Thermal threshold and interspecific competition help explain intertidal hermit crab assemblages

Habitat heterogeneity promotes species diversity by providing a variety of abiotic and biotic conditions, whose impact on performance varies among species. Then, mobile species would be expected to move to areas whose conditions maximize their fitness. However, biotic pressures such as interspecific competition can push subordinate species into suboptimal areas, impeding this matching. The tropical hermit crab Clibanarius albidigitus occupies mostly upper intertidal sites where they can experience extreme environmental conditions. Meanwhile, its stronger agonistic competitor, Calcinus californiensis, mostly inhabits more moderate conditions at the mid intertidal. We estimated the avoidance threshold of the two hermit crab species to increasing water temperatures to help explain their intertidal distribution. We also compared the avoidance threshold of Cli. albidigitus to rising temperatures when presented alone and in the presence of chemical cues of its competitor to assess potential competitive niche exclusion. The avoidance threshold was measured in experimental tanks with a ramp that led from the water to an air-exposed platform; the threshold was defined as the temperature at which individuals emerged and remained air-exposed. Clibanarius albidigitus emerged at a higher temperature than its competitor, showing a higher thermal tolerance and potentially explaining its distribution in the upper intertidal. In the presence of Cal. californiensis, Cli. albidigitus emerged at lower temperature than when alone, likely as a strategy to reduce competition with stronger agonistic competitors, even at the cost of coping with harsh conditions. Our results support the hypothesis that competitive habitat exclusion contributes to explaining hermit crab assemblages.

Automated summary

Habitat heterogeneity promotes species diversity. The hermit crab Cli. albidigitus has higher thermal tolerance and occupies upper intertidal sites, while its competitor Cal. californiensis inhabits more moderate conditions. Cli. albidigitus emerged at lower temperature in the presence of Cal. californiensis, suggesting competitive niche exclusion.