Competitive Dynamics of a Tropical Hermit Crab Assemblage

The competitive dynamics of a three—species littoral hermit crab assemblage in Panama are demonstrated. Calcinus obscurus is found in the middle to low intertidal habitat, Clibanarius albidigitus occurs in the middle to high intertidal zone, and Pagurus sp. (undescribed) is found exclusively in the low intertidal habitat. When either Pagurus or Clibanarius are sympatric with Calcinus they occupy shells of poorer quality than they do when they occur alone, while the shell supply of Calcinus is not negatively influenced by sympatry. Experimental addition of empty shells to sympatric and allopatric populations of Calcinus and Clibanarius showed that while Clibanarius exploited empty shells better than Calcinus, Calcinus obtained the new shells over time from Clibanarius via active interference competition. The added shells were also found to move from smaller to larger crabs within 4 d of their introduction. Habitat segregation between Calcinus and Clibanarius is facilitated by the active competitive displacement of Clibanarius by Calcinus. The mechanism of this competitive displacement is shown to be an active escape response of Clibanarius to Calcinus individuals. These results indicate that active competition for a limited supply of shells directly influences the shell quality and spatial distributions of these sympatric hermit crabs. Since shell quality regulates growth and reproduction in hermit crabs, active competition ultimately influences growth and reproduction, demonstrating a fitness cost and benefit to competitors.