Description: A thin somewhat slimy sheet on rocks or shells. Patches up to 15cm diameter have been seen, 2-3mm thick. Usually grey-brown. Also reported to be sometimes pale or deep "yellow, orange-yellow, violet or bright red". Surface : With pores scattered fairly evenly all over. Smooth. Obvious excurrent channels may be present. Oscules scattered over surface of sponge, with conical raised edges.

Habitat: Very common, e.g. on valves of Aequipecten opercularis and other shells; rock surfaces especially in the kelp zone.

Distribution: "Arctic, Atlantic coast of Europe, Mediterranean" Known recently from E. Scotland, St. Abbs; W. Scotland, Tiree; Strangford Lough; Lough Hyne, Co. Cork; Saltee Is., Co. Wexford.

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

iNaturalist: Hymedesmia brondstedi at iNaturalist World Species Observations database.