IS THE ACROPORA PROLIFERA HYBRID NEW? A CASE STUDY FROM THE EARLY HOLOCENE ENRIQUILLO VALLEY

Acropora spp. corals are among the most threatened species in the Caribbean, arguably suffering up to 98% mortality since monitoring began in the 1970s. Acropora prolifera, the F1 (first filial generation) coral hybrid of Acropora cervicornis and Acropora palmata, has been increasingly identified in environments once solely dominated by the parent species. It has been suggested that the hybrid might be better suited to recent environmental conditions than both parent species, although the timing of initial hybridization is still in question. Has A. prolifera recently emerged in response to anthropogenic changes in climate and the lethality of White Band Disease? Or did the hybrid emerge long ago as a function of natural evolutionary processes without being recognized as taxonomically distinct from A. cervicornis until recently? To complicate matters, while these corals can be distinguished at the genetic/molecular level, there is some degree of morphological plasticity in the hybrid. This study examined 214 samples previously collected from a 9-meter-thick stratigraphic section from a virtually monospecific A. cervicornis-dominated outcrop (dated from 9.38-7.32 ka) in the Enriquillo Valley, Dominican Republic, to determine if any A. prolifera specimens had previously been misidentified as A. cervicornis. Each coral fragment was checked for distinct morphologies of A. prolifera-like traits and assigned a value on a relative scale for coral thickness (diameter), taphonomic grade, and morphology (i.e. corallite spacing, tip shape) to assess the survivability of features that might aid the identification of these similar coral fossils. Although initial geochemical data suggest changing environmental conditions during coral deposition leading up to the Holocene Thermal Maximum, across the 214 samples, no A. prolifera fossils could be identified. This suggests a lack of A. prolifera within this region in the early and mid-Holocene.