The site of the seabeds of Aci Castello is of volcanic nature with rocky substrates, including columnar basalts at medium depths of about 20-40 meters. Photophilic biocoenoses, with Cystoseira, are found in the upper infralittoral, while the Coralligenous with encrusting red algae prevails at greater depths. A Posidonia meadow is located between the island of Lachea and the coast. The Posidonia meadow is well developed between the port of Aci Castello and Acitrezza. There are stretches of rock with encrusting calcareous Rhodophyceae and also sea urchins. Tropical species such as Caulerpa racemosa and Lophocladia lallemandii are present at depths of up to 20-25 meters, colonizing the Sicilian coasts. The site coincides with the Marine Protected Area “Isole Ciclopi”. The vegetal populations of hard substrate in the infralittoral show high biodiversity, with species rare for the Mediterranean like Serpula israelitica. The bryozoans Setosella vulnerata and Setosellina capriensis, which are rare in the Mediterranean, are present in depths of 50-100 meters.