Mutualism And Coral Persistence: The Role Of Herbivore Resistance To Algal Chemical Defense - Statistical Data Included

Ecology, Sept, 1999 by John J. Stachowicz, Mark E. Hay

Using field, mesocosm, and laboratory manipulations, we demonstrate a mutually beneficial association between Oculina and the majid crab Mithrax forceps that is similar to some ant-plant mutualisms in terrestrial environments (e.g., Janzen 1966, 1969, review in Beattie 1985). The coral provides the crab with dietary supplements and shelter from predators while the crab defends the coral from overgrowth by encroaching competitors, thus enhancing coral growth and survivorship. We also experimentally examine how variation in external environmental conditions affects the outcome of this interaction, highlighting the context-dependent nature of mutualistic interactions. Our results suggest that this type of protective mutualism has population and community implications that extend beyond the participant species. The crab directly alters the local benthic community, enhancing the growth and survival of its host and ensuring the persistence of the diverse host-associated community by increasing the production of biogenic structure.

METHODS

Study sites and organisms

Field experiments were performed at Radio Island Jetty near Beaufort, North Carolina, USA (34 [degrees] 42 [minutes] N, 76 [degrees] 41[minutes] W). This rock jetty runs from just subsurface to 9 m depth, and field experiments were deployed at 2.0 and 6.0 m. Although the jetty is man-made, its flora and fauna are similar to that of natural rock ledges along the North Carolina coast (Richardson 1978, Link 1980, Peckol 1982, see Results). In the summer and fall, the shallow, well-lit portion of the jetty is dominated by brown seaweeds like Sargassum, Padina, and Dictyota, whereas the deeper portions are dominated by colonial invertebrates, including the scleractinian coral Oculina arbuscula, the upright bryozoan Bugula neritina, the hydroid Eudendrium carneum, several species of tunicates, and a few sponges. Abundant omnivorous fishes include the spottail pinfish Diplodus holbrooki, the pinfish Lagodon rhomboides, and the sheepshead Archosargus probatocephalus. The predatory black sea bass Centropristis striata is also abundant. In the winter and spring, colder temperatures induce an offshore migration of most fishes and reduce activity of resident urchins (Arbacia punctulata), shifting community dominance toward more palatable red and green seaweeds and an increased abundance of fleshy invertebrates (Hay 1986, Hay and Sutherland 1988).

Oculina arbuscula is a branching, scleractinian coral endemic to North and South Carolina. Oculina growing in sunlit waters possess symbiotic zooxanthellae, but this relationship is facultative. In dark habitats, colonies are azooxanthellate, yet continue to grow (Miller 1995). A diverse invertebrate fauna hosted by the coral includes the majid crab Mithrax forceps (McCloskey 1970). This crab occurs from Cape Hatteras, North Carolina south through Florida and the Caribbean to Brazil on rocky shores and reefs from the intertidal to a depth of 90 m (Williams 1984). Mithrax has been found associated with various species of sponges and branching corals, as well as intertidally on oyster shells, and its geographic range extends well south of that of Oculina, so association with this coral is not obligate. Little is known of the ecology or feeding habits of M. forceps, although other crabs in the genus are herbivorous and enter into facultative associations with corals and calcified seaweeds (Coen 1988, Stachowicz and Hay 1996). To avoid biasing the results of field experiments through excessive disturbance, crabs and corals for all field, mesocosm, and laboratory experiments were collected at a depth of 9 m from the deck of the Liberty Ship, a large sunken ship [approximately]10 km from Radio Island (34 [degrees] 41[minutes]10 [seconds] N, 76 [degrees] 43 [minutes] 30 [seconds] W).