FACULTATIVE MUTUALISM BETWEEN AN HERBIVOROUS CRAB AND A CORALLINE ALBA - ADVANTAGES OF EATING NOXIOUS SEAWEEDS

Because encrusting coralline algae rely on herbivory or low light leve ls to prevent being overgrown by competitively superior fleshy algae, corallines are relatively rare in shallow areas with low rates of herb ivory. In contrast to this general trend, the branching coralline alga Neogoniolithon strictum occurs primarily in shallow seagrass beds and along the margins of shallow reef flats where herbivory on macrophyte s is low. This alga apparently persists in these habitats by providing refuge to the herbivorous crab Mithrax sculptus al mean densities of 1 crab per 75 g of algal wet mass. When crabs were removed from some h ost corallines, hosts without crabs supported 9 times the epiphytic gr owth of hosts with crabs after only 30 days. Crabs without access to a coralline alga were rapidly consumed by reef fishes, while most of th ose tethered near a host alga survived. These results suggest that the crabs clean their algal host of fouling seaweeds and associate with t he host to minimize predation. However, to effectively clean the host, the crab must consume the wide array of macroalgae that commonly co-o ccur with coralline algae in these habitats, including chemically defe nded species in the genera Halimeda, Dictyota, and Laurencia. Crabs di d readily consume these seaweeds, which were avoided by, and are chemi cally defended from, herbivorous fishes. Even though crabs readily con sumed both Halimeda, and Dictyota in whole-plant feeding assays, chemi cal extracts from these species significantly reduced crab feeding, su ggesting that factors other than secondary chemistry (e.g., food value , protein, energy content), may determine whole-plant palatability. Ha ving the ability to use a wide variety of foods, and choosing the most profitable rather than the least defended foods, would diminish forag ing time, increase site fidelity, and allow the crab to function mutua listically with the host alga. Despite the obvious benefit of associat ing with N. strictum, M. sculptus did not prefer it over other habitat s offering a structurally similar refuge, suggesting that these crabs are not N. strictum specialists, but rather occupy multiple habitats t hat provide protection from predators. Structurally complex organisms like N. strictum may commonly suppress competitors by harboring protec tive symbionts like M. sculptus. It is possible that diffuse coevoluti on has occurred between these two groups; however, this seems unlikely because both herbivore and host appear to respond most strongly to se lective pressures from predators and competitors outside this associat ion.