Determinants of substrate selection in lotic mayfly larvae: is cryptic coloration important?

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Crypsis, or resemblance with the background, is known to be important for many aquatic organisms, but its role in lotic predator-prey systems is poorly known. In a combination of laboratory experiments and field observations, we studied whether two species of lotic mayfly larvae (Baetis rhodani, Heptagenia dalecarlica) show cryptic behavior (i.e., prefer dark substrates), and how this is affected by predation risk and food availability. Both species were able to distinguish between differently colored substrate patches, as well as between patches of variable food reward. A majority of larvae (56-78 %) chose dark substrates over light-colored ones, both in the laboratory and in natural streams. This behavior, however, was differently modified in the two species by variation in predation risk and food reward. For Baetis larvae, predation risk and food reward had no effect on the preference for dark substrate patches. Similarly, the proportion of larvae on dark substrates did not differ between streams with and without fish. In fishless controls, Heptagenia used dark patches with low food reward less than predicted, whereas in the presence of fish, light-colored patches were under-selected, especially those with high food reward. We suggest that differences in patterns of substrate choice between Baetis and Heptagenia are related to their different life-styles, although other antipredator behaviors are probably more important in most field situations.