We model the mutualism interactions between gobies and shrimp based on recent experimental work on the shrimp Alpheus floridans and the facultative and obligate gobies Ctenogobius saepepallens and Nes longus in the Bahamas. We show that the model is consistent with observations, and suggest that obligate mutualism may favour rapid speciation in gobies due to their restricted spatial range. We calculate the resilience of the goby-shrimp systems to evaluate the robustness of the mutualist interactions to parameter choices. While experimental evidence has noted the ubiquity of obligation in gobies, our theoretical investigation predicts the ubiquity of facultative mutualism in shrimp despite the many obligate mutualism strategies open to them. Future experimental work might be usefully directed at evaluating the ubiquity of facultative versus obligate interactions of shrimp in goby - shrimp mutualist populations, and examining whether facultative and obligate strategies are correlated with food availability.