Donate Take Action

Join us


Marine Animal Encyclopedia

Irish moss Mastocarpus stellatus

Irish moss is a tough red seaweed that is common on exposed shores, often forming a dense turf on the lower shore. Its frond is attached to rock by a disk-shaped holdfast, from which arises a narrow stipe (stalk) that gradually expands into a divided blade, which is slightly rolled to form a channel with a thickened edge. Reproductive structures housed in small nodules on the Irish moss blade’s surface produce a very different seaweed in the form of a thick black crust (it was originally named Petrocelis cruenta because it was thought to be an entirely different species). Spores from this crust grow back into the erect form, in a typical two-phase life history. Mastocarpus stellatus and the similar Chondrus crispus are both known as Irish moss or carrageen moss and are collected on an industrial scale on both sides of the north Atlantic to produce the gelling agent carrageenan.

Irish mosszoom image
  • Kingdom Phaeophyta
  • Length 7 in (17 cm)
  • Habitat Lower shore and subtidal rocks
  • Water_temperature 32–77°F (0–25°C)
  • Distribution Coasts of northeastern North America, northwestern Europe, and Mediterranean
Irish moss habitat mapzoom image