The effects of shell availability on intraspecific competition in experimental populations of the hermit crab, Pagurus longicarpus Say

Abstract

It has been proposed in the literature that intraspecific competition in hermit crabs, mediated by a succession of shell exchanges, would lead to members of a given size class experiencing approximately the same level of shell adequacy. This hypothesis was modified and extended to predict that intraspecific competition should result in the maximization of the correlation between hermit crab size and shell size, regardless of the level of shell adequacy. The prediction has been tested by establishing laboratory populations of the hermit crab Pagurus longicarpus Say, and simulating these populations using a computer program which assumed constant motivational states and dominance relationships. The populations at the beginning of the experiment consisted of 19–21 individuals inhabiting shells as a result of a process that did not allow actual shell selection. The populations were divided among three treatments based on the availability of 0, 10, or 20 shells in addition to those inhabited initially. At the end of the first week, all populations displayed an increased correlation between hermit crab weight and shell weight, and these values were similar to those generated by the computer simulation. The values at the end of the second week, however, showed a noticeable decline in the populations with no empty shells available. In addition, these populations had the largest number of injuries, which could only be the result of aggressive encounters between hermit crabs. The results of these experiments are used to formulate a general model of hermit crab behavior under different levels of shell availability and shell adequacy.