Theses and Dissertations

Defense Date

12-10-2014

Document Type

Thesis

Degree Name

M.S. Marine Biology

Second Degree Name

M.S. Coastal Zone Management

Department

Oceanographic Center

First Advisor

David S. Gilliam

Second Advisor

Jose V. Lopez

Third Advisor

Bernhard Riegl

Abstract

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.

Comments

This work was partially supported by funding from a contract with the Broward County Marine Resources Section. Sponge removal was conducted in accordance with Special Activity License Number: SAL-11-1358-SRP, granted by the Florida Fish and Wildlife Conservation Commission.

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