Food choice of two bacteria-feeding nematode species dependent on food source, food density and interspecific competition

Feeding selectivity and flexibility are generally poorly understood behaviours of free-living nematodes. In the present study, the food choice and feeding habits of two bacteria-feeding, free-living species, Panagrolaimus cf. thienemanni and Poikilolaimus sp. (Nematoda), isolated from Movile Cave (Romania), were examined with respect to food source, food density and interspecific competition. These two species coexist in a very stable environment, compete for the same food resources, and play a major role in Movile Cave's food web. Panagrolaimus and Poikilolaimus were found to favour significantly the bacterium Escherichia colt as a food source compared with glucose or algae. High densities of E. colt (10(9) cells ml(-1)) were preferred by Panagrolaimus, in contrast to the low densities (10(6) cells ml(-1)) preferred by Poikilolaimus. This result held true when the two species were exposed to intra- and interspecific competition, when Panagrolaimus significantly preferred E. colt at densities of 10(9) cells ml(-1), whereas Poikilolaimus, while not significantly preferring a fixed bacterial density, was nonetheless attracted to densities of 10(5) - 10(7) cells ml(-1). These density-related differences in food choice could explain the ability of these species to coexist without competitive exclusion in the same stable environment of Movile Cave.