Why deep-sea corals in the Red Sea could be forced into deeper waters

Warm waters run deep

Hundreds of metres down in the Red Sea, the water stays at a balmy 21°C all year round. Most ocean depths are close to freezing. Here, specialised life thrives in total darkness. Because this sea floor is warm, it offers an early glimpse into how global warming could change deep seas elsewhere.

Yet even animals used to warm sea floors have limits. A recent modelling study checked what happens to deep-sea corals and mesophotic corals as carbon emissions rise. Mesophotic refers to the ocean depth zone between 30 and 150 metres where light is dim but still supports life. By linking 653 sightings from six sea cruises with seabed maps, the team built an ecological niche model for four coral groups. An ecological niche model is a computer algorithm that matches species sightings with environmental data to map out viable living space.

The dive to escape the heat

The team tested five future climate scenarios, called Shared Socioeconomic Pathways, or SSPs. These are standard future tracks that model how greenhouse gas emissions may change this century. Under every track, viable space for all four coral groups shrank by 2090–2100. Losses were worst under SSP5-8.5, the highest fossil-fuel path. Under this run, the stony coral Madracis interjecta lost over 45% of its viable range, falling from 29,317 square kilometres to 15,893 square kilometres. The black coral Bathypathes thermophila fared best, losing about 2.3%.

As upper layers heat up, viable zones shift down into the dark. The lace coral Stylaster tritoni shifted from an average depth of 424 metres down to 675 metres under SSP5-8.5. Yet moving down brings little comfort. As these animals move deeper, temperatures at those depths will still climb. Today, shared zones where all four groups thrive cover 16,960 square kilometres, including spots near the Farasan Bank. Under extreme warming, these shared refuges shrink.

Finding the rocky ridges

Corals cannot grow on soft mud. They need hard ground where swift currents bring food. The models showed that seafloor shape was vital. A measure called Vector Ruggedness Measure—a score showing how rough and uneven the seabed is—drove over 64% of living space predictions for the sea fan Acanthogorgia. The team checked model accuracy using the True Skills Statistic, a test grading how well a tool predicts both presence and absence. Scores reached 0.86, showing strong accuracy.

Finding seafloor with the right temperature and slope does not prove corals can live there. While models map viable zones, young corals may fail to cross wide stretches of open sea to settle there.

What we still do not know

Gaps remain. Sightings came mostly from Saudi waters, leaving western parts less well mapped. Deep-sea cameras also view only small patches of floor, giving few sightings. Finally, scientists do not know the exact heat limits of these corals, nor if they can adapt fast enough before 2100.