The secrets of coralligenous habitats

A coralligenous habitat often looks like rock covered in animals and algae.

Yet part of this 'rock' was built by living things.

Calcareous red algae produce calcium carbonate and form hard crusts.

Over generations, these crusts accumulate. Other organisms settle on top and also contribute to construction.

A coralligenous habitat is therefore a bioconstruction, meaning a structure built largely by living things.

Calcareous organisms gradually add material.

Some organisms cover surfaces and strengthen parts of the structure.

New generations settle on top of earlier ones.

They develop on vertical or steeply sloping walls.

Organisms colonise walls, cracks, cavities and overhangs.

Sea fans, sponges and other organisms make the landscape very complex.

They can also develop on more horizontal surfaces.

The structure then often consists of blocks, bumps, depressions and cavities.

Light and sediment deposits do not act in exactly the same way as on a vertical wall.

Red, orange, yellow, pink or purple: coralligenous habitats can be very colourful.

These colours come from pigments in the different organisms.

But underwater, not all colours remain equally visible.

With increasing depth, reds and oranges are strongly reduced and look much darker.

When a diver switches on a torch, these colours become visible again.

The torch therefore does not create the colours: it allows us to see them again.

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