Reproductive Strategy of Siderastrea radians in the St. Martins Keys, Florida

Flexible reproductive strategies promote reproductive success of corals in the face of climate change, eutrophication, and other pressures. Methods to assess such flexibility include comparative studies across species' geographic ranges. Our study evaluated the reproductive strategy of Siderastrea radians (Pallas, 1766) near the northern edge of its range, the St. Martins Keys, Florida, and compared results to existing data from elsewhere. In 2006 and 2007, surveys indicated that colonies were aggregated (mean Morisita's index = 3.7) at densities of 0.0–86.0 colonies m⁻² (mean = 7.0 colonies m⁻²). Size-frequency distributions were skewed, with most colonies being in the lowest one-fifth of the overall size range. Histological data suggested a gonochoric population, but reproductive mode remains unconfirmed because neither planulae nor spawning were observed. Samples indicated that 50% of female colonies reached sexual maturity at ∼32 mm maximum diameter and sex ratios were not significantly different from 1:1 for aggregations at scales of tens and hundreds of meters. By contrast, Caribbean populations reached puberty at 20 mm and exhibited a female-biased sex ratio. Statistically significant linear regressions showed that densities of solitary polyps (i.e., recruits) increased with increasing densities of all conspecifics, including colonies above and below puberty size. The relationship with colonies above puberty size was weakest, which indicated that settlement and/or survival of planulae increased near any conspecific and suggested that recruitment was not localized around natal colonies. Overall, results pointed to variation in the reproductive strategy of S. radians across its geographic range.