Effect of the 1995-1996 bleaching event on polyp tissue depth, growth, reproduction and skeletal band formation in Montastraea annularis

Polyp tissue depth, skeletal extension rate, reproduction and density band formation were studied in 15 colonies of Montastraea annularis from October 1994 to September 1997 as part of a long-term field study. With the commencement of a bleaching event in September 1995, the effects of bleaching on these biological parameters were examined. Bleaching affected all the parameters investigated, but the speed of recovery of each parameter to pre-bleaching values differed. Tissue depth recovered the fastest, within 4 to 8 wk of regaining normal pigmentation. Once pigmentation and tissue depth had recovered, skeletal growth resumed at its pre-bleaching rate. Reproduction took the longest to recover and reproductive failure occurred in severely bleached colonies 1 yr after bleaching. Even after 2 yr, reproductive output remained reduced in previously bleached colonies when compared with previously unbleached ones, although all colonies had successfully completed gametogenesis and spawning. Among the reproductive parameters measured, gonad size recovered faster than the number of gonads per polyp (i.e. polyps produced a smaller number of full-sized gonads 2 yr after bleaching). In all colonies, bleached and unbleached, the 1995 annual high-density (HD) band, which formed at the time of the bleaching event, appeared more prominent and in some cases wider than the HD bands for the years that preceded and succeeded bleaching. Bleaching reduced skeletal extension rate, and the HD band that formed at the time of the bleaching event was most pronounced in colonies in which skeletal extension rate was reduced for the longest period of time (i.e. severely bleached colonies). In 1996, the year following the bleaching event, colonies that did not complete gametogenesis (i.e. severely bleached colonies) produced no distinct HD band, while those that completed gametogenesis (i,e. mildly bleached colonies) produced a distinct HD band. These findings suggest that an inverse relationship between skeletal extension rate and HD band prominence may occur in Montastraea annularis, and that the failure of severely bleached colonies to produce a HD band in 1996 might be related, not to bleaching, but rather to the lack of gonad development and spawning in these colonies.