Abstract. Most studies on coral reefs have focused on shallow reef (,30 m) systems due to the technical limitations of conducting scientific diving deeper than 30 m. Compared to their shallow-water counterparts, these mesophotic coral reefs (30-150 m) are understudied, which has slowed our broader understanding of the biodiversity, ecology, and connectivity of shallow and deep coral reef communities. We know that the light environment is an important component of the productivity, physiology, and ecology of corals, and it restricts the distribution of most species of coral to depths of 60 m or less. In the Bahamas, the coral Montastraea cavernosa has a wide depth distribution, and it is one of the most numerous corals at mesophotic depths. Using a range of optical, physiological, and biochemical approaches, the relative dependence on autotrophy vs. heterotrophy was assessed for this coral from 3 to 91 m. These measurements show that the quantum yield of PSII fluorescence increases significantly with depth for M. cavernosa while gross primary productivity decreases with depth. Both morphological and physiological photoacclimatization occurs to a depth of 91 m, and stable isotope data of the host tissues, symbionts, and skeleton reveal a marked decrease in productivity and a sharp transition to heterotrophy between 45 and 61 m. Below these depths, significant changes in the genetic composition of the zooxanthellae community, including genotypes not previously observed, occur and suggest that there is strong selection for zooxanthellae that are suited for survival in the light-limited environment where mesophotic M. cavernosa are occurring.

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Some coral species have extremely long life spans. Many corals have the capacity to regenerate from fragments or pieces of tissue; thus, they rely, at least in part, on asexual reproduction for the maintenance of their populations. As a first step in understanding the mechanism underlying the long life span of corals and their possible rejuvenation during asexual reproduction, we studied whether the somatic tissues of the coral Galaxea fas cicularis had telomerase activity. We quantified the level of telomerase activity in various parts of polyps using stretch PCR. The outer and inner wall tissues of the polyps showed similar levels of telomerase activity, which were comparable with that of HeLa cells, on a unit protein basis. Because the outer wall tissue does not contain gonads, it is suggested that somatic tissues of the coral possesses telomerase activity. Telomerase activity was detected throughout the year in the polyps, regardless of the gonad maturation stage. This study reports that the somatic tissue of the coral has telomerase activity.