Soft CoralsAlcyonacea

A soft coral holds its shape with seawater.

Push a finger into a broccoli coral and there is nothing solid in there. Instead of the limestone cup a stony coral builds around itself, a soft coral carries thousands of microscopic limestone needles called sclerites scattered loose through its tissue, like gravel through wet concrete, and the rest of the stiffness is pressure. The colony pumps itself full of water and stands up. Every polyp on it has eight feathery tentacles, always eight, and that count is what puts soft corals in the octocorals alongside the sea fans and the sea pens.

It also makes them the only large animals on the reef that change size. A tree coral on a current-swept wall at night is full, bristling and standing well off the rock; by day the same colony has let the water out and hangs there as a slumped rag a fraction of the size, and divers regularly fail to recognize it as the thing they photographed twelve hours earlier.

One of them moves. The pulsating xenid opens and closes its eight tentacles over and over, every polyp beating on its own schedule, all day, without stopping and without going anywhere. Measurements on this species show the pulsing pays for itself: it stirs the thin film of still water sitting on the tissue and sweeps away the oxygen that photosynthesis keeps piling up there, and the algae living inside the coral work better for it. It is the only coral you can watch working.

Soft corals do much of the visual work on 🌊 Red Sea reefs and are usually the thing in the foreground of somebody's photograph: leather corals spread flat over the shallows, broccoli coral in fields on sheltered slopes, and tree corals in pink and red on the walls where the current runs hardest.