Description of Solitary ascidians, including [Ascidia mentula] and [Ciona intestinalis], with [Antedon] spp. on wave-sheltered circalittoral rock
This variant occurs on circalittoral bedrock or boulder slopes in generally wave-sheltered conditions (often in sea lochs) with little tidal flow. It is frequently found on vertical or steeply-sloping rock. Apart from the large ascidians, Ascidia mentula and Ciona intestinalis, the rock surface usually has a rather sparse appearance. Scyphistomae larvae are often present on any vertical surfaces. Grazing by the sea urchin Echinus esculentus leaves only encrusting red algae (giving the bedrock/boulder substratum a pink appearance), cup corals Caryophyllia smithii and the keelworm Pomatoceros triqueter. There may be a few hydroid species present, such as Nemertesia spp. and Kirchenpaueria pinnata, occasional Alcyonium digitatum and occasional Metridium senile. Barnacles Balanus spp. and the colonial ascidian Clavelina lepadiformis also occasionally occur. At some sites, echinoderms such as the crinoid Antedon spp., the starfish Crossaster papposus and Asterias rubens and the brittlestar Ophiothrix fragilis (in low densities) may be found. The squat lobster Munida rugosa is likely to be found in crevices, under boulders, and the hermit crab Pagurus bernhardus may be observed moving around the rock surface. The brachiopod Neocrania anomala is frequently observed (especially where this biotope occurs shallower than unit A4.314 for example). The saddle oyster Pododesmus patelliformis may occasionally be seen attached to the rock/boulder face. Situation: This biotope is typical of one found in sheltered sealochs. On slightly more wave and tide exposed sites, a transition to the more diverse A4.313 biotope will occur. Below A4.3111, you may find the A4.314 biotope (especially situated on the sills of sealochs). Temporal variation: The abundance of C. intestinalis tends to fluctuate seasonally, so it may appear absent at a site at one time of year and then be present at other times, altering the visual appearance of the biotope. Other solitary ascidian species such as A. mentula and Ascidiella aspersa tend to be longer-lived (approximately 7 years and 3 years respectively).