Le Scléractiniaire Dendrophyllia cornigera en eau peu profonde, à Ouessant (Bretagne, Atlantique NE) en l'absence de barrière thermique

During a diving survey at Ushant island, the Scleractinian Dendrophyllia cornigera (Lamarck) was discovered at a depth of 30 m, below the laminarian zone. This is the shallowest record at present of this species, which is better known from the rocky bottoms between 200 and 500 m. Conditions favouring this occurrence in shallow water, as well as the occurrence of the species around 60-90 m in other coastal sites (Sein, Audierne, Penmarc'h, Glenan, Belle-Ile, Vigo), are examined, with special reference to temperature. D. cornigera tolerates temperatures ranging from 8 degrees C at its northern limit in the SW of Ireland, to 14 degrees C at its southern limit, in the Cape Verde Islands. Within its entire distribution range, the species displays some stenothermy, with an annual temperature variation ranging from 1-2 degrees C in the deep offshore sites to 2-3 degrees C in the shallow coastal sites. This narrow temperature range in shallow waters results from the upward movement of cold water: the Galician upwelling, the south Armorican cold pod (''bourrelet froid''), the Ushant thermic front. At Ushant island, strong currents, eddies and swell all increase this phenomenon. However, the location of D. cornigera in the east of the island, appears to be related to the comparative degree of shelter from the strongest western surges.