Influence of temperature on hydrodynamic costs of morphological defences in zooplankton: experiments on models of Eubosmina (Cladocera)

1. If swimming speed is correlated to fitness (e.g. by affecting food intak e or the chance to evade predators) or considerable energy is expended in s wimming, zooplankton wearing protruding structures las predator defence) th at significantly increase drag resistance must pay a cost for the better pr otection against predators that these traits imply. 2, In an experiment with plastic models, the drag and energy consumption of swimming in two species of Eubosmina were examined. Eubosmina longispina h as a typical Bosmina morphology with a low carapace and short antennule, wh ereas E. coregoni gibbera has a very high carapax and long antennule. 3, At 5 degrees C, E, c. gibbera had 32-45% higher drag than E. longispina. At 20 degrees C, the difference is 20-45%. 4. A mathematical model of swimming predicts that these differences in drag should result in 18-20 (at 5 degrees C) or 14-16 (at 20 degrees C) percent age lower speed for E. c. gibbera than for E: longispina if they use the sa me amount of energy in swimming. 5. The relative difference in drag or swimming speed between the two specie s was highest at low Reynolds number (i.e, low speed or low temperature). T hese results show that hydrodynamic costs of extreme morphology may increas e with decreasing temperature. 6. The increased cost of morphological antipredator defence at low temperat ures may be enlightening with regard to the role of temperature in the indu ction of cyclomorphic traits in zooplankton. This may be one explanation fo r why extreme farms of E. c. gibbera and some Daphnia are only found in the summer when water temperature is high.