The sea bed at Cap d'Agde contains 2000 ha of posidonia fields benefiting from a protected status at the European level (Natura 2000). It grows between a depth of 35 m and the surface.
It is one of the few underwater meadows of the Languedoc-Roussillon, an indispensable link in the food chain, a habitat in which many fish species live and lay their eggs. It is an invaluable breeding ground, also sheltering molluscs, for instance the world's second biggest shell : the noble pen shell, a giant mussel that lives most often among the sea-grass, with its tip fixed in the sand.
The "Direction du Milieu Marin de la Ville d"Agde" takes care of the protection of the posidonias of Le Cap d'Agde.