Feeding behaviour and limb morphology of two cladocerans with small intersetular distances

Abstract

Length-weight relationships, feeding rates, and surface area and morphology of the third and fourth thoracic limbs of Daphnia carinata and Ceriodaphnia quadrangula s.l. were examined. Weight (W) increased between the square and cube of body length (L). W= 7.88 × 10^-7 L^2.23, W= 1 .7 × 10^-6 L^2.26 for D. carinata and C. quadrangula s.l., respectively. Both cladocerans had small (<0.3 µm) intersetular distances. Surface areas (A) of limbs 3 and 4 increased as a power function of body length: A = 5.35 × 10^-2 L^2.0 (D. carinata); A = 1 .22 L^1.6 (C. quadrangula s.l.). Reynolds number was < 1; boundary layers surrounding the setules overlapped at beat rates of 5 and 16 cycles s^-1. Filtering rates (K) for both species increased with increasing body length according to K = 1 .8 × 10^-4 L^2.0 (D. carinata) and K = 1.3 × 10^-5 L^2.2 (C. quadrangula s.1.). K also increased as a power function of limb surface area. Filtering rates on Ankistrodesmus falcatus decreased above cell concentrations of c. 10⁴ cells ml^-1. Hourly feeding rates (No. of cells per animal) increased with increasing cell concentration up to the incipient limiting concentrations and thereafter remained relatively stable. Weight-specific feeding rates for D. carinata decreased with increasing body size but for C. quadrangula s.l. remained relatively constant. D. carinata positively selected particles in the size range 4.8-7.0 µm, whereas C. quadrangula s.l. showed no particle preference within the size range 3-12µm. Both cladocerans showed differences in their feeding behaviour when offered a range of phytoplankton species but both fed most efficiently on A. falcatus. Although these cladocerans had limb morphologies that differentiated them from their Northern Hemisphere counterparts, this was not reflected in their feeding behaviour.