Multifocal bleaching of Millepora dichotoma in the Gulf of Aqaba (Eilat), Red Sea

The zooxanthellate hydrocoral Millepora dichotoma is an important framework builder of shallow reefs in the Gulf of Aqaba (Red Sea). In this study, we document and characterize the emergence of multifocal bleaching (MFB), a new syndrome in M. dichotoma. MFB is characterized by bleached spots scattered over the colony. The establishment of MFB in M. dichotoma at the study site was surprisingly rapid. We first noticed this phenomenon and assessed its prevalence in February 2010, and re-assessed it 1 yr later, in February 2011. The prevalence of MFB along the reef crest at a depth of 0.5 m is extremely high (63 and 67% in 2010 and 2011, respectively), significantly higher than along the fore-reef at a depth of 2.5 m (16 and 31% in 2010 and 2011, respectively). Spatial analyses were used to characterize the distribution of MFB across 2 spatial scales on the reef: among-transects scale (<800 m) and within-transect scale (<60 m). The results clearly show that the affected hydrocorals are aggregated over these 2 scales. While several factors may contribute to the observed pattern, the aggregated spatial distribution of the affected hydrocorals suggests that local and inter-colonial transmission play a significant role in the spread of MFB.