Nematode movement along a chemical gradient in a structurally heterogeneous environment. 1. Experiment Deplacement des nematodes suivant un gradient chimique dans un milieu a structure heterogene. 1. L'experimentation

The interaction between soil structural heterogeneity and chemical gradients, and their effect on the movement of free-living nematodes was investigated. Four experimental treatments were used. These consisted of a nematode (Caenorhabditis elegans) on a homogeneous layer of nutrient agar in a Petri dish, with or without a localised bacterial food source (Escherichia coli) acting as an attractant. Structural heterogeneity was then introduced by adding a monolayer of sand grains onto both of the homogeneous treatments. All trails were recorded using time-lapse video, and subsequently digitised prior to analysis. Turning angle distributions and the fractal dimension of the trails were calculated for each treatment. There was a statistically significant effect (P ≤ 0.01) of all treatments on the movement of the nematode. In the presence of the attractant, nematode movement was more linear and directed towards the bacterial source. Structural heterogeneity caused the nematode to have more linear movement compared to a homogeneous environment. The fractal dimension of the nematode trails was significantly higher (P ≤ 0.01) for the treatment without structure or bacteria, than for the other treatments. The results, for the first time, quantify the degree to which nematodes carry out random foraging type behaviour in a homogeneous environment and produce more directed non-random movement in the presence of attractant. Finally, when structure is present the foraging strategy becomes more of an avoidance strategy, allowing the nematode to escape structural traps, such as 'dead-end' pores, and then continue to react to attractant gradients.