This encrusting sponge may grow as a thin sheet or cushion up to 14 mm thick when contracted and often forms patches up to 30 cm across. The colour is very variable from grey through to dark red. The amount of pigment present generally reflects prevailing water clarity and illumination levels. On the west coast of Ireland it is normally bright red, while in turbid water in Strangford Lough it is pale pink. The surface is smooth to slippery, with circular depressions containing inhalant pore sieves similar to those in Hemimycale. This species could be confused with Hemimycale columella but its raised oscular rims, conspicuous oscules and the lack of white rims to the uniformly sized depressions make it quite easy to distinguish from Hemimycale.
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