Mastocarpus is a genus of red algae (Rhodophyta) in the family Phyllophoraceae, order Gigartinales, class Florideophyceae. These are multicellular marine macroalgae — commonly called red seaweeds — found attached to rocks in the intertidal and subtidal zones of cold to temperate coastal seas. The genus is distributed nearly worldwide, with species recorded from the Pacific and Atlantic coasts of North America, Europe, and beyond.
Members of Mastocarpus are morphologically complex, exhibiting a striking heteromorphic life cycle: the conspicuous upright thallus (gametophyte stage) alternates with a crustose encrusting phase historically described as a separate genus, Petrocelis. The upright fronds are typically dark red to purplish-black, cartilaginous, and blade-like, often bearing surface papillae or bumps — a feature reflected in the genus name. The crust-forming tetrasporophyte stage is cryptic, looking nothing like the leafy gametophyte, and was only recognized as the same organism through culture studies in the 20th century.
Well-known species include Mastocarpus papillatus, common on Pacific North American rocky shores, and Mastocarpus stellatus, widespread on North Atlantic and European coastlines. The genus was previously placed within Gigartina, and older literature may refer to its species under that name.
Distribution
Mastocarpus has a near-cosmopolitan distribution along cold to temperate rocky coastlines. Species occur on both the Pacific and Atlantic coasts of North America (notably M. papillatus on the Pacific coast and M. stellatus on the Atlantic and European coasts), as well as on the coasts of Europe and elsewhere in the Northern Hemisphere.
Ecology
Species of Mastocarpus are intertidal and subtidal macroalgae, growing attached to rocks in wave-exposed to moderately sheltered coastal environments. They are often zonally distributed on rocky shores, tolerating periodic emersion and desiccation. The crustose tetrasporophyte (formerly Petrocelis) persists in areas where the upright gametophyte cannot, giving the genus resilience across microhabitats. These algae form part of the structural habitat for a range of invertebrates in the rocky intertidal community.
Taxonomy Notes
Mastocarpus was historically included within the genus Gigartina (family Gigartinaceae); species such as M. stellatus were long known as Gigartina stellata and M. papillatus as Gigartina papillata. Morphological and molecular studies reorganized these algae into Phyllophoraceae. A key discovery was that the encrusting "Petrocelis" stage is the diploid tetrasporophyte of Mastocarpus, making the life cycle heteromorphic — a now-classic example in algal biology.
