Predation rates and prey selectivity in two predacious estuarine nematode species

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 Q 10 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.

Can field distributions of marine predacious nematodes be explained by sediment constraints on their foraging success?

“…obs.). As a result, encounter probabilities between predator and prey largely depend on the predator's own motility (Moens et al 2000). Thus, the observed sediment effects on predation rates likely result from changes in the behaviour of the predator rather than of its prey.…”

“…It is important to note that neither prey species is typical of the sedimentary habitats frequented by Enoploides longispiculosus. Both species have, however, been shown to be suitable prey for E. longispiculosus in laboratory experiments (Moens et al 1999, Hamels et al 2001, the functional response of E. longispiculosus to these prey nematodes has been established (Moens et al 2000), and there are obvious advantages to using prey from standardized cultures over non-culturable prey isolated from a variable environment (Moens & Vincx 1998).…”

“…The total prey densities in the present experiments roughly corresponded to 200 prey 10 cm -2 in a 2 cm sediment horizon, which is at the lower end of prey nematode densities on the Paulina tidal flat (see 'Results' section). However, functional responses of E. longispiculosus are most pronounced at still lower prey densities, and predator-prey encounters at this prey density are probably largely in excess of actual predation rates (Moens et al 2000). Microcosms consisted of 3.5 cm diameter Petri dishes containing azoic sediment.…”

“…We hypothesized that (1) the preference of E. longispiculosus for fine and medium sandy sediments could be explained by grain-size effects on predation efficiency, (2) the presence/abundance of silt may further constrain the suitability of sediments for E. longispiculosus, and (3) published vertical migration behaviour of E. longispiculosus in intertidal sediments as a function of the tides (Steyaert et al 2001) may reflect an impact of sediment water content on the predator's foraging efficiency. Since E. longispiculosus is a selective feeder (Moens et al 2000, Hamels et al 2001, we further hypothesized that prey preferences may be modified by the same 3 sediment characteristics. We therefore performed controlled laboratory experiments to assess the effects of each of these sediment characteristics separately on the foraging efficiency and prey selectivity of E. longispiculosus.…”

“…Recent laboratory experiments have shown that this predator feeds selectively on other nematodes (Moens et al 1999(Moens et al , 2000 and ciliates (Hamels et al 2001) at rates sufficient to influence prey biomass and species composition. However, these laboratory results are yet to be translated to field conditions, where a variety of sediment-and hydrodynamics-related factors may affect E. longispiculosus foraging success and hence its impact on prey communities and on microbial-meiofaunal foodwebs.…”