The benefit of obligate versus facultative strategies in a shrimp–goby mutualism

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Obligate mutualists are predicted to benefit more from mutualism than facultative mutualists and harbor traits that help derive this extra benefit. I tested these predictions with a shrimp–goby mutualism. Individual shrimp (Alpheus floridanus) construct burrows that are shared with individual gobiid fishes that warn shrimp when emergence from burrows is unsafe. The benefit to gobies is refuge from predation. I compared predator avoidance performance of obligate (Nes longus) and facultative (Ctenogobius saepepallens) shrimp-associated gobies with access to a shell, a shrimp burrow, or no shelter. The two gobies performed similarly with shells and no shelter, but N. longus outperformed C. saepepallens with shrimp burrows. Thus, N. longus benefits more from mutualism than C. saepepallens. Also, N. longus has four behavioral traits that likely allow it to benefit from mutualism more than does C. saepepallens: (1) diving into the nearest shrimp burrow when confronted with predators, (2) long flight initiation distance, (3) long time-until-re-emergence after taking refuge, and (4) remaining close to shelter. The latter three likely come with a cost to foraging or mate acquisition as they limit searching range outside the burrow and detract from time spent outside burrows. Thus, N. longus likely harbors traits that allow it to deal with these costs. I discuss a framework for determining whether traits crucial to obligate association with shrimp (both for reaping benefits and dealing with costs) are pre-adaptations or are created through coevolution with shrimp.