Coexistence of Congeneric Ciliates (Karyorelictida: Loxodes) in Relation to Food Resources in Two Freshwater Lakes

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Abstract
Two species of the ciliated protozoon Loxodes (L. magnus Stokes and L. striatus Penard) coexist in the hypolimnia of 2 biologically productive lakes. L. magnus was usually more numerous in Esthwaite Water; L. striatus was always more numerous in Priest Pot. L. magnus was considerably larger than L. striatus (mean cell volumes 960 .times. 103 and 196 .times. 103 .mu.m3) and its mouth was much bigger (mean lengths 80.7 and 31.7 .mu.m). There was no interspecific overlap in either cell size or mouth size. Both species are raptorial feeders, living principally on small (5-50 .mu.m) algae. L. magnus was dominant in Esthwaite where most algae were large and many were too large for L. striatus. In Priest Pot, large food particles were relatively scarce and there was considerable overlap in the spectra of particle sizes ingested by the 2 spp. L. striatus was dominant chiefly because of its higher weight-specific feeding rate and population growth rate. Laboratory experiments confirmed that the size and shape of particles were the only qualitative features determining whether or not food particles were ingested. Food resource partitioning appeared to be the chief mechanism facilitating coexistence of these 2 spp.