Description of biotope or habitat type

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   Polyclinum aurantium and Flustra foliacea on sand-scoured tide-swept moderately wave-exposed circalittoral rock

Description

This variant is typically found on the upper face of moderately exposed, moderately tide-swept, circalittoral bedrock or boulders. Sand and silt are periodically re-suspended in the water column, resulting in scour-tolerant species being characteristic of these areas. There is a dense covering of the scour-resistant bryozoan Flustra foliacea attached to the bedrock plains and boulders. The colonial ascidian Polyclinum aurantium commonly covers the rock surface at most locations within this biotope - itself incorporating sand grains into its surface to give it the appearance of sandy rock nodules. Other ascidians that may occur in this crust are the flat, encrusting colonial Botrylloides leachii, Botryllus schlosseri and the colonial ascidian Clavelina lepadiformis, although in varying quantities at each location. A short turf of other bryozoans such as Alcyonidium diaphanum, Crisularia plumosa and Bugulina flabellata occur amongst the ascidians. Other species found in this biotope are the sponges Cliona celata, Leucosolenia botryoides and Sycon ciliatum, the hydroids Tubularia indivisa, Nemertesia antennina, Halecium halecinum and the anthozoans Alcyonium digitatum and Urticina felina. Echinoderms which may be present include the starfish Asterias rubens, Crossaster papposus and the brittlestar Ophiothrix fragilis. Crustaceans such as the crab Cancer pagurus, the hermit crab Pagurus bernhardus and the lobster Homarus gammarus may be observed in crevices and under boulders. The palps of the polychaete Polydora spp. may be observed whilst the nudibranch Antiopella cristata may be seen preying on the hydroid/bryozoan turf. This variant is commonly found on the Northumberland coast, Flamborough Head and the Lleyn Peninsula.

Situation

As FluCoAs.Paur tends to occur in waters with a sediment load, the resulting light penetration is reduced, meaning that kelp forests such as Lhyp, normally found shallower than FluCoAs.Paur in the infralittoral zone, are present over a narrower depth range.

Temporal variation

Not known

Characterising species

Similar biotopes or habitat types

CR.HCR.XFa.Mol

This biotope occurs on more heterogeneous substrata, composed of bedrock ridges with cobbles and sand plains, rather than predominantly bedrock in the case of CR.HCR.XFa.FluCoAs.Paur. Both biotopes contain F. foliacea and a similar hydroid turf, but CR.HCR.XFa.Mol lacks the diversity of sponges and the dense polyclinid ascidians. It is the dense aggregations of Molgula manhattensis which distinguish this biotope from others.

CR.HCR.XFa.FluCoAs.SmAs

This variant occurs on slightly more wave-exposed sites but subject to similar tidal streams. Both biotopes are found on the upper faces of circalittoral bedrock and boulders. Although both biotopes feature F. foliacea as a dominant feature of their fauna, CR.HCR.XFa.FluCoAs.SmAs has a much more diverse associated sponge fauna and lacks the abundant sheets of the colonial ascidian Polyclinum aurantium.

CR.HCR.XFa.FluCoAs.X

This variant occurs on slightly more wave-exposed sites but subject to similar tidal streams. While CR.HCR.XFa.FluCoAs.Paur is found on circalittoral bedrock or boulders, CR.HCR.XFa.FluCoAs.X occurs on mixed substrata (boulders, cobble, pebble and gravel). Although both biotopes feature F. foliacea as a dominant feature of their fauna, CR.HCR.XFa.FluCoAs.X has a much more diverse associated fauna and lacks the abundant sheets of the colonial ascidian Polyclinum aurantium.

CR.MCR.EcCr.FaAlCr.Flu

This biotope is found under similar exposure and tidal stream regimes as CR.HCR.XFa.FluCoAs.Paur but occurs in slightly deeper water depths. CR.MCR.EcCr.FaAlCr is also found on similar rocky substrata as CR.HCR.XFa.FluCoAs.Paur. While F. foliacea is the dominant bryozoan present in both, it is the presence of dense aggregations of Polyclinium aurantium which distinguish CR.HCR.XFa.FluCoAs.Paur from this biotope.

Classification history of this biotope or habitat type