Selective attraction of marine bacterivorous nematodes to their bacterial food

This paper explores the role of selective attraction to food in determ~ning the spatial (micro)distribution of closely related nematode species. The attractiveness of 3 different bacterial strains to 4 species of Monhysteridae, Diplolaimelloides rneyli. Diplolaimella dievengatensis, Monhystera sp. and Geomonhystera disjuncta, was studied in a multiple choice design. In our study area, the 4 nematode species considered are associated with Spartina anghca detritus decay and have partially overlapping microhabitat preferences. As they all belong to the same feeding guild, they are potential competitors for food. Each of the 4 nematode species was attracted to the bactenal strain B1, but important interspecific differences were noted in the nematodes' response to live or heat-killed bacteria, to bacteria at different cell densities or of different age, and to the filtered supernatant of B1 culture. While the responses of D. meylj to the Gram-positive bacteria Halobacillus trueperi and to the Gramnegative Eschenchia col1 \yere similar, D. dievengatensis and Monhystera sp. were preferentially attracted to H. trueperi and E. coli, respectively. This opposite preference influenced both the numbers and their relative abundances of D, dievengatensis and Monhystera sp. inside bacterial patches in experiments with a mixed 2-species nematode inoculum. Bacterial cell density strongly influenced the nematode response, \nth D. meyliinvariably preferring the highest cell densities offered, while D. dievengatensis and Monhystera sp. had a peak response at lower cell densities. Though chemotaxis IS suggested as an underlying mechanism, the nature of the nematodes' response remains unproved. The present results strongly support the importance of food patchiness in determining the heterogeneous distribution of nematodes, and extend the concept in such a way as to allow for small differences in mlcrohabitat choice between closely related species. They also support the view that nematodes are special~st feeders, though they probably select spots where suitable food is plentiful rather than individual food particles. Finally, the present study offers a baseline for an understanding and further study of patterns of succession among nematode specles associated with decaying Spartina anglica detritus in terms of highly specific relationships with different strains, growth stages, and densities of bacteria involved in the mineralization of Spartina anglica-derived organic matter.