Ecological and genetic aspects of the population biology of the littoral rotifer Euchlanis dilatata

Littoral rotifers are associated with vegetation and thus live in highly structured habitats. This structural complexity increases the possibilities of forming local subpopulations differing in (a) plant substrate used, (b) biotic interactions, and (c) genetic structure. I explore some of these topics in populations of the littoral rotifer Euchlanis dilatata; I investigated several aspects of egg-laying behavior in 3 clones of E. dilatata. Individuals lay their eggs randomly within culture dishes in the absence of substrate, while eggs laid in the presence of an artificial substrate are deposited in clumps by 2 of the 3 clones. When given a choice among artificial substrates with or without eggs and eggshells of conspecifics, rotifers lay most eggs on those substrates containing eggs followed by those with eggshells; I found E. dilatata most often associated with the macrophyte Myriophyllum exalbescens at 3 sites in Devils Lake (Lincoln Co., OR). In general, rotifers laid most of their eggs on this macrophyte species in laboratory preference experiments. I also assessed habitat-specific predation susceptibilities for rotifers cultured in the presence of aquatic macrophytes (M. exalbescens, Elodea canadensis, or Ceratophyllum demersum) and two invertebrate predators (damselfly nymphs Hydra). Rotifer survival was greatest on Myriophyllum. This population consisted of individuals of two distinct size classes. Females of the larger size class are up to 1.5 times longer than those of the smaller size class. I determined chromosome numbers for individuals of each morphotype. Large morphotype females average 21 chromosomes, small morphotype females 14. Individuals of both ploidy levels were most often associated with Myriophyllum. While there was no evidence of spatial habitat segregation, their temporal distributions varied. Diploid individuals were present only in late spring and summer collections, while triploids were present throughout the year. These distribution patterns were reflected in life history characteristics under various temperature treatments. Diploid clones produced significantly more young and lived longer under high temperature regimes (25{dollar}\sp\circ{dollar}C) whereas triploid individuals survived longer at low temperatures (12{dollar}\sp\circ{dollar}C). In addition, diploid and triploid also differed in their relative susceptibilities to invertebrate predators. Both damselfly nymphs and Hydra were more effective in reducing the survival of the larger triploid rotifers