The Ivory Tree Coral Oculina varicosa, forms extensive bioherms (reefs) of azooxanthellate colonies at depths of 70-100m along the edge of the Florida Hatteras slope. Deepwater Oculina reefs support invertebrate and fish communities as diverse as those of tropical coral reefs, and are a critical spawning habitat for a number of commercial fisheries species. A different morphological variant of O. varicosa inhabits the near-shore limestone ledges, and these shallow-water colonies were included in the study as an ecological comparison of conspecific populations exploiting different habitats, and as a preliminary reproductive model for the less accessible deep reefs. Colonies from the two populations were confirmed as conspecific using Internal Transcribed Spacer (ITS) sequences of the nuclear ribosomal gene from deep and shallow colonies.

Growth rates (skeletal deposition) of samples from both populations were measured under varying temperature and light regimes. The results showed a significant difference between the two populations under different light conditions. Temperature had no significant effect on growth however. This warrants further investigation, since photosynthetically enhanced calcification did confer an advantage to the shallow zooxanthellate samples, but temperature had no consistent effect on calcium deposition.

Oculina varicosa is a gonochoristic broadcast spawning species, with small eggs (<100 micro-m) and a high fecundity of approximately 4,000-8,000 eggs per cm-2 of skeletal surface area for both populations. The gametogenic cycle begins in the early summer and spawning occurs during late summer and fall, with no obvious relationship to lunar or tidal phase. Histological analysis of gonad sections revealed concurrent gametogenic cycles in both populations; however, the deeper populations generally spawned later (September) than their shallow counterparts (July-August).
Embryos developed quickly (<6 hours at 25oC), producing small (160 micro-m), active planulae, which swim to the water surface immediately on hatching. After 24-48 hours the planulae begin to swim throughout the water column, after a week they become demersal and settlement occurred 21 days after spawning. Planktonic duration was integrated with hydrodynamic information to estimate larval dispersal potential. It appears that larvae not only have the potential to be transported between the deep reef tracts, but may also contribute larvae to near-shore zooxanthellate populations during summer upwelling events.

The deep banks have been under federal protection since 1984, but large areas of this important habitat have been destroyed. The objective of this study was to describe aspects of O varicosa, reproductive ecology and provide insight into recovery potential of the damaged reef. The coral produces many highly dispersive larvae, but re-colonisation of damaged areas is extremely slow. Three-dimensional modeling of larval transport, and molecular studies of clonal variability within the systems would provide valuable information on potential natural recruitment rates.