What is Dulse Seaweed? Ecology and Human History of Palmaria palmata

Description

Palmaria palmata is in the family of red seaweeds that includes nori and Porphyra, Irish moss, and Gracilaria.  P. palmata is a relatively small seaweed, with palmate fronds that grow to about 50 cm (20”) long and 3-8 cm (1”-3”) wide.  The healthy mature plants are famous for their beautiful crimson, purplish, or reddish-brown color.  The fronds are soft, leathery, and supple with a skin-like texture.  They extend from a short, almost indistinguishable stipe attached to a rather small and fragile, disk-like holdfast.  A typical plant starts out as a single flattened frond that gradually expands and divides into multiple segments, giving the impression of a hand with fingers.  However, the size and shape of P. palmata fronds can be quite variable, especially under sheltered, silty conditions, where the fronds are often narrow and finely dissected.  Plants with short broad fronds or with longer straplike fronds are also found throughout its range. 

Dulse is fairly easy to identify in the field and mature plants are rarely mistaken for other species.  Grateloupia turuturu is morphologically similar to P. palmata and could be mistaken for it in areas where they overlap.  This may become more of a problem as the range of Grateloupia continues to expand into areas where it wasn’t previously found, including into the Gulf of Maine.

Scientific & Common Names

Palmaria palmata was formerly known by the Latin name Rhodymenia palmata. Its current scientific name is rooted in the Latin word palma for hand or palm of the hand, which is repeated in both parts of the name to emphasize the palm-like shape of the plant.  Palmaria mollis is a closely related species found in the Pacific, where it’s known as Pacific dulse or red ribbon.

In English, P. palmata is usually known as dulse, which itself comes from the Gaelic word duileasc (or duileasg).  In Spanish the word dulce means sweet, and although dulse seaweed doesn’t have the subtle sweet flavor of sugar kelp, people often enjoy it as a snack.  Dulse is also known as dillisk, dilsk, red dulse, sea lettuce flakes, or creathnach.  Creathnac is an Irish word for a ‘feminine’ form of dulse that is smaller, narrower, and darker than the ‘masculine’ variety known as duileasc. Creathnac is also known as shell dulse and it usually grows on mussel shells or attached to kelp stipes. Dulse is also sometimes known by the poetic name of Neptune’s girdle.

Life History & Ecology

Dulse is an arctic to cool temperate North Atlantic species found on both the European and North American coasts.  In Europe it’s found as far north as Spitzbergen, a large island in the Arctic Sea off the north coast of Norway, and as far south as Portugal.  In North America it’s found as far north as Arctic Canada and south to Long Island, NY.  P. palmata grows best between 6°C to 15°C (44°F to 59°F) and can survive freezing temperatures.  The upper lethal temperature is about 20°C (68°F), though strains found at the southern limits of distribution are more adapted to warmer temperatures. P. palmata has been reported to grow on the coast of Hokkaido, but this hasn’t been confirmed with genetic analysis and these reports might actually be referring to Palmaria mollis, a closely related species with a similar northern distribution in the North Pacific.  Older publications identified P. mollis as a variety of P. palmata but it is now known to be a separate species. 

Dulse is a perennial that can regrow new fronds every year from the holdfast.  The maximum life span of individual fronds or holdfasts is unknown other than they can persist through the winter.  Marginal outgrowths of new fronds can emerge as buds from the primary blade or holdfast.  This allows dulse to be vegetatively propagated in a lab setting.  Older growth is often covered with bryozoans and other organisms.

The reproductive cycle of P. palmata wasn’t well understood until about 1980, when researchers first succeeded in growing it in the lab. Prior to this achievement, no one had found female plants in the wild, leading many to conclude that dulse reproduced asexually.  We now know this isn’t the case.

Unlike most other red algae with triphasic life cycles, Palmaria palmata has a biphasic life cycle consisting of a sexual gametophyte alternating with an asexual tetrasporophyte. Dulse is dioecious, meaning separate male and female plants. Female plants are found only as microscopic gametophytes, and the dulse beds we harvest for food consist of either male haploid gametophytes with a half set of chromosomes, or asexual diploid tetrasporophyte plants with a full set of chromosomes.  These forms occur together and are indistinguishable in the field until the tetrasporophytes become reproductive after their first season. At this time, they develop sorus tissue visible as dark red, slightly elevated patches on the frond margins.  Under a microscope it can be seen that this tissue contains spores clustered together in groups of four…hence the name “tetraspores”.  Each tetraspore contains half the full set of chromosomes.

After tetraspores are released, they quickly adhere to any suitable substrate (rock, shell, or kelp stipes).  The tetraspores then develop into haploid gametophyte plants, with a sex ratio of about 1:1. Male tetraspores don’t become sexually mature plants until after 9-12 months of vegetative growth. Female tetraspores, on the other hand, sexually mature within days as microscopic crustose plants.  There’s speculation that some microscopic females may lodge in the holdfast of established plants, which is one reason why harvesters advise to leave holdfasts behind. Female plants grow reproductive structures called carpogoniums that each carry an egg, along with tubular filaments called trichogynes to receive spermatia.  These structures persist for up to 5-6 months but then gradually decline before the female plant dies.

The life history of dulse is unique in that females from each generation are reproductive their first year but males aren’t reproductive until their second year.  This means females can only be fertilized by older males from previous years, which may help prevent inbreeding within the same generation while also ensuring a new crop of plants each season.  Upon fertilization the egg develops into a new tetrasporophyte that overgrows the female plant and releases tetraspores the following year to begin the cycle anew.  Tetrasporophytes and male gametophytes become fertile in late fall/early winter and their reproductive season can extend for several months.

Dulse is rather unusual in that it can grow either attached to rocks and shells or as an epiphyte on other macroalgae, usually on the stipe of large kelp species such as Laminaria spp.  Dulse may be found either as solitary plants or in extensive beds, usually in the upper sub-tidal zone, where it’s submerged at all but the lowest of tides.  Dulse grows to a maximum depth of about 10 meters in the Gulf of Maine (33’). 

Dulse is common throughout its range, though it’s not especially abundant except under certain conditions and it’s often completely absent from exposed coasts.  This means that harvestable dulse beds are found only in a few special locations, such as the coasts of Grand Manan Island. Dulse favors rocky, north facing coasts and is most plentiful on beaches covered with ‘boulder fields’ of medium to large boulders.  Dulse beds provide food and habitat for sea snails and other small invertebrates, though no species is known to depend entirely on dulse for survival.  Sea urchins are quite fond of dulse but don’t appear to heavily impact it, perhaps because dulse grows in the upper sub-tidal zone where urchins are less frequent.  Dulse has no known diseases, though galls produced by nematodes, copepods, or bacteria may occasionally occur on the fronds.  The closely related Palmaria mollis can become infected by a fungal-like Oomycete known as Petersenia palmariae, but apparently P. palmata is not susceptible to this parasite.

History of Use

The first written record of Europeans eating dulse dates back to about 563 AD.  A poem attributed to St Columba refers to the monks of Iona, a small island off the western coast of Scotland, collecting dulse off the rocks for food.  “Let me do my daily work/Gathering dulse/Catching fish/Giving food to the poor.”  According to the Icelandic sagas dating back to 961 AD, sol (old Norse word for dulse) was a highly valued food source.  Dulse makes its first appearance in Icelandic legal documents in 1150 AD, which state that it is “perfectly legal to collect and eat another man’s dulse when traveling across his property” (from The Uses of Seaweed in Iceland by S.V. Hallson).  We think this is a code well worth honoring today.

References to dulse as food and medicine have appeared in northern European texts many times since. Although seaweed in general was considered a poverty food, dulse was widely enjoyed by rich and poor alike and was even used for trade or barter for goods exchanged with inland residents.  Once dried, dulse became a form of hard currency. Durable, lightweight, and with a long shelf life, it could be taken on long journeys.  It’s said that Celtic warriors ate dried dulse for stamina as they marched and that British sailors used it in lieu of chewing tobacco to help prevent scurvy. The Auld Lammas Fair, held every August since 1606 in Ballycastle Northern Ireland, may represent the longest tradition of dulse trading.  Visitors to the fair will find a profusion of market stalls offering dulse for sale. Dulse was popular in Scotland right up through the 1800’s, as observed by Charles Dickens in 1858 when he reminisces about the ‘dulse wives’ selling dulse in Aberdeen.  Irish and Scottish immigrants brought their special love of dulse to the new world, especially to the hard scrabble coasts of Maritime Canada, where a dulse tradition thrives to this day.

The tradition of eating dulse, and other seaweeds in general, fell out of favor with many Europeans through the 1900’s with the rise of the “western diet” and the widespread availability of processed foods.  Of course, eastern cultures never abandoned seaweed as food and it remained popular with some coastal European populations, particularly those of Iceland, Norway, Scotland, Ireland, and Nova Scotia. Today, sea vegetables are enjoying a resurgence in popularity as people rediscover their culinary, nutritional, and medicinal benefits.  Dulse in particular has become extremely popular with consumers.

Culinary Attributes

Dulse is considered highly delectable by seaweed aficionados.  It can be eaten fresh from the rocks but is preferred dried because as it dries and ages it develops more interesting and complex flavors. We currently offer eighteen dulse products in various dried whole leaf or milled forms.

Dried dulse can be eaten raw and straight from the bag or included as an ingredient in any number of recipes.  It has a mildly salty flavor and a tangy umami essence that some describe as slightly smoky. This attribute lends itself well to our smoked dulse, currently the only smoked seaweed we offer. Dulse’s savory, smoky essence is enhanced and it becomes crispy when it’s lightly pan-fried, leading to comparisons with bacon.  Dried dulse is an easy on-the-go snack food.  It’s best served somewhat soft and supple, when it has a moisture content between 10-20%. At this moisture level our dulse fronds are soft and supple and feel somewhat moist. In Atlantic Canada and Ireland, dulse lovers prefer what is called “hard dulse”, still delicious but drier, less tangy and chewier. Dulse that appears too dry for your liking can be softened with one or two fresh apple slices in a closed container. 

Some Irish old timers insist that dried dulse is best eaten cooked.  They like to boil it, eat the softened dulse, and then drink the broth to get the nutrients released from cooking.  Home cooks have found many creative ways to include dulse in recipes.  Traditionally, the simplest way to eat dulse in Ireland was as a condiment with bread, butter, and milk.  Another popular use was to lightly cook pieces or flakes of dulse in oil or butter and then mix it in with boiled potatoes, turnips, or vegetables.  Dried dulse flakes can also be included in bread recipes.  Traditionally, this was done to extend flour in households with tight budgets, but today it’s usually to add flavor and minerals.  Dulse can be added to a variety of soups, stews, and casseroles, though it tends to disintegrate and lose its red color with long cooking time, leaving just flavor and minerals behind. Our recipe book Sea Vegetable Celebration offers many ways to use dulse, and recipes can also be found on the recipe page of our website. 

Nutritional & Medicinal Attributes

Dulse has an excellent nutritional profile, containing protein, minerals, and vitamins.  On average it contains about 13% protein, though depending on season and location of harvest it can contain as much as 30%.  Dulse protein is considered high quality because it has all of the essential amino acids. One of the more abundant amino acids found in dulse is glutamic acid, which imparts a savory umami flavor. As with most plant-based proteins, dulse protein is bound to indigestible carbohydrates and cooking or boiling makes it more bio-accessible.

Like all seaweeds, dulse has high mineral content, and it’s especially rich in iron.  One seven-gram serving of whole leaf dulse (about ⅓ cup) contains about 14% of the Recommended Dietary Intake (RDI).  The same serving also contains 9% RDI potassium, 9% RDI copper, and other minerals.  Dulse contains several important B vitamins (B1, B2, B3, and B9), vitamin A (as beta-carotene), and vitamin E. Dulse’s vitamin C content (9% RDI in a 7g serving) is why British sailors used it like chewing tobacco to ward off scurvy.  Compared to brown seaweeds, dulse contains moderate levels of iodine, but even so, just 3 grams of dulse flakes provides about 425 micrograms iodine, or 284% of the RDI.

Irish people observed in the 1800’s that eating dulse could prevent or even cure people of parasitic intestinal worms.  Prannie Rhatigan, in her book Irish Seaweed Kitchen, tells of how parents in County Sligo would collect dulse at low tide from a boulder known as worm rock, boil it, and feed the broth to their afflicted children.  It was believed that this rock, and only this rock, could grow dulse with the power to cure worms, but today we know that all dulse contains some amount of the anthelmintic (worm killing) compound kainic acid.  The Japanese use a related red alga, Digenea simplex, for the same purpose.

Icelandic people used to apply dulse in poultices to fight wound infection.  As with many folk remedies, this traditional practice is supported by modern science showing that dulse contains anti-bacterial compounds.  These largely consist of structurally complex, bioactive polysaccharides similar to those found in other seaweeds and having anti-cancer, anti-microbial, and anti-inflammation properties.  Seaweed polysaccharides play an especially important role in human digestion as dietary fiber, and they may act as prebiotics to favor the growth of beneficial bacteria in the colon.  Another potential benefit of seaweed polysaccharides is that they may absorb and remove toxic heavy metals as they pass through the intestinal tract.  Laboratory studies have shown that seaweed polysaccharides have an affinity for heavy metals, absorbing and binding them within an indigestible matrix.  This has led some to promote the theory that eating dulse can “detoxify” heavy metals from the human body, though at this point there’s no hard scientific evidence to support this claim.

Wild Harvest & Processing

Most of the wild dulse harvested from the North Atlantic comes from just a small number of locations in Europe and Maritime Canada.  Because dulse is a relatively short plant that’s usually only plentiful on boulder fields, it can’t be easily harvested from boats using hand rakes or mechanical devices. The traditional method of scrambling over slippery boulders and hand cutting, as practiced for hundreds of years, remains the norm today.  Harvesting can only be done during a few hours each day at low tide when the beds are no longer submerged, and on many days the tides don’t go out far enough.  The season in the northern Gulf of Maine generally begins in May and lasts until about October.

The tide limited access and low technology approach used to harvest dulse act as natural checks on excessive harvesting, but they also mean the harvest is subject to weather conditions and the willingness of harvesters to venture out when the tide is low, sometimes in the very early morning.  Consequently, some harvest years are better than others, which is why we sometimes run out of stock or have to ration sales of certain dulse products.

Harvesters and the grounds they harvest from are held to organic certification standards that further ensure sustainability.  Although we occasionally offer non-certified dulse, that doesn’t mean it’s held to a lower standard.  Our non-certified dulse is harvested from the same grounds and following the same sustainable harvesting practices as our certified organic dulse.  It just hasn’t yet undergone the rigorous inspection and paperwork process required under third-party certification.

After it’s been harvested the dulse is transported inland to drying fields, usually in clean permeable bags or baskets in the back of a truck.  Here, it’s spread in a single layer on clean rocks and cobble to be solar dried.  Ideally, dulse is dried within a day of harvest, which means harvest timing has to coincide with both low tides and sunny days.  When the two don’t quite align, harvesters will sometimes store freshly harvested bags of dulse submerged in the sea, but this can only be done for a few days at best.  Our blog post “Where’s the dulse” describes the harvesting and processing of dulse in more detail.

Dulse drying field

Aquaculture

A great deal of research has gone into developing dulse aquaculture, but so far commercial success has been limited. Dulse can be grown as a free-floating plant in bubbling tanks of seawater; it doesn’t necessarily have to grow attached to rocks or other substrate like it does in the wild. Scientists at Oregon State University have investigated and promoted P. mollis tank farming since the 1980’s, and this species is now raised as food for both abalone and humans.  Their work recently became widely known following news reports that described fried P. mollis as tasting like bacon. The phrase “dulse, tastes like bacon!” caught fire on social and national media, somewhat to the chagrin of the OSU researchers.

Although dulse seems well suited for tank culture, only a few companies have had commercial success. Big Island Abalone in Hawaii and The Cultured Abalone Farm in California grow P. mollis in tanks to feed their farmed abalone, but not for human consumption.  A few small-scale operations on the US west coast grow dulse in tanks at artisanal scale, which they sell at local markets and to innovative chefs.  As of 2021, dulse tank farming remains confined to these few small-scale, experimental or pilot ventures.  In part, this may be attributed to the expense of coastal real estate and the high capital and operating costs of land-based tank farming in general.  There are also few available sources of dulse hatchery seed stock, and the majority of what is farmed today originates from wild stocks.  This lack of hatchery seed stock means limited opportunities for market experimentation and innovation.

Sea-based dulse farming has proven equally challenging.  Dulse can be propagated either vegetatively, or sexually from reproductive tetrasporophytes.  Most dulse tank farming ventures use vegetative propagation, which is relatively straightforward.  Basically, large plants are cut into smaller pieces and suspended in tanks of flowing, bubbling seawater, sometimes with added fertilizer, to regrow as larger plants. However, vegetative propagation is impractical for growing dulse at large scale at sea.  At this scale it is far more efficient to seed nets or line with hatchery produced spores, as is done with other red seaweeds such as nori.   Although spore production methods have been developed for P. palmata, some technical challenges remain unresolved.  These include the ability to consistently manipulate tetrasporophyte reproductive timing; reducing reliance on wild plants as the source of hatchery seed; poor survival of hatchery spores; and inconsistent seed line densities.  At sea, a particular challenge has been heavy overgrowth, or fouling, of dulse lines by epiphytes.  Scientists are working to address these issues, notably in Ireland, Norway, and the US, and sea-based dulse farms could soon become a reality.  In 2021, the pioneering seaweed company Springtide Seaweed was awarded a federal grant to develop dulse farming methods at a mariculture site located in downeast Maine.