Description of [Flustra foliacea] and colonial ascidians on tide-swept exposed circalittoral mixed substrata
This variant is typically found on very exposed to moderately exposed, circalittoral mixed substrata subject to moderately strong tidal streams. It most frequently occurs between 10m and 20m water depth. This variant is characterised by a dense hydroid and Flustra foliacea turf, along with other scour-tolerant species, growing on the more stable boulders and cobbles which overlie coarse muddy sand and gravel. Although Nemertesia antennina is the dominant species within the hydroid turf, other species such as Halecium halecinum, Nemertesia ramosa and Hydrallmania falcata may also be present. Other bryozoans found amongst the hydroid and Flustra turf include Cellepora pumicosa, Bugula flabellata, Bugula turbinata, and a crisiid turf. Encrusting red algae, the polychaete Pomatoceros triqueter and barnacles such as Balanus crenatus may be found on the smaller cobbles and pebbles, which may become mobile during extreme storms. Echinoderms such as Asterias rubens and Ophiothrix fragilis may be present on the boulders, or the coarse sediment in between. On the larger, more stable boulders, isolated sponge communities may develop, with species such as Scypha ciliata, Dysidea fragilis, Hemimycale columella, Esperiopsis fucorum and Stelligera rigida. In addition, small Alcyonium digitatum, various ascidians (Clavelina lepadiformis, Botryllus schlosseri), Pododesmus patelliformis and top shells (Calliostoma zizyphinum, Gibbula cineraria) may colonise the upper faces and vertical sides of larger boulders. At some shallower sites, the foliose red algae Hypoglossum hypoglossoides may be found on the tops of larger boulders. Within the coarse sediment underlying these boulders and cobbles, anemones such as Cerianthus lloydii and Urticina felina may be recorded. Under-boulder fauna typically consists of terebellid worms, and crabs such as Pisidia longicornis and Cancer pagurus. Situation: This variant is found in wave-exposed locations, resulting in kelp forest in the infralittoral zone being dominated by Alaria esculentus and Laminaria hyperborea. When the substratum becomes rockier, this biotope will graduate into the variant A4.1342.