Description: Thinly encrusting with irregular outlines when growing on rock. Can become a thick cushion when growing on living Queen scallops Aequipecten opercularis (though possibly this is a separate species). Alive, the encrusting form is normally pale yellow or green. The form on Aequipecten varies from brown to purple or "vivid red". When preserved it is "yellow or brown". Soft, easily crushed. Oscules are clearly visible in living sponges, slightly raised above the general level of body, with transparent rims which collapse on collection. In thinly encrusting specimens transparent excurrent channels can be seen converging on oscules. Cushions on Aequipecten may only have a single oscule.

Habitat: Littoral to at least 40m, encrusting mainly on Pecten and Aequipecten. On rock faces and wrecks in both sheltered and exposed conditions. Apparently fast growing. "Also on Laminaria and hydroids."

Distribution: Common along the Channel coasts of "British Isles; France". Also "Belgium"; Atlantic coasts of "France; Spain"; and "Mediterranean". Common on Aequipecten in Strangford Lough and Scottish sea lochs. The encrusting form has been found in Strangford Lough entrance, in Lough Hyne and at St Kilda.

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

iNaturalist: Mycale macilenta at iNaturalist World Species Observations database.