Coexistence of similar species: evidence of a resource and microhabitat sharing in two intertidal hermit crab species

Abstract

Theory predicts that species can only coexist if they are sufficiently different in their resource and/or microhabitat utilization; if their needs are too similar, the stronger population will exclude the weaker, unless the two species are equally strong competitors. This hypothesis is difficult to assess in most species because populations are limited by multiple resources simultaneously. In hermit crabs, however, fitness is overwhelmingly limited by gastropod shells, which are their only shelter from external pressures, from predation to harsh environmental conditions. Several hermit crab species have similar shell requirements and occupy the same habitat. Here, we examine the degree of overlap in shell use and microhabitat occupation between two species, one of which (Calcinus californiensis) is clearly dominant over the other (Clibanarius albidigitus) in direct fights over shells. We hypothesized that these species should show microhabitat separation or shell partitioning. We sampled hermit crabs’ presence and shell use in the high- and mid-intertidal. The species overlapped in the two intertidal sites, especially at the higher intertidal, and they occupied preferred and suboptimal shells with similar frequency. The co-occurrence and strong overlap in shell use of these two species could be explained by a resource exploitation versus interference trade-off, relaxed selection pressure on shell characteristics due to small body size, spatial/temporal niche segregation, or dominance by Ca. californiensis that has yet to result in habitat displacement of Cl. albidigitus. Further research is needed to determine which of these hypotheses explain the apparent coexistence of ecologically similar species.