Movement and Microdistribution of Sida Crystallina and Other Littoral Microcrustacea

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Abstract
Littoral invertebrates of Cochran Lake, Michigan were either highly plant—associated, and found infrequently away from aquatic macrophytes, or free—swimming among the plants. Plant—associated taxa were distributed according to available surface area. Numbers of periphyton and detritus feeders, chiefly chydorid Cladocera and chironomid midge larvae, were highly correlated with diatom density, whereas filter—feeding Cladocera attached to the plants (Sida, Simocephalus) were not. Cyclopoid copepods characteristic of the littoral during the day (Macrocyclops, Eucyclops, and Microcyclops) were also found primarily on aquatic plant surfaces. Abundances of the chydorid Cladocera increased slightly at night, while other taxa showed little diurnal variation. Free—swimming species in the littoral (Bosmina, Ceriodaphnia) sometimes formed large patches covering many square metres. Large species (Mesocyclops, Diaptomus, Epischura) greatly increased the numbers and diversity of this community at night. Sida crystallina, a filter feeder typically attached to plant surfaces, nonetheless swims frequently and well if disturbed. Juveniles moved between substrates more frequently than adults, and were found nearer the surface. Sites for reattachment were located visually. Frequent and often extensive episodes of swimming, especially by juveniles, provided a plausible explanation for the high concentrations of Sida observed on isolated plants, particularly those on the margins of stands of aquatic vegetation.