Cold Water Coral Grows In The Dark And Grows Slowly

Coral is usually pictured in warm shallow water. A large share of the world's coral lives deep, cold and entirely without light, on structures built over thousands of years.

No light means no algal partner

Tropical reef corals house algae in their tissues and obtain much of their energy from them. That partnership requires sunlight and therefore shallow water.

Cold water corals have no such partner. They capture food directly from the water with their tentacles, which frees them from any depth limit imposed by light.

They occur far below the reach of the sun, in cold water on continental slopes, seamounts and the edges of submarine canyons.

Current supplies the food

Without light, the constraint becomes delivery. These corals grow where currents concentrate sinking organic particles and drive them repeatedly past the polyps.

This is why they cluster on ridges, mounds and steep breaks in slope. Flow accelerates over such features, and the same spot is supplied continuously.

The colonies then modify the flow themselves, creating turbulence that traps more material, so an established mound tends to keep the conditions that formed it.

Structure is the reason they matter

The corals build a hard, branching framework, and dead skeleton beneath the living surface accumulates into mounds standing well above the surrounding seabed.

That framework provides shelter and attachment on a plain that otherwise offers neither. Sponges, brittle stars, crustaceans and fish concentrate around it.

Several commercially important fish species aggregate on these grounds, which is precisely why the areas attracted fishing effort in the first place.

Growth is measured in millimetres a year

Cold water slows metabolism, and these colonies extend at rates of a few millimetres annually. Individual colonies can be centuries old and mounds far older.

Damage is therefore effectively permanent on any human timescale. A framework broken in an afternoon will not be rebuilt within a lifetime, or many lifetimes.

Bottom trawling has been the main cause, since gear dragged across a mound reduces a three-dimensional structure to rubble in a single pass.

Protection depends on knowing where they are

These grounds are difficult and expensive to locate, requiring acoustic mapping and remote vehicles. Much of the deep seabed has never been surveyed at this resolution.

Closures can only be drawn around mapped features, so unmapped reefs receive nothing, and the mapping often follows fishing rather than preceding it.

Deep water also changes chemically as it absorbs more carbon dioxide, which makes it harder for these corals to build skeleton at all, independently of any physical damage.