Competition by obligate and facultative mutualists for partners in a shrimp-goby association

Mutualist species compete intra and inter-specifically for the resources provided by their partners. Because obligate mutualists are more reliant than facultative mutualists on the resources that their partners provide, they are expected to compete more strongly for those resources. Here, I examined interference competition in two goby fishes: Nes longus (an obligate mutualist) and Ctenogobius saepepallens (a facultative mutualist). Both gobies associate with the shrimp, Alpheus floridanus. Shrimp provide gobies with refuge from predators (a burrow in the sand), and gobies provide shrimp with a warning signal when predators are near. Using an aquarium experiment, I examined the behavior of a pair of gobies with access to a single shrimp burrow. I used four different goby pairings: large N. longus and small N. longus, large N. longus and small C. saepepallens, large C. saepepallens and small N. longus, and large C. saepepallens and small C. saepepallens. When paired with large N. longus individuals, small gobies of both species were less likely to occupy the single burrow than when paired with large C. saepepallens individuals. In addition, large N. longus individuals were less likely to co-occupy the single burrow with smaller gobies than were large C. saepepallens individuals. These results seem to indicate that large N. longus individuals exclude smaller gobies from burrows, while large C. saepepallens individuals do not. This study adds evidence to the supposition that obligate mutualists in general compete more strongly for mutualist partners than do facultative mutualists.