Does the evasive behavior of Hexarthra influence its competition with cladocerans?

Abstract

We tested the hypothesis that evasive movements by Hexarthra mira reduce adverse effects of interference competition with cladocerans permitting coexistence. To do this we studied the population growth of Hexarthra mira and two non-evasive Brachionus species in the presence of one of either two cladocerans (Daphnia similoides, Ceriodaphnia cornuta) at three food (Chlorella) levels (0.5, 2 and 4 x 106 cells ml-1) at 25°C. The non-evasive, but larger-sized B. calyciflorus was suppressed by D. similoides at all food levels tested, and by C. cornuta at high food levels only. The smaller B. angularis showed similar trends with D. similoides, but with C. cornuta it persisted and increased in population size at the medium and high food levels. Hexarthra was able to coexist with both the cladocerans regardless of food level. However, population growth rate of Hexarthra was affected significantly in presence of D. similoides, but not in the presence of C. cornuta. We suggest that evasive behavior of Hexarthra helps it coexist with large cladocerans by reducing the frequency of its being drawn into their branchial chambers.