Appearance of an anomalous black band disease at upper mesophotic depths after coral bleaching.

Abstract

In the summer of 2016, extensive coral bleaching occurred on the coral reefs of Okinawa, Japan, which was often lethal in shallow waters. In November 2016, after the coral bleaching event, an anomalous black band disease (ABBD) was noticed during regular monitoring of the Pachyseris-dominated upper mesophotic Ryugu Reef in Okinawa. We subsequently conducted 10 × 1 m belt transects in deeper (30-35 m depth, n = 5) and shallower areas (27-29 m, n = 7) of the reef and obtained data on coral percentage coverage, Pachyseris percentage coverage, occurrence (numbers of lesions) of ABBD, and ABBD percentage coverage on Pachyseris. Both depths showed high live coral coverage (>90%), indicating little mortality from the summer 2016 bleaching event, and Pachyseris percentage coverage was significantly higher in the deeper area (mean ± SD = 48.6 ± 45.0%) compared to the shallower area (5.1 ± 5.0%). Additionally, although numbers of ABBD occurrences (= lesions) were significantly higher in the deeper area (81.0 ± 52.8 as opposed to 8.3 ± 6.7 at shallower depths), total ABBD percentage coverage on Pachyseris was not significantly different between shallow and deep areas (7.0 ± 3.2% versus 4.7 ± 3.6%). ABBD was observed to be Pachyseris-specific at Ryugu Reef. These results indicate that similar to shallower reefs, mesophotic reefs can experience increased disease occurrences subsequent to non-lethal coral bleaching events.