ABSTRACT: Enoploides longispiculosus and Adoncholaimus fuscus are representatives of nematode genera prominent in sediments of the North Sea and adjacent estuaries. Both are predatory nematodes, although predation is facultative in the latter. The present study investigates functional responses and prey selectivity in both species through the use of controlled laboratory experiments. Both predators had strongly prey density-dependent predation rates. A maximal predation rate of 4 monhysterid prey nematodes per predator per 24 h was 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, indicating that this species was prey-limited at all prey densities tested. Predation rates were strongly affected by temperature, with a Q10 close to 2 between 10 and 20°C. Incubation in the light resulted in a similar decrease in predation rate compared to dark incubations, as did a temperature decrease from 20 to 10°C. E. longispiculosus exhibited a clear preference for some nematode prey over others. An encounter probability model indicated that preferences could not be explained by encounter rates. Strike rates were low (<10%) in E. longispiculosus, and exceptionally low (<<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 standing stock and production at the 2 sampling sites used in this study, where E. longispiculosus dominates the nematode community in abundance and, especially, biomass. A. fuscus may mainly derive food from feeding modes other than predation; E. longispiculosus may be prey-limited in its natural habitat. Since this nematode also feeds on other metazoans, it may also impact temporary meiofauna. The high predation rates and prey selectivity of predacious nematodes may be important structuring factors to meiofaunal communities.

KEY WORDS: Nematodes · Predation · Prey selectivity · Encounter probability · Top-down control

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Published in MEPS Vol. 205 (2000) on October 19
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000