| This dissertation highlights the importance of positive interactions as ecological and evolutionary forces in marine communities. In chapters one and three, I show that slow-growing, structurally complex, plants and sessile animals may commonly persist in areas of intense competition by sheltering herbivorous symbionts to suppress faster-growing competitors. In chapter 1, I demonstrate one such mutually beneficial relationship between the herbivorous crab Mithrax forceps and the coralline alga Neogoniolithon strictum. In chapter 3 I show that the coral Oculina arbuscula provides the crab Mithrax forceps with dietary supplements and shelter from predators while the crab defends the coral from overgrowth by encroaching competitors, thus enhancing coral growth and survivorship. I also demonstrate the interaction is only mutually beneficial where competition between corals and seaweeds is intense, highlighting the conditional nature of these mutualisms.;In chapters two and four I show how chemical defenses can provide associational refuge for small herbivores in the same manner as the structural defenses of corals and calcified seaweeds. In chapter 2, I demonstrate that predation pressure drives specialization by the decorator crab Libinia dubia on a chemically defended alga, Dictyota menstrualis. Crabs decorated with this alga experience less predation in the field than crabs decorated with a palatable seaweed. Libinia prefers D. menstrualis over all seaweeds, and this preference is cued by dictyol E, a metabolite specific to D. menstrualis that deters feeding by omnivorous fishes that prey on both seaweeds and crustaceans. Thus, in effect, Libinia behaviorally sequesters the chemical defenses from this plant. However, in chapter four I point out that Dictyota and Libinia do not have completely overlapping ranges. At sites where Dictyota menstrualis occurs (North Carolina and Alabama), crabs strongly prefer this alga for camouflage, while crabs collected outside the range of this alga (New Jersey) do not prefer D. menstrualis over several other seaweeds. Such adaptation to local associations may provide an important means of population differentiation in the marine environment where few obvious geographic barriers exist. |
