Coral reefs ecosystems are deteriorating and facing dramatic changes. These changes suggest a shift in dominance from corals to other benthic organisms. Particularly in the Caribbean Sea, with corals dying, sponges have become the leading habitat-forming benthic animals. However, little is known about what life-history traits allow organisms to proliferate in a marine system that is undergoing change. Thus, the objective of this dissertation was to try to understand the current increase of encrusting excavating sponges on deteriorating Caribbean coral reefs through the study of reproduction, recruitment and dispersal potential of the widely distributed and currently expanding species, Cliona delitrix. Different methodological approaches were used, such as histology, electron microscopy, quantification of sponges in the field, genetics, and mathematical modeling. Results are presented in four different chapters. It was found that Cliona delitrix has an extended reproductive cycle in Florida, USA, from April - May to around November - December depending on a >25°C sea-water temperature threshold. C. delitrix gametogenesis is asynchronous and it has multiple spawning events. C. delitrix is recruiting abundantly on Caribbean coral reefs, preferentially on recent coral mortality than on old coral mortality. The increase in C. delitrix and other excavating sponges can be explained by the repeated spawning and by the coincidence in time and space of larval production with the availability of new dead coral, which tend to overlap during the warmest months of the year. Eggs or larvae of C. delitrix appear to survive enough to be transported by currents over larger distances. It was found that dispersal ranges for Cliona delitrix may reach as far as ~315 km in the Florida reef track, and over ~971 km in the South Caribbean Sea, between Belize and Panama. Thus, reproduction, dispersal, and recruitment patterns of C. delitrix along with oceanographic currents, and eddies that form at different periods of time, are sustaining the spread of this sponge on coral reefs. According to mathematical models carried out, C. delitrix increase on reefs fluctuates depending of coral mortality events and available space on old dead coral (colonized by algae and other invertebrates). However, under temperature anomalies, these sponges will 2 tend to increase and take over the reef system only if heat stress and coral mortality is moderate. Under massive mortality events both corals and sponges will tend to decline, although sponges at a slower rate than corals. In general, coral excavating sponges have been favored by coral mortality, especially during past few decades. However as bioeroders, their success is also limited by the success of calcifying corals. In a reef management context and based on this dissertation’s findings, it is suggested that excavating sponges, and especially Cliona delitrix, should be more formally included in reef monitoring programs. Their increase can be used to track coral mortality events on reefs (past and future), and also can be used as another major bioindicator of health on coral reefs.

Bioerosion is a major process that affects the carbonate balance on coral reefs, and excavating sponges from the genus Cliona are some of the most important bioeroders on Caribbean reefs. The orange boring sponge, Cliona delitrix, is an abundant excavating sponge offshore southeast Florida that frequently colonizes dead portions of live stony corals, killing live coral tissue as it grows. With the recent decline in coral cover attributed to combined environmental and anthropogenic stressors, the increasing abundance of excavating sponges poses yet another threat to the persistence of Caribbean coral reefs. In the first part of this study, I explored distributional patterns of C. delitrix offshore southeast Florida and compared yearly sponge growth/corresponding coral tissue loss rates across habitats of different depths. C. delitrix densities and growth rates were significantly higher on the outer reef, where coral colonies also showed some of the fastest tissue retreat rates. More sponge individuals were found on sites with higher coral densities, likely resulting from the higher availability of preferred coral skeleton substrate. C. delitrix showed a clear preference for boulder stony coral species, which could alter the coral community composition in the future and allow an increase in branching and foliose species. The growth rates of C. delitrix offshore southeast Florida are slower compared to rates from other locations, likely a result of intense fouling of the coral-sponge interface by other spatial reef competitors. These results suggest that outer reef sites with high boulder coral density offshore southeast Florida are most vulnerable to C. delitrix colonization and may continue to suffer the greatest impacts of coral bioerosion. Excavating sponges are also strong competitors for space on coral reefs; able to colonize, excavate, and kill entire live stony corals. Despite the known negative effects of excavating sponges on stony corals very few studies have experimentally tested the competitive nature of this interaction. In the second part of this study, I examined the effect of manual removal of the excavating sponge, Cliona delitrix (Pang 1973), on tissue loss of the stony coral Montastrea cavernosa (Linnaeus 1767), and its possibility as a restoration technique. A total of 33 M. cavernosa colonies colonized by small C. delitrix sponges (up to 10 cm in diameter) were examined. Sponges were removed using a hammer and chisel from 22 of the affected colonies, and 11 colonies were left alone as controls. After sponge removal, the resultant cavities in the coral skeletons were filled to minimize future colonization by other bioeroders and promote coral tissue growth over the excavation. Cement was used as fill material on 11 of the colonies, and the remaining 11 cavities were filled with epoxy. Standardized photos of each colony were taken immediately, at 6 months and 12 months after sponge removal. Results show a significant reduction in coral tissue loss in colonies where sponge was removed, and both fill materials performed similarly reducing coral tissue loss. I also found that a majority of experimental corals showed no return of C. delitrix to the colony surface a year after removal. This study demonstrates that eliminating the bioeroding sponge competitor may promote recovery of the affected stony coral. Additionally, the sponge removal technique can be applied to any stony coral colonized by C. delitrix to preserve, or at least slow the loss of, remaining live tissue.