What is Irish Moss Seaweed? Ecology and Human History of Chondrus crispus

Description

Chondrus crispus is classified as a red seaweed, and in fact it often is a deep red or dark purple color, though coloration can range from pale yellow to green purplish red to almost black. Chondrus has a short and often frilly or curly appearance somewhat resembling parsley. It likes to grow on rocks and boulders in the mid to low intertidal zone, where it can be exposed at especially low tides.  Chondrus is usually most abundant 4-7 meters (13-23 feet) below mean low water but it also grows at shallower depths or as deep as 38 meters below mean low water, where it is always completely submerged.  Chondrus can be found in thick beds, patches, or as solitary plants mixed in with other seaweed.

Chondrus is a small plant that rarely exceeds 15-20cm (6-8 inches).  It has a crustose discoid holdfast that conforms tightly to hard surfaces, much like moss or lichen. The holdfast bears a number of erect, foliose fronds. These have dichotomous (paired) branching, where each branch divides into two equal parts as it grows. This leads to the characteristic frilly appearance, though less frilly forms having broader or narrower fronds are also found. These varieties can make it challenging to identify Chondrus in the field. Nine or ten varieties have been described, with the most extreme forms having either a broad, lacy appearance or a narrow, sticklike appearance.

Scientific & Common Names

Chondrus (pronounced shondroos) is a common, widely distributed seaweed bearing many common names due to its long history of human use. Chondrus crispus is most often known as Irish moss, reflecting its long history in Ireland and its short, moss-like growth habit on rocks. The Japanese know Chondrus as ‘tsunomata’ and it’s marketed as Hana Tsunomata’ in Japan by the only company to commercially grow Chondrus in tanks. In English it’s also known as carrageen, carrageen moss, curly moss, gristle moss, curly gristle moss, pearly gristle moss, Dorset weed, jelly moss, lichen, moss, rock moss, sea moss, sea pearl moss, and white wrack.  Similar red seaweeds of tropical origin are also called moss, sea moss, or Irish moss, which can be confusing to consumers.  We address this confusion in our blog post Irish Moss vs. Sea Moss vs. Irish Sea Moss; Are They The Same Thing?

The name carrageen for Irish moss is popularly attributed to a place name in Ireland where Irish moss is especially abundant, although ethnobotanists haven’t found strong evidence to support this claim.  Some suggest that the name carrageen comes from the Gaelic word cairgain, which translates simply as “an herb”.  Regardless of origin, carrageen gave rise to carrageenan, probably the most well-known product derived from Irish moss.

Irish moss. Photo by S. Redmond

Life History & Ecology

Chondrus crispus is the only Chondrus species found in the Atlantic, with eight other Chondrus species found in the Pacific. C. crispus is a northern seaweed that doesn’t grow any further south in the Atlantic than Portugal on the European coast and New Jersey on the American coast. It’s more heat tolerant than some other cold water seaweed species and can survive temperatures as warm as 29°C (84°F). Its preferred temperature range is between 15°C to 20°C (59°F to 68°F).

Its short stature and sturdy holdfast help Irish moss withstand strong currents along semi-exposed coastlines.  The most extensive populations are found on horizontal or sloping surfaces of massive boulders and ledge.  However, Irish moss can also be found in sheltered areas as individual plants or patches attached to small rocks, or even as unattached, free-floating mats.  The capacity to grow without a holdfast in a free-floating form helps make Irish moss well-suited for tank aquaculture.

Like many seaweeds, Irish moss reproduction has an alternation of generations consisting of a sexual haploid (half set of chromosomes) gametophyte, an asexual diploid (full set of chromosomes) carposporophyte, and another diploid stage known as a tetrasporophyte.  Male and female gametophytes mature as separate plants.   Male spermatia are released and drift until they encounter a carpogonium (egg) on a female plant.  The fertilized egg amplifies within a structure called the cystopcarp to produce thousands of genetically identical diploid carpospores, which are released from the cystocarp into the environment to germinate into free-living tetrasporophytes.  Mature tetrasporophytes produce thousands of haploid tetraspores, which are also then released into the environment to germinate into gametophytes (haploid plants).

The male and female haploid stages, the carposporophytes (found only on females), and the tetrasporophytes are all found as mature plants and all may be present within a population at the same time.  Seaweed specialists can identify the various forms through subtle characteristics, the most obvious being when cystocarps become visible as reddish nodules on the fronds of female plants.  Reproduction occurs year-round but peaks in summer and autumn for male and female gametophytes, and from autumn to spring for tetrasporophytes. Like many seaweeds, Irish moss also reproduces vegetatively.  New fronds can germinate from holdfasts as well as from any part of the thallus. 

Irish moss grows throughout the year but growth rate depends upon the interplay between daylength, temperature, and nutrient availability.  Longer days and warmer temperatures are conducive to faster growth but may also be times of lower nutrient availability.  Growth is often fastest in the fall, when there is optimum synergy between these three factors.  Irish moss is a perennial seaweed and frond longevity is believed to be about 4-5 years. The holdfast is thought to survive for longer periods than fronds, but for just how long remains a subject for study. 

Irish moss beds offer food and refuge to small crustaceans, snails, amphipods, and worms, along with juveniles of larger animals such as sea urchins, mussels, crabs, and sea stars. True limpets (a type of sea snail) will graze upon Irish moss and sea urchins will devour it when preferred species such as sugar kelp or Alaria aren’t available.  Irish moss tends to get heavily colonized through summer months by epibionts such as bryozoans and ascidians, as well as by epiphytic algae.  Irish moss sheds it’s outer layer (epidermal shedding) to rid itself of these colonizing organisms.  Surf action and cold winter temperatures also help clean the fronds.  Several species of bacteria and fungi are known to infect Irish moss and adversely affect its growth and survival.  Large die-offs caused by pathogens are rarely observed in nature but they can be a problem when Irish moss is cultured in the lab or in aquaculture.

History of Use

Irish moss has been harvested by humans for many hundreds of years.  Its attractive red color undoubtably encouraged early coastal residents to try it for food and other purposes. Like other abundant, inter-tidal seaweeds, Irish moss was often used by coastal homesteaders as a feed supplement or extender for horses, cattle, pigs, and other domesticated animals.  The residents of Ireland, where Chondrus crispus is especially abundant, found a great many uses for it, including as mattress stuffing and to thicken printing inks. 

Irish moss has long been especially prized for its medicinal properties.  Popular accounts and newspaper advertising from the 1800’s show a well-established Irish moss industry in the British Isles.  The earliest known advertisement for the food and medicine properties of Irish moss is in a Dublin, Ireland publication dated Oct 22, 1829: “Carrageen or Irish Moss so much appreciated and recommended as a Dietetic Remedy for Invalids and as a substitute for Isinglas in making Blamonge, Jellies &co. are enabled to supply it to the Public at a price considerably lower than that at which it has hitherto been sold.“ (from Mitchell and Guiry, “Carrageen: A Local Habitation or a Name?”).

Irish moss’s usefulness as both food and medicine is due in large part to its mucilaginous, gel forming properties.  When Irish moss is boiled and then cooled it forms a gelatinous substance known as carrageenan, with a number of useful applications.  This led to the rise of an Irish moss industry in Europe in the 1800’s, and Irish immigrants to the US and Canadian Atlantic coasts, where Chondrus crispus is abundant, brought that industry with them.  The town of Scituate, MA was an early center of the US Irish moss industry, attracting people of Irish ancestry and earning it the title “Most Irish Town in America”. The industry provided jobs in Scituate well into the 1960’s and also boomed in Canada.  In just one year, Canadian Irish moss production rose from 261,000 pounds (dry weight) in 1941 to over 2 million pounds by 1942.

Culinary Attributes

Before the discovery and commercial production of carrageenan it was common to use isinglass, obtained from the swim bladders of fish such as sturgeon, cod, and hake, to thicken fruit jellies, jams, and desserts.  Isinglass was also used to clarify fermented spirits such as beer and wine.

Carrageenan has largely (though not entirely) replaced isinglass because it can be used in the same manner but it’s cheaper and easier to produce.  Blancmange, a French sweet dessert made from milk or cream and sugar, was originally thickened with isinglass but is now usually thickened with carrageenan.  Click here for a delicious Irish moss blancmange recipe.  Carrageen moss pudding is the Irish version of blancmange, made from milk, sugar, egg, vanilla, and Irish moss.  Carrageenan has become a widely used thickener in a variety of dairy based and other processed foods, although today most carrageenan comes not from Irish moss but from other red seaweed species, mostly farmed in the tropics.  We discuss carrageenan and the carrageenan industry in more detail in our blog post “There’s carrageenan in my seaweed!”.  Irish moss is still preferred by many home cooks to thicken puddings, custards, and soups.

Maine Coast Sea Vegetables sells Irish moss as  whole leaf, flakes, and powder.  Whole leaf is a popular choice to make traditional blancmange or moss puddings. Whole leaf can be boiled to make carrageen gel, which can be stored in the refrigerator for up to two weeks and used as needed to thicken foods, in medicinal applications, or for homemade skin care products.  Flakes and powder can be used in similar ways as whole leaf but offer additional convenience.  Both are easily added (1-2 tsp) to smoothies and soups.  Powdered Irish moss dissolves in water and can be substituted for corn starch in gravies and other recipes where corn starch is used.

People are sometimes surprised by the strong, earthy aroma of Irish moss, which one web site describes as smelling like the garden aisle at Home Depot.  However, Irish moss is practically odorless and tasteless once it’s cooked and cooled to make carrageen or prepared into recipes. 

Nutritional & Medicinal Attributes

Besides its food thickening properties, Irish moss is nutrient dense and it was famously used as a starvation food during the Irish potato famine of 1846-48.  Irish moss contains every mineral needed for human health, including respectable levels of calcium, iron, and potassium, the three minerals listed on the FDA Nutrition Facts panels.  Irish moss also contains appreciable levels of microminerals such as magnesium, manganese, and copper.  Just 1 tsp (about 3.5g) of Irish moss powder provides 8% of the RDI for iron and 11% of the RDI for manganese.  Although it’s low in fat (about 3%), Irish moss is one of the few non-animal sources of polyunsaturated fatty acids, which promote good brain and cardiovascular health.  The protein content of Irish moss varies between 12-20% and averages about 15%.  Irish moss protein is considered high quality because it contains every essential amino acid in digestible form. Irish moss contains about 300 micrograms iodine per gram, which is more than most other red seaweeds but less than most brown seaweeds.  Finally, Irish moss is a good source of fiber, and its unique polysaccharides are thought to benefit intestinal health.

Although carrageenan has little to no nutritional value, folk medicine has long believed it has medicinal properties. Irish doctors have prescribed carrageenan jelly or broth since the early 1800’s to treat intestinal disorders, ulcers, sore throats, coughs, and other respiratory ailments.  Its mucilaginous properties helped to soothe these and similar maladies that caused inflammation and irritation of mucous membranes.  Irish moss was also widely used as an expectorant, a substance that loosens congestion and encourages more productive coughing, so that each cough more effectively clears phlegm and mucus from the lungs. 

Modern science is discovering new pharmacological properties of Irish moss.  Lab studies show that carrageenan inhibits several viruses, including the human papillomavirus that causes genital warts and cervical cancer.  Other studies show that a carrageenan nasal spray can be prophylactic against coronaviruses, including the common cold and COVID-19. Carrageenan may trap the viral particles within its molecular structure, which mimics receptors the virus uses to gain entry into human cells.  In the lab, it’s been found that carrageenan inhibits proliferation and metastasis of cancer cells.  Recent research in the online publication Neuroscience News showed that carrageenan may modulate the immune response, potentially making it useful for people with overactive or imbalanced immune systems. 

It’s important to recognize that some of these findings were found in vitro (outside the living body; in a lab). Carrageenan is not digested by humans so it passes through the intestinal tract largely intact, thus limiting its pharmacological action to the digestive tract when eaten. 

Skin and body care are other applications where the soothing properties of Irish moss carrageenan are appreciated.  Irish moss extracts form hydrocolloids (water gels) used in cosmetics and having sunscreen and anti-ageing components as well. One can purchase Irish moss lotions, creams, and soap or make them at home using our Irish moss powder or whole leaf.  This simple recipe uses Irish moss powder, water, aloe vera gel, and almond oil to make a soothing lotion.

Wild Harvest & Processing

Irish moss is readily harvested by cutting or plucking it from rocks on exposed shoreline at very low tides.  A sharp knife or garden shears can be used to cut or clip the fronds just above the holdfast. However, Irish moss is most abundant in the lowest portion of the intertidal and the shallow subtidal, where it is usually submerged even at low tides.  This makes it impractical, if not impossible, to harvest large quantities by hand cutting individual fronds.

The rise of the carrageenan industry in the early 1900’s led to the need for more efficient harvesting methods involving hand rakes with long handles.  At first, harvesting was done from a skiff at low tide by dragging the rake along the bottom, but the process became semi-mechanized in the mid-1900s when drag rakes were towed behind motorized boats.  This facilitated industrial-scale harvesting for carrageenan production in several regions in the North Atlantic, including Ireland and the Canadian Maritimes.  Irish moss was also collected in large amounts from the beach when it was cast ashore by storms. Canadian landings peaked in the mid 1970’s at about 50,000 wet metric tons, providing about 75% of the world’s carrageenan production.  Overharvesting caused immense damage to the standing stock, but beginning at about the same time the world’s first seaweed farm to grow Eucheuma, a red alga that can also be processed for carrageenan, was established in the Philippines.

Today, seaweed aquaculture of Eucheuma and related species provides most of the world’s carrageenan, and wild harvest of Irish moss has returned to pre-industrial, sustainable methods using hand rakes.  The rake is designed to remove only larger fronds, leaving the immature fronds and holdfast behind for regeneration.  The freshly harvested moss is brought to shoreside drying facilities where it’s dried in a similar fashion as dulse by spreading it in a single layer on clean rocks and cobble, or tarps, for solar drying.  Greenhouses may also be used.

Aquaculture

Researchers began investigating cultivation methods for Chondrus and other related red algal species when it became evident that wild stocks couldn’t be sustainably harvested to meet the growing global demand for carrageenan. Early research in the 1960’s and 70’s worked to develop ocean farming for tropical red alga species such as Eucheuma, and tank farming for Chondrus.  The first published account of tank-cultured Chondrus was in 1971, when A.C. Neish and collaborators described propagating Chondrus in a wooden tank located at Sandy Cove, about 30 minutes from Halifax, Nova Scotia.

Their report sparked interest from two major international carrageenan producers, who within a short time began their own tank cultivation trials.  A flurry of research activity ensued over the 1970’s but eventually it became apparent that growing Chondrus in tanks at large scale for carrageenan production couldn’t compete with ocean farming of the tropical red species.  The only company still successfully growing Chondrus in tanks is Acadian Seaplants Ltd. in Canada.  The company originally targeted carrageenan production but switched to cultivating an edible Irish moss variety with unique color and texture qualities, which they market as 'Hana Tsunomata’ in Japan.

Open water cultivation of Chondrus has been even less successful.  Again, this is largely due to inability to demonstrate it can compete with faster growing tropical species.  A unique strain of Chondrus crispus was studied for cultivation potential in estuaries and basins of Atlantic Canada. Known as giant Irish moss because of its very broad fronds, this strain lacks a holdfast and is naturally found only in a sheltered lagoon in the southeastern Gulf of St Lawrence.  It only reproduces vegetatively and it depends on attached blue mussels to keep it anchored.  Although research in 1999 showed it had cultivation potential, no commercial interest developed, and since that time the wild population of giant Irish moss has dramatically declined.  Recent research done in 2019 blames this decline on an invasive green crab species that predates upon the blue mussels that the moss relies upon to keep it anchored.  This is yet another example of how each sea vegetable species exists in a special relationship with its local environment and the creatures inhabiting it.