An experimental study of the role of gradient and substratum in the distribution of two stream‐dwelling triclads, Crenobia alpina (Dana) and Polycelis felina (Dalyell) in North Wales

Summary A laboratory stream is described which has simulated steep‐ and shallow‐gradient bottoms whose gradients can be varied. It was found that Polycelis felina, the principal triclad of shallow‐gradient streams had a much reduced ability to colonize the simulated steep‐gradient bottoms when compared to Crenobia alpina, which is the principal triclad of the steep‐gradient bottoms in the field. P. felina is unable to move in the steep‐gradient bottoms to any great extent, while C. alpina showed considerable ability to do this. When a reduction in flow was made this did not increase the proportion of P. felina to be found in the steep‐gradient regions. The triclads were also tested for possible acclimatory responses and it was found that their responses were more fundamentally based. On the basis of the above evidence it is suggested that the absence of P. felina from the steep‐gradient regions of streams in North Wales is due to a behavioural inability to tolerate the current regimes present in the steep‐gradient bottoms.