Geographic variation in interactions between size classes of the limpet Cellana tramoserica

We examined competition between different size classes of the limpet Cellana tramoserica at two locations in southeastern Australia. At Williamstown, on a sheltered basalt shore, small limpets showed the same competitive superiority over larger conspecific individuals as has been reported previously. In contrast, when the experiment was repealed on a limestone shore (Cheviot Beach), this pattern disappeared, and there were strong interactions within size classes, but weak interactions between size classes. Large limpets lost weight as density rose, while small limpets lost weight and had a higher risk of mortality. Intra-class interactions at Cheviot Beach were generally weaker than inter-class effects. The results are consistent with predictions made in an earlier study, in which it was hypothesized that the competitive interaction at Williamstown was the result of an interaction between the texture of the rock and the morphology of the radula. These results emphasize the importance of ecological divisions within species, and add complexity, because not only to the competitive abilities of these size/age classes of limpet differ, but their relative abilities vary between shores. It is possible that similar differences occur in other species. We also tested whether animals with a history of competition could compete successfully against animals that had previously been at low density. Large limpets performed well, with previously competing animals growing better than animals just introduced to higher densities. For small limpets, a history of competition resulted in animals growing much more slowly, suggesting that under sustained high densities, the condition of small limpets will continue to decline.