But this 'rock' is not always ordinary rock.
Part of the structure we see is built by living organisms, layer by layer, over centuries and sometimes millennia.
This is the coralligenous habitat.
Contrary to what its name might suggest, corals are not its main architects. The essential role belongs to inconspicuous encrusting calcareous red algae able to deposit calcium carbonate in their tissues.
Accumulating generation after generation, they help build a complex mineral structure on which an extraordinary diversity of organisms settles.
Observing a coralligenous habitat therefore means looking simultaneously at a landscape, an ecosystem and an ongoing biological construction.
BUILDING
Building organisms gradually add material to the formation.
STRENGTHENING
STARTING AGAIN
The coralligenous structure thus grows through successive accumulation.
WALL CORALLIGENOUS HABITATS
Organisms can colonise walls, overhangs, cracks and cavities.
The structure becomes very complex.
Sea fans, sponges, bryozoans, sea squirts and other upright organisms then turn the formation into a truly three-dimensional landscape.
PLATFORM OR BANK CORALLIGENOUS HABITATS
Light and especially sedimentation do not act in exactly the same way as on a vertical wall.
The profile may be lower, irregular and broken into blocks, bumps, depressions and cavities.
But the principle remains the same: biological construction, slow growth and continual competition for space.
Red, orange, yellow, pink, purple… Coralligenous habitats are among the Mediterranean's most colourful landscapes, even though they develop in dim light.
These colours are not necessarily intended to be seen. They come from pigments that may support photosynthesis in algae, help protect cells or play a part in various biological mechanisms in sponges, sea fans, sea squirts or bryozoans.
And contrary to a common misconception, fish do not all see in the same way. Their colour perception varies among species and, above all, with the available light.
At depth, reds and oranges are strongly attenuated and appear dark. A diver's torch reintroduces these wavelengths, revealing a palette that was present but had become almost invisible.
The torch does not create the colours of the coralligenous habitat: it reveals them.
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