Description: This sponge may be massive-lobose, hemispherical to irregularly rounded and up to 30cm or more across. It is usually grey to off-white in colour but may be completely white in dark caves. Rounded evenly spaced oscules are obvious all along the tops of the ridges.

Habitat: Common on bedrock and stable boulders from the littoral to 300m. A common species in sites with strong water movement, but also found under overhangs in extreme shelter.

Distribution: A common species throughout most of the British Isles but apparently absent from North sea coasts.

Similar Species: Unlikely to be confused with other species.

Key Identification Features:

• The slate grey colour.
• Rounded lobes and evenly spaced oscules.

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

iNaturalist: Pachymatisma johnstonia at iNaturalist World Species Observations database.