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Sublittoral stable cohesive sandy muds occurring over a wide depth range may support large populations of semi-permanent tube-building amphipods and polychaetes. In particular large numbers of the amphipods Ampelisca spp. and Photis longicaudata may be present along with polychaetes such as Lagis koreni. Other important taxa may include bivalves such as Nucula nitidosa, Chamelea gallina, Abra alba and Kurtiella bidentata and the echinoderms Echinocardium cordatum and Acrocnida brachiata. In some areas, polychaetes such as Spiophanes bombyx and Polydora ciliata may also be conspicuously numerous. This community is poorly known, appearing to occur in restricted patches. In some areas it is possible that SS.SMu.ISaMUuAmpPlon may develop as a result of moderate organic enrichment. A similar community in mud has also been reported in the Baltic which is characterised by large populations of amphipods such as Ampelisca spp., Corophium spp. and Haploops tubicola (see Petersen 1918; Thorson 1957) and it is not known if SS.SMu.ISaMu.AmpPlon is a UK variant of this biotope. Additionally, in organically enriched areas, the community may be characterised by capitellids and Mediomastus fragilis.

Sublittoral marine sand in moderately exposed or sheltered inlets and voes in shallow water may support large populations of semi-permanent tube-building amphipods and polychaetes. Typically dominated by Crassicorophium crassicorne with other tube building amphipods such as Ampelisca spp. also being common. Other taxa include typical shallow sand fauna such as Spiophanes bombyx, Urothoe elegans, Bathyporeia spp. along with various polychaetes including Parexogone hebes, and Lanice conchilega. Polydora ciliata may also be abundant in some areas. At the sediment surface, Arenicola marina worm casts may be visible and occasional seaweeds such as Saccharina latissima may be present. As many of the sites featuring this biotope are situated near fish farms, it is possible that it may have developed as the result of moderate nutrient enrichment, which may lead the biotope to transition into an organically enriched variant, related to SS.SMx.CMx.KurThyMx. The distribution of this biotope is poorly known and like the muddier SS.SMu.ISaMu.AmpPlon, to which it is related, appears to have a patchy distribution.