A study of coexistence of gastropods in the River Hull system

Storey, Robert Albert Sill (1982). A study of coexistence of gastropods in the River Hull system. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001011b

Abstract

A qualitative study conducted on the gastropod snails in the River Hull system provided evidence of spatial separation of some species. Computer analysis showed that the snails could be used to classify stretches of the river. Unifying environmental characteristics of each site were then used to identify discontinuities between snail groups. From this evidence, temporary drying-up and direct or indirect influence of flow appeared to affect snail occurrence. At the tidal limit, the wide species diversity was found to be a temporary phenomenon, for there appeared to be a 'rock' and ’weed' community which were seasonally displaced probably due directly to the flow over a weir. The 'rock' species were found to possess differences in microdistribution pattern since they were selective with regard to the size of stone, its aspect and its distance from the weir. The 'weed' species showed relatively little separation with regard to plant quality, but there was a seasonal distinction between the pulmonates and the prosobranchs. There was some evidence, during periods of spatial overlap, of resource partitioning in their diets also. The general observations that food in the gut reflected that which was available on the substratum suggested either a broad diet or possibly substratum selection.

There were differences both in radula structure and application which corresponded to substratum differences, and different diurnal activity patterns were also demonstrated.

Although it is possible that predation, competition with other invertebrates and other factors could have been instrumental in limiting population size, there is ample evidence from this study that sufficient differences existed along the niche dimensions of space, nutrition and time, to have reduced the degree of overlap between species to a level where competitive exclusion would not occur, even with large population sizes.