Lithophyllum incrustans Philippi
Description: Thick, dull chalky, yellowish, pink or lavender calcareous crusts forming irregular concretions, to 40 mm thick, margins ridged where crusts meet.
Habitat: On rock, lower intertidal semi-exposed and exposed pools, subtidal to 30 m. west and south-west coasts, common, especially in shallow subtidal. On the W and SW coasts of Ireland, this species is frequently associated with intertidal populations of the purple sea-urchin Paracentrotus lividus, the pink encrustations of the alga contrasting beautifully with the purples and greens of the urchins, which frequently dominate exposed shore limestones.
Similar species: Many crustose corallines occur in the intertidal and shallow subtidal, but require expert examination to confirm their identities.
Video from the west coast of Ireland at Black Head, Co. Clare showing the crustose corallines below the Laminaria digitata: