Selective deep water coral bleaching occurs through depth isolation.

Climate change is degrading coral reefs around the world. Mass coral bleaching events have become more frequent in recent decades, leading to dramatic declines in coral cover. Mesophotic coral ecosystems (30-150 m depth) comprise an estimated 50-80 % of global coral reef area. The potential for these to act as refuges from climate change is unresolved. Here, we report three mesophotic-specific coral bleaching events in the northern Red Sea over the course of eight years. Over the last decade, faster temperature increases at mesophotic depths resulted in ~50 % decline in coral populations, while the adjacent shallow coral reefs remained intact. Further, community structure shifted from hard coral dominated to turf algae dominated throughout these recurrent bleaching events. Our results do not falsify the notion of the northern Red Sea as a thermal refuge for shallow coral reefs, but question the capacity of mesophotic ecosystems to act as a universal tropical refuge.

Coral fluorescence: a prey-lure in deep habitats.

Fluorescence is highly prevalent in reef-building corals, nevertheless its biological role is still under ongoing debate. This feature of corals was previously suggested to primarily screen harmful radiation or facilitate coral photosynthesis. In mesophotic coral ecosystems (MCEs; 30-150 m depth) corals experience a limited, blue-shifted light environment. Consequently, in contrast to their shallow conspecifics, they might not be able to rely on photosynthates from their photosymbionts as their main energy source. Here, we experimentally test an alternative hypothesis for coral fluorescence: a prey-lure mechanism for plankton. We show that plankton exhibit preferential swimming towards green fluorescent cues and that compared to other morphs, higher predation rates are recorded in a green fluorescing morph of the mesophotic coral Euphyllia paradivisa. The evidence provided here - that plankton are actively attracted to fluorescent signals - indicates the significant role of fluorescence in amplifying the nutritional sink adjacent to coral reefs.