Description: Thin sheet covering rock or bivalve shells, to thin crust 1-5mm thick. Light red, reddish-orange, "blood-red". Brown when dried. Difficult to determine in thin specimens, but rather brittle in thicker ones. Oscules have transparent converging excurrent channels.

Habitat: On bivalve shells on muddy sediments, rocks and boulders in sheltered sites, and also on bedrock in strong tidal streams. Intertidal to 180 m.

Distribution: "N. Atlantic": White Sea to Mediterranean. Coasts of Ireland, southern and western Britain, Channel Isles. Recently known from Calf of Man, Strangford Lough, Lough Hyne, Skomer Is.

Distribution Map from NBN: Microciona armata at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Microciona armata at iNaturalist World Species Observations database.