Predation rates and prey selectivity in two predacious estuarine nematode species

Enoploides longispiculosus and Adoncholaimus fuscus are representatives of nematode genera prominent in sediments of the North Sea and adjacent estuar ies. Both are predatory nematodes, although predation is facultative in the latter. The present study investigates functional responses and prey selec tivity in both species through the use of controlled laboratory experiments . Both predators had strongly prey density-dependent predation rates. A max imal predation rate of 4 monhysterid prey nematodes per predator per 24 h w as found in E. longispiculosus at prey densities of 200 ind. per petri dish and higher; no such maximal predation rate was found for A. fuscus, indica ting that this species was prey-limited at all prey densities tested. Preda tion rates were strongly affected by temperature, with a Q(10) close to 2 b etween 10 and 20 degreesC. Incubation in the light resulted in a similar de crease in predation rate compared to dark incubations, as did a temperature decrease from 20 to 10 degreesC. E. longispiculosus exhibited a clear pref erence for some nematode prey over others. An encounter probability model i ndicated that preferences could not be explained by encounter rates. Strike rates were low (<10 %) in E. longispiculosus, and exceptionally low (<much less than>1%) in A. fuscus, indicating that many encounters did not result in attack, or that a portion of the attacks did not result in prey capture . The observed predation rates cannot be supported by prey nematode standin g stock and production at the 2 sampling sites used in this study, where E. longispiculosus dominates the nematode community in abundance and, especia lly, biomass. A. fuscus may mainly derive food from feeding modes other tha n predation; E. longispiculosus may be prey-limited in its natural habitat. Since this nematode also feeds on other metazoans, it may also impact temp orary meiofauna. The high predation rates and prey selectivity of predaciou s nematodes may be important structuring factors to meiofaunal communities.