Cystoseira mediterranea is a brown (Phaeophyceae) algae of the Fucales order. It is a multicellular organism composed by a thallus, with a single main axis that is soft compared to the outer extremities, and some contain holdfast, stipe, blade and lamina. This species can grow to a height of 40 cm. The organism has a very distinct basal and apical regions. The receptacles, known as pneumatocysts, are air vesicles, filled with air, making the algae to float in strong currents.