Description: Small hemispherical cushions up to 2 cms in diameter and 4 mm high. Consists of erect branched filaments in a mucilaginous matrix. Branching dichotomous.

Habitat: Epilithic or on Ralfsia in the littoral.

Distribution: South-western distribution in the British Isles, recorded from south-west England, south Wales, south and west Ireland and western Scotland. Europe: Portugal, Spain, France. Further afield: the Canary Islands.

Similar Species: There are other small algae appearing as spots or stain-like patches. All difficult to identify and require microscopic examination.

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

iNaturalist: Petrospongium berkeleyi at iNaturalist World Species Observations database.

WoRMS: Petrospongium berkeleyi at World Register of Marine Species.