Mastocarpus spp. (Petrocelis phase)

Tar spot alga

Thallus is a thick (to 2 mm or 0.08 in), brownish to blackish crust that is actually the alternate reproductive phase (tetrasporophyte) of the Turkish Washcloth (Mastocarpus). The crust is firm but not hard: one can sink one’s fingernail into it. Its surface is smooth but sometimes shows faint concentric scalloping.

Habitat

This very resilient, perennial phase can be extremely long lived, to 90+ years; common on high to low intertidal rock in protected to semi-exposed habitats.