An unattached, fragile, alga with a calcareous skeleton. It is very similar to and often confused with Phymatolithon calcareum. Its form is very variable but it commonly occurs as highly branched nodules forming a 3-D lattice. Individual plants may reach 4 - 5 cm across and are bright pink in colour when alive but white when dead.Maerl is a generic name for certain coralline algae that grow unattached on the sea bed. Only two instances of the crustose form of Lithothamnion corallioides have been recorded from the British Isles; in Dorset and Devon.