Coral-associated invertebrates as indicators of reef health in the Caribbean

Coral reef biodiversity is partially comprised of macro-invertebrates that live partially embedded inside the skeletons of live corals, from where they may potentially cause extensive damage to their host’s external polyp tissue. In the present study, six dominant groups of Caribbean coral associates were investigated for adverse effects on their host’s health. Internal damage of the coral’s skeleton was not considered. The associates can be categorized into two groups, i.e., those with small coral wounds (boring mussels of the genus Leiosolenus, coral barnacles of the family Pyrgomatidae, and gall crabs of the family Cryptochiridae) and those with larger coral injuries (feather duster worms of the genus Anamobaea, worm snails of the genus Petaloconchus, and serpulid worms of the genus Spirobranchus). A positive relationship was found between the surface area occupied by the associated invertebrate and the size of the surrounding damage. Damage caused by one particular associate could also vary among host corals, probably indicating host-specific resistance to symbiont-related damage. Some coral species showed pigmented tissue adjacent to the symbiont, possibly indicative for regenerative growth, disease, or a chemical defence mechanism. Hence, this study clarifies the nature of the symbiotic relationships between various coral-associated invertebrates and their hosts and demonstrates the variation in harmful impact of widespread coral-associated fauna in the Caribbean. This information helps to clarify the potential use of coral-associated invertebrates as bioindicators of reef health: their presence is a contribution to reef biodiversity, but when they reach high densities (like in eutrophic conditions), some of them are capable of causing serious harm to their host corals.