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This biotope complex occurs on wave-sheltered, variable salinity bedrock and cobbles, subject to moderately strong to weak tidal streams. This complex contains a suite of sponges able to tolerate the variable salinity conditions like Hymeniacidon perlevis, Suberites ficus, Halichondria panicea, Halichondria bowerbanki, Cliona celata and Leucosolenia botryoides. The barnacle Balanus crenatus is frequently recorded in this complex. A sparse hydroid/bryozoan turf composed primarily of Nemertesia antennina, Nemerteis ramosa, Plumularia setacea, Alcyonidium diaphanum and Crisularia plumosa is often recorded. Other species recorded are the ascidians Clavelina lepadiformis, Morchellium argus and Dendrodoa grossularia, the anemones Metridium senile and Sagartia troglodytes, the starfish Asterias rubens and the crab Carcinus maenas. Two biotopes have been identified within this complex: CuSpH (cushion sponges with hydroids) and HbowEud.

This sub-biotope typically occurs in a mixture of turbid, full and variable salinity water, on wave-sheltered and moderately exposed bedrock or boulders. Tidal streams are typically moderately strong but may vary considerably. This sub-biotope occurs in relatively shallow water (typically 5m to 11m water depth) and is dominated by cushion sponges, hydroids and ascidians. On the silty, rocky substrata, large growths of sponge are usually associated with this biotope (Suberites ficus, Hymeniacidon perlevis, Cliona celata, Halichondria panicea, Raspailia ramosa). The tasselled form of Amphilectus fucorum is also notably present. Other epifauna present includes silty hydroids such as Nemertesia antennina, Nemertesia ramosa, Plumularia setacea, Hydrallmania falcata and Halecium halecinum. Individual colonies of dead mans fingers Alcyonium digitatum and plumose anemones Metridium senile may be seen attached to the tops of boulders and ridges. At some sites, whole sides of rocks may be colonised by the anemones Cylista elegans, Sagartia troglodytes and Actinothoe sphyrodeta. Within crevices in the rocky substratum and at the base of boulders Urticina felina and Cereus pedunculatus may be found. Ascidians such as Clavelina lepadiformis, Morchellium argus, Dendrodoa grossularia, Diplosoma listerianum and Distomus variolosus may all be observed. Other ubiquitous species which may be recorded include Polydora, terebellid worms, Balanus crenatus, Alcyonidium diaphanum and Asterias rubens.

This biotope typically occurs on circalittoral mixed substrata (bedrock, boulders, cobbles, pebbles and gravel) in the moderately strong, tide-swept narrows near the entrance of Loch Etive, although not in the extremely tide-swept Falls of Lora. This sea loch is unique in having a substantial freshwater input from the surrounding moorland, yielding the most brackish, large sea loch in Scotland. Large growths of the brackish-tolerant sponge Halichondria bowerbanki cover the cobble and boulder seabed, interspersed with Amphilectus ovulum, the hydroid Eudendrium arbuscula and the bryozoan Alcyonidium diaphanum which are particularly characteristic of these conditions. Tufts of the bryozoan Eucratea loricata are occasional in most areas. Other species recorded include Carcinus maenas, Asterias rubens, Crossaster papposus, Buccinum undatum, Pagurus berhardus, Henricia spp., Onchidoris bilamellata and Palio dubia, tolerant of the low salinity, are found in the circalittoral throughout this area. Ascidians such as Ascidiella scabra and Corella parallelogramma may also be present. A very impoverished low salinity version is present in the upper basin of Loch Etive. The biotope CuSpH is similar in several respects to this biotope and will develop in less brackish situations where species-richness is generally greater.

This biotope is found in variable salinity environments and tends to occur on the upper faces of circalittoral bedrock and boulders, in sheltered sites subject to moderately strong tidal streams. This biotope is characterised by aggregations of cushion sponges such as Hymeniacidon perlevis, Halichondria panicea, Halichondria bowerbanki and Cliona celata, other sponges (Leucosolenia botryoides and Suberites ficus) along with occasional hydroid tufts of Nemertesia antennina, Nemertesia ramosa and Plumularia setacea. Other species that may be present include the colonial ascidians Clavelina lepadiformis and Morchellium argus, Dendrodoa grossularia, the anemones Metridium senile and Sagartia troglodytes, the barnacle Balanus crenatus, Asterias rubens, Carcinus maenas and Crisularia plumosa. Two variants of this biotope have been recorded: CuSpH.VS and CuSpH.As.

This sub-biotope typically occurs in turbid, variable salinity water, on wave-sheltered bedrock in estuaries subject to strong tidal regimes where circalittoral communities occur in relatively shallow water (typically 5m to 8m water depth). Cushion sponges, hydroids and ascidians dominate the biotope. Large growths (often up to 50cm across) of the sponges Halichondria panicea mixed with Halichondria bowerbanki almost entirely cover the bedrock, appearing in places like a continuous cushion. Haliclona oculata, Suberites ficus, Leucosolenia botryoides, various hydroids such as Plumularia setacea, Nemertesia antennina, Nemertesia ramosa and various bryozoans such as Crisularia plumosa, Bugulina turbinata and Amathia pustulosa protude through the Halichondria spp. sponge growth. Colonial ascidians such as the lightbulb ascidian Clavelina lepadiformis and Morchellium argus may also be observed. Other more ubiquitous species include Balanus crenatus, Carcinus maenas, Asterias rubens, Metridium senile, Cylista elegans and Ophiothrix fragilis.