Description: Thallus small and encrusting often as specks or patches which may become confluent to several cms across. Firmly attached.

Habitat: Common. Epilithic and on shells and corallines in the lower littoral in shallow rock pools to 30 m.

Distribution: Widespread in the British Isles. Europe: Black Sea, Germany, Denmark, Baltic Sea, Sweden, Norway, Faroes and Iceland. Greenland. Atlantic coast of North America: Canada, Maine and New Hampshire. Further afield: Atlantic and Pacific South America, Alaska, Canada and California.

Similar Species: All other small brown encrusting algae are similar and require microscopic examination for determination.

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

iNaturalist: Petroderma maculiforme at iNaturalist World Species Observations database.

WoRMS: Petroderma maculiforme at World Register of Marine Species.