L Journal of Experimental Marine Biology and Ecology 243 (2000) 137–154 www.elsevier.nl / locate / jembe Temperature, salinity and food thresholds in two brackish- water bacterivorous nematode species: assessing niches from food absorption and respiration experiments * Tom Moens , Magda Vincx University of Gent, Biology Department, Marine Biology Section, K.L. Ledeganckstraat 35, B-9000 Gent, Flanders, Belgium Received 12 January 1999; received in revised form 20 July 1999; accepted 29 July 1999 Abstract Respiration and food assimilation of two estuarine bacterivorous nematodes, the rhabditid Pellioditis marina and the monhysterid Diplolaimelloides meyli, were measured at a range of temperatures, salinities, and food densities. The aim of this study was to identify the fundamental niche of both species in their natural habitat, and to investigate the relative importance of food and abiotic factors in determining presence and success of nematode species in the highly dynamic estuarine tidal environments of macrophyte detrital habitats. Of the three factors studied, salinity least impacted P . marina and D. meyli. Respiration and assimilation in both species showed only minor variation in the salinity range of 10 to 30‰. Respiration decreased at marine salinities in P . marina, and increased in both species at oligohaline salinities down to a stress-induced maximum around a salinity of 5‰, then steeply declined towards freshwater conditions. Temperature heavily affected both species, but Q -values in D. meyli were considerably higher than in P . marina, 10 suggesting the former species to be particularly well adapted to fine-tuning its energy expenditure as a function of temperature. The highest respiration and assimilation rates in both species were at 258C. At still higher temperatures, metabolic rates were depressed, but while P . marina was entirely inactivated above 308C, D. meyli continued to respire and assimilate food up to 358C. The scope for production, calculated as the net difference between assimilation and respiration rates (both expressed in units of C), was 0 at 58C and increased to a maximum at 258C in both nematodes; it declined at higher temperatures, but remained positive up to 358C in D. meyli. 8 Significant food assimilation in both nematodes occurred only at bacterial densities above 10 cells 21 8 21 ml . Assimilation rate reached a maximum at 5 3 10 cells ml in D. meyli, and remained constant at higher densities. P . marina, by contrast, had a well defined peak assimilation at a food 9 21 density of 2.5 3 10 cells ml , with lower rates at both lower and higher food densities. This contrasts with observations on ingestion rate, and suggests a food density-dependent assimilation efficiency. The present results suggest that tolerances of exposure to salinity and temperature *Corresponding author. Fax: 132-9-264-5344. E-mail address: tom.moens@rug.ac.be (T. Moens) 0022-0981 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(99)00114-8