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Irish moss
A-D Chondrus crispus ; E-F Mastocarpus stellatus
Scientific classification
Kingdom: Archaeplastida Phylum: Rhodophyta Class: Rhodophyceae Order: Gigartinales Family: Gigartinaceae Genus: Chondrus Species: C. crispus
Binomial name
Chondrus crispus Stackhouse

Chondrus crispus, known under the common name Irish moss, or carrageen moss (Irish carraigín, "moss of the rock"), is a species of red alga which grows abundantly along the rocky parts of the Atlantic coast of Europe and North America. In its fresh condition the plant is soft and cartilaginous, varying in colour from a greenish-yellow, through red, to a dark purple or purplish-brown. The principal constituent of Irish moss is a mucilaginous body, made of the polysaccharide carrageenan of which it contains about 55%; the plant also has nearly 10% of protein and about 15% of mineral matter, and is rich in iodine and sulphur. When softened in water it has a sea-like odour, and because of the abundant cell wall polysaccharides it will form a jelly when boiled, containing from 20 to 100 times its weight of water.

Chondrus crispus is a relatively small red alga little more than 20cms long growing from a discoid holdfast and branching in a dichotomous, fan-like, manner four or five times. The morphology is very variable, especially the broadness of the thalli. The branches are 2 - 15 mm broad, firm in texture and dark reddish brown in colour bleaching to yellowish in sunlight. The gametophytes (see below) often show a blue iridescence and fertile sporophytes show a spotty pattern. Mastocarpus stellatus (Stackhouse) Guiry is a similar species which can be readily distinguished being strongly channelled and often somewhat twisted. The cystocarpic plants of Mastocarpus show reproductive papillae quite distinctively different from Chondrus.^[1] When washed and sun-dried for preservation it has a yellowish translucent horn-like aspect and consistency.

Chondrus crispus is found growing on rock from the middle intertidal zone downwards. It is common all around the shores of the British Isles and can also be found along the coast of Europe from Iceland, the Faroe Islands ^[2] western Baltic Sea to southern Spain.^[1] It is found on the Atlantic coasts of Canada^[1][3] and recorded from California in and the United States and Japan.^[1] However, any distribution outside the Northern Atantic needs to be verified. There are also other species of the same genus in the Pacific Ocean, for example, C. ocellatus Holmes, C. nipponicus Yendo, C. yendoi Yamada et Mikami, C. pinnulatus (Harvey) Okamura and C. armatus (Harvey) Yamada et Mikami.^[4]

Chondrus crispus is a source of carrageenan, which is commonly used as a thickener and stabilizer ^[5]in milk products such as ice cream^[6] and processed foods including luncheon meat, in Europe it is indicated as E407 or E407b. It may also be used as a thickener in calico-printing and for fining beer or wine. Irish moss is frequently mixed with Mastocarpus stellatus (Gigartina mammillosa), Chondracanthus acicularis (G. acicularis) and other seaweeds with which it is associated in growth. Carrageenan and agar-agar are also used in Asia for gelatin-like deserts such as almond jelly.
Presenetly the major source of carrageenan is tropical seaweeds of the genera Kappaphycus and Eucheuma. Irish moss is also a beverage popular in the Caribbean (most notably Jamaica), and is made by boiling the Irish moss for about an hour in water, and a flavouring, among which Vanilla or Strawberry are popular, and then milk or sweetened condensed milk is added, or sometimes it is made flavourless and once the milk has been added rum and spices are then added. It is usually served chilled, is very thick and is sometimes thought to have aphrodisiac qualities. It is also now available ready made, tinned. The Irish Moss used in the Caribbean is most often Gracilaria spp.

Chondrus crispus undergoes an alternation of generation life cycle common in many species of algae (see figure below). There are two distinct stages: the sexual haploid gametophyte stage and the asexual diploid sporophyte stage. In addition there is a third stage- the carposporophyte, which is formed on the female gametophyte after fertilization. The male and female gametophytes produce gametes which fuse to form a diploid carposporophyte, which forms carpospores, which develops into the sporophyte. The sporophyte then undergoes meiosis to produce haploid tetraspores (which can be male or female) that develop into gametophytes. The three stages (male, female and sporophyte) are difficult to distinguish when they are not fertile; however, the gametophytes often show a blue iridescence.

The life cycle of Chondrus crispus. Below the life stage are indicated if the life stage is haploid(n) or diploid (2n) and the type of carrageenan present.

How the life cycles of Chondrus crispus might look in nature. The gametophytes show blue iridescence and the fertile sporophytes exhibit a spotty pattern.

Chondrus crispus is, compared to most other seaweeds, well-investigated scientifically. It has been used as a model species to study photosynthesis, carageenan biosynthesis, and stress responses.

The portion of the 65th plate of Ernst Haeckel's Kunstformen der Natur (1904), depicting Florideae Chondrus crispus, i.e. Irish moss

When cultured in the laboratory Chondrus can have a morphology similar to the Haeckel plate, in Nature this is; however, rarely seen

• Museum of London - Caribbean London - Catalogue
• Summers, Adam (July 2006). Keep Me Hanging On : Surviving in the intertidal zone tests the rubbery limits of algae. Natural History 115 (6): 26-27. 
• Seaweed Site
• AlgaeBase
• Marine Life Information Network
• Innovations
• Rhode Island
• Irish Moss industry on Prince Edward Island[1]