An experimental field evaluation of healing in damaged, unbleached and artificially bleached star coral, Montastraea annularis

To examine the effects of bleaching on the ability of coral to resist algal invasion and to regenerate damaged tissue, ramets of Montastraea annularis from a single unbleached genet were broken from narrow bases underwater an d separated into four treatments(12 injured and artificially bleached; 12 i njured and unbleached; six artificially bleached controls; and six unbleach ed controls). The unbleached treatments were left in ambient light and the artificially bleached treatments were shaded until the ramets were white (z ooxanthellae numbers reduced 98%, confirmed histologically). The injured tr eatments were then rasped with forceps, damaging a 2.27 cm(2) +/- 0.06 area on each ramet and leaving shredded tissue in place. The controls were left undamaged. After 120 d, all of the unbleached damaged ramets regenerated a nd healed the damaged area, while only four of the bleached damaged ramets recovered. Unbleached ramets healed faster (12 ramets healed by 80 d, mean = 64 +/- 12 d) than bleached ramets (4 ramets healed by 90 d, mean = 79 +/- 9 d). Two of the bleached damaged ramets died and the remaining six did no t heal but regained full coloration by 120 d. Only the damaged areas of 11 of 12 bleached damaged ramets became covered with sediment and then coloniz ed by algae. None of the other ramets became sedimented or were colonized b y algae. Bleaching clearly debilitated some of the ramets since they were l ess able to heal the damaged area. The results suggest that symbiotic zooxa nthellae in normal densities may facilitate (possibly by providing energy f or mucus production and ciliary action) sediment removal, prevent algal col onization and promote tissue regeneration and healing. In addition, the met hod of bleaching used in this study causes the least amount of stress to th e coral compared to bleaching in field conditions ("natural" bleaching), th erefore the negative effects of natural bleaching must certainly be greater than those revealed in this study.