Vulnerability of Daphnia middendorffiana to Parabroteas sarsi predation: the role of the tail spine

The predaceous calanoid copepod Parabroteas sarsi and Daphnia middendorffiana co-exist in South Andes ponds. Daphnia middendorffiana juveniles have a tail spine with negative allometric development. A series ofexperiments was carried out with tailed and tail-removed juveniles of three different instars. In all cases tested, feeding rates were significantly higher on tail-removed prey. Direct observations showed a higher frequency of unsuccessful attacks on spined juveniles when compared with tail-removed juveniles. The proportion of dorsal attacks also increased in spined juveniles. Prey total length was a much better predictor of feeding rate than prey body size. Three groups of juveniles with equal total length, but differentage, body size and biomass, showed no significant differences in their vulnerability to predation.