Intrinsic microhabitat preference promotes coexistence in two competing intertidal hermit crabs

Environmental complexity can reduce overlap in resource use by generating fine-scale spatial variation, allowing species to occupy distinct microhabitats. Mixed intertidal shores, where rocky substrates alternate with sandy patches, are highly heterogeneous systems that provide benthic organisms with wide physical, chemical, and structural heterogeneity. In such environments, substrate use may reflect species-specific microhabitat preferences that facilitate species coexistence. However, species distributions can also be influenced by their tolerance of proximity to conspecific and heterospecific competitors. Here, we examined how social conditions affect microhabitat use in two competing intertidal hermit crab species. We first determined each species' preference among four intertidal microhabitats defined by substrate type and exposure conditions (sand, upper submerged rock surfaces, under-rock microhabitats, and air-exposed rock surfaces) under solitary conditions. We then evaluated how their microhabitat use changed as a function of competition with conspecifics versus heterospecifics, and as a function of competitor density. The two species differed significantly in their microhabitat preferences: Clibanarius albidigitus, which is typically associated with the upper intertidal, primarily occupied the upper side of submerged rocks, whereas the mid-intertidal species Calcinus californiensis preferred under-rock microhabitats. Both species largely avoided sandy and fully air-exposed microhabitats. The microhabitat use of C. californiensis was largely unaffected by competition, while C. albidigitus occupied its preferred habitat significantly less often under intra-specific competition but was not affected by inter-specific competition. The contrasting microhabitat preferences exhibited by the two species likely limit direct interspecific competition. Therefore, habitat segregation is largely structured by environmental conditions and intrinsic microhabitat preferences rather than direct interference, while population density exerts little influence on spatial organization.