freshwater copepods have underestimated maximal clearance rates and the degree of size selectivity. All five species of copepods exhibited strong selection for an intermediate-sized flagellate (25-mu-m Carteria) or a large-sized nonmotile alga (80-mu-m Pediastrum) over a small-sized flagellate (6-mu-m Chlamydomonas). The weight-specific clearance rate for Tropocylops prasinus feeding on motile Carteria (271 ml mg-1 h-1) was about twice that of three diaptomid copepods and more than an order-of-magnitude higher than the estimate for Epischura lacustris feeding on its preferred alga, Pediastrum. Assuming that distance chemoreception ('smell') is important in remote detection, we predicted that the addition of high concentrations of 'algal odors' would obscure any chemical gradients emanating from individual algal cells and would thereby hinder the remote detection and active capture of large algae. Contrary to this hypothesis, the addition of amino acids, sucrose, and algal extracts had no effect on the clearance rates and selectivity of Diaptomus birgei. These results, together with recent cinematographic studies (Vanderploeg et al., 1990), suggest that mechanoreception is the primary mechanism for the remote detection of large particles by diaptomid copepods. A raptorial cyclopoid, Tropocyclops prasinus, exhibited strong preferences for motile algae, whereas a suspension-feeding calanoid, D. birgei, did not select between motile and nonmotile cells. Motility appears to be an important factor in algal detection for small cyclopoid copepods but not for suspension-feeding diaptomids.