Coral communities have persistent patterns of depth zonation. What are the ecological causes of these patterns? In the Caribbean Sea, the direct effects of predators on community structure are equivocal. While some invertebrates have episodic effects at local scales (for example, snails in the genus Drupella) parrot fish in the genus Sparisoma have the ecological abundance and functional morphology to strongly impact individual corals and limit the development of coral populations.

Remarkably, only a few studies have experimentally evaluated the direct effects of Caribbean parrotfish on community structure, with conflicting outcomes. Steve Barnes and I designed and executed a field experiment in the Bocas del Toro region to test the role of parrotfish predation on coral zonation. We chose to manipulate three conspicuous species of the shallow-water community on Caribbean reefs: the branching species Porites furcata, the free-living species Manicina areolata, and the massive brain coral Diploria clivosa. We also used a sham treatment in this design of dead coral, to determine if parrotfish preferred to consume live coral, when faced with a dead coral alternative of similar morphology. Replicates of the three species + shams were transplanted from 1.5 to 12 m, while control treatments that remained at the shallow depth. To exclude large fish predators, Steve caged ½ of the replicates within treatments at each depth. As the figure above shows for the branching Porites furcata, parrotfish can have striking effects on these species at the deep depth, always greater on live coral than on shams. Videotaping of deep treatments captured the stoplight parrotfish Sparisoma viridae consuming corals, and that the specific life-history phase of S. viridae was correlated with live coral preference. Our experimental approach shows that parrot fish play key roles in the development and zonation of hard corals. These effects may be direct and ecological, if coral larvae settle throughout these zones but recruits fail to establish because of the limiting effects of predation, or they may have ultimate effects in driving natural selection on larval behavior that increases settlement in predator free zones. Narissa Bax has lead an effort to determine if larval behavior can control vertical settlement in the Favia fragum complex and has indeed found evidence that the Tall ecomorph has larval swimming and settlement behaviors that increase recruitment to shallow habitats where predators are rare. Narissa is currently working this data up for publication.