The secrets of coralligenous habitats

Imagine a wall covered in sea fans, sponges and many small animals.

You might think that all these living things simply settled on a rock.

But part of the structure was itself built by living organisms.

Small calcareous red algae make a hard material containing calcium carbonate.

Over time, layers accumulate and gradually form the coralligenous habitat.

Organisms make and add hard material.

Other organisms cover some surfaces and help strengthen the whole structure.

New generations grow on top of earlier ones.

They can develop on a large vertical wall.

Animals live on the walls, in cracks, beneath overhangs and inside cavities.

Large sea fans sometimes give the impression of an underwater forest.

Coralligenous habitats can also develop on more horizontal seabeds.

They then form bumps, blocks, hollows and cavities.

In both cases, the landscape has been slowly built by living things.

Red, orange, yellow, pink, purple…

Coralligenous habitats can be very colourful.

Yet underwater, some colours become difficult to see with increasing depth.

Red and orange in particular become much darker.

When a diver switches on a torch, these colours appear again.

They were already there: the torchlight simply makes them easier to see.

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