Increase of Excavating Sponges on Caribbean Coral Reefs: Reproduction, Dispersal, and Coral Deterioration
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.