Competitive dynamics and natural history of the shell-inhabiting amphipod Polynesoecetes kekeae (Myers 1975)

Abstract

Polynesoecetes kekeae (Myers 1975) is a gastropod shell-inhabiting amphipod that lives with a micro-hermit crab (Pagurixus sp.) and shell-inhabiting tanaid on backreefs of Moorea, French Polynesia. Field samples revealed that all three crustaceans occupied the same species of prosobranch shells and there was an overlap in shell size utilization between P. kekeae and Pagurixus sp. and between P. kekeae juveniles and tanaids. Empty shells were rarely observed, and P. kekeae inhabited damaged shells more frequently than Pagurixus sp. A shell selection experiment indicated that P. kekeae also inhabit smaller shells than preferred. Behavioral experiments found that a Pagurixus sp. hermit crab without a shell was capable of forcefully evicting P. kekeae from its shell. It is suggested that shells are a limiting resource and exploitative competition likely occurs among P. kekeae, Pagurixus sp. and tanaids. There is evidence that interference competition may also occur between the amphipod and hermit crab. This shell-inhabiting crustacean assemblage is an interesting example of convergent evolution of adaptations specific to inhabiting gastropod shells and is the first documentation of hermit crabs competing with non-hermit crabs for shells.