Selective attraction of marine bacterivorous nematodes to their bacterial food

This paper explores the role of selective attraction to food in determining the spatial (micro)distribution of closely related nematode species. The a ttractiveness of 3 different bacterial strains to 4 species of Monhysterida e, Diplolaimelloides meyli, Diplolaimella dievengatensis, Monhystera sp. an d Geomonhystera disjuncta, was studied in a multiple choice design. In our study area, the 4 nematode species considered are associated with Spartina anglica detritus decay and have partially overlapping microhabitat preferen ces. As they all belong to the same feeding guild, they are potential compe titors for food. Each of the 4 nematode species was attracted to the bacter ial strain B1, but important interspecific differences were noted in the ne matodes' response to Live or heat-killed bacteria, to bacteria at different tell densities or of different age, and to the filtered supernatant of B1 culture. While the responses of D. meyli to the Gram-positive bacteria Halo bacillus trueperi and to the Gramnegative Escherichia coli were similar, D. dievengatensis and Monhystera sp, were preferentially attracted to H. true peri and E. coli, respectively. This opposite preference influenced both th e numbers and their relative abundances of D. dievengatensis and Monhystera sp, inside bacterial patches in experiments with a mixed 2-species nematod e inoculum. Bacterial cell density strongly influenced the nematode respons e, with D, meyli invariably preferring the highest cell densities offered, while D. dievengatensis and Monhystera sp, had a peak response at lower cel l densities. Though chemotaxis is suggested as an underlying mechanism, the nature of the nematodes' response remains unproved. The present results st rongly support the importance of food patchiness in determining the heterog eneous distribution of nematodes, and extend the concept in such a way as t o allow for small differences in microhabitat choice between closely relate d species. They also support the view that nematodes are specialist feeders , though they probably select spots where suitable food is plentiful rather than individual food particles. Finally, the present study offers a baseli ne for an understanding and further study of patterns of succession among n ematode species associated with decaying Spartina anglica detritus in terms of highly specific relationships with different strains, growth stages, an d densities of bacteria involved in the mineralization of Spartina anglica- derived organic matter.