Susceptibilities of Ten Rotifer Species to Interference From Daphnia Pulux

Culture experiments showed that 10 rotifer species differed greatly in their susceptibilities to mechanical interference from Daphnia pulex. Direct observations and videographic analyses of rotifers entrained in Daphnia's inhalant feeding current provided some mechanistic explanations for these differences in susceptibility. In general, well—protected species either: (1) were too large to enter Daphnia's branchial chamber (Conochilus unicornis), (2) regularly escaped from Daphnia's inhalent current (Polyarthra remata), or (3) were rejected from Daphnia's branchial chamber after very short residence times (<2 s) (Asplanchna priodonta, Keratella crassa, K. testudo, Synchaeta pectinata). Highly susceptible species were rejected from Daphnia's branchial chamber only after relatively long residence times (6—15 s) and were either small in size (Keratella cochlearis, Synchaeta ablonga) or delicate (Ascomorpha ecaudis). Species with the longest residence times probably are those that are least likely to irritate Daphnia's branchial chamber and hence the most likely to be eaten or manipulated and damaged before being detected and rejected. In natural zooplankton communities, Daphnia interference has the potential to impose high morality rates on susceptible rotifer species and, therefore, to shift the species structure of rotifer assemblages in favor of resistant species. The ability of Daphnia to suppress most rotifer species in natural communities probably is often due to the combined effects of interference and exploitative competition. Large rotifer species least susceptible to Daphnia interference should be the most susceptible to declining food availability.