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 intake or the chance to evade predators) or considerable energy is expended in swimming, zooplankton wearing protruding structures las predator defence) that significantly increase drag resistance must pay a cost for the better protection 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 has a typical Bosmina morphology with a low carapace and short antennule, whereas 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) percentage lower speed for E. c. gibbera than for E: longispina if they use the same amount of energy in swimming. 5. The relative difference in drag or swimming speed between the two species was highest at low Reynolds number (i.e, low speed or low temperature). These results show that hydrodynamic costs of extreme morphology may increase with decreasing temperature. 6. The increased cost of morphological antipredator defence at low temperatures may be enlightening with regard to the role of temperature in the induction of cyclomorphic traits in zooplankton. This may be one explanation for why extreme farms of E. c. gibbera and some Daphnia are only found in the summer when water temperature is high.