OTHER ENVIRONMENTAL-FACTORS AFFECT THE SIZE OF CRESTS DEVELOPED BY DAPHNIA-CARINATA (CLADOCERA, DAPHNIIDAE) IN RESPONSE TO NOTONECTID PREDATORS

1. Crest size and body size was measured in Daphnia carinata King sampled from field enclosures in which environmental conditions and notonectid predator (Anisops hyperion Kirkaldy) levels had been manipulated, and from control treatments which had predator levels and other environmental factors equivalent to those in the natural environment. 2. Individuals of D. carinata developed larger crests in treatments with predators than in treatments without predators. 3. The environmentally manipulated treatment was considered to have lower levels of food but was also likely to have undergone other changes in physicochemistry and resource variability that may have affected crest size. The effect of the manipulation cannot be confirmed, therefore, as the result of variation in food levels. Animals in the treatments considered to have lower amounts of food had smaller crests than in the control treatment irrespective of whether predators were present. The effects of environmental manipulation and predators were additive. 4. Enclosures without predators, but in which water from the natural, predator-rich, environment was exchanged through the enclosure walls, showed no evidence of a chemical induction effect. Chemical induction effects may have been countered by a reduced food supply resulting from the higher D. carinata population densities that developed in these enclosures. 5. This work provides a second example of an alteration of the morphological response of daphnid prey to predators by environmental factors, probably food, and suggests this phenomenon may be general in cladoceran species displaying predator-induced changes in morphology.