What is Truffle Seaweed? The Ecology and Human History of Vertebrata lanosa

Description

Vertebrata lanosa is a red alga that grows almost exclusively as an epiphyte on rockweed (Ascophyllum nodosum) and less often on fucoids such as bladderwrack. It rarely grows free living attached to rocky substrates, and not as well as when attached to rockweed, so for this reason it may best be considered a semi-obligate epiphyte. V. lanosa usually appears on rockweed as dense, bushy tufts of multiple filamentous segmented fronds (i.e., polysiphonous) forking from several main branches. It attaches to its host with creeping rhizoids that penetrate the frond, mainly at points of damage. Its texture is often described as cartilaginous or wiry, and its color is typically a deep reddish brown or purplish red, or almost black when dried. V. lanosa grows to a length of about 75mm (3”) and is commonly found wherever rockweed grows. It sometimes forms lush patches on rockweed, but rarely to the point of overwhelming or smothering it.

There are about 30 species in the genus Vertebrata, all of which are smallish, bushy algae with filamentous segmented thalli. However, only V. lanosa, and less often V. fucoides, are found attached to rockweed fronds. It can be challenging for the untrained eye to tell the two apart, but V. lanosa tends to form spherical tufts, whereas V. fucoides tufts tend to be looser, more elongated, and darker in color. Also, the base of the main axis and branches in V. fucoides is often covered with corticating cells, which are scaly in appearance, whereas in V. lanosa the cortication is absent. Unlike V. lanosa, V. fucoides is a lithophyte and not a semi-obligate epiphyte, and it thrives attached to rocky substrates.

Common Names

Vertebrata lanosa was previously known as Polysiphonia lanosa, with the genus name Polysiphonia assigned on the basis of the polysiphonous fronds. P. lanosa was reclassified as Vertebrata lanosa in 1967, though most scientists persisted in referring to it as Polysiphonia until about two decades ago. Of the thirty or so Vertebrata species, only V. lanosa is often known by one or more common names. In Ireland it’s often referred to in Gaelic as Olann dhearg, which is said to translate as polysiphonia. In English it's usually called wrack siphon weed, but the names ocean truffle, sea truffle, truffle seaweed, or truffle of the sea are becoming increasingly popular as more people learn of its culinary properties. V. lanosa has also been sometimes referred to as tandy or mistletoe of the sea. We prefer the common name truffle seaweed as opposed to truffle of the sea, which usually refers to pepper dulse, or sea truffle, which refers to truffle clams.

Life History & Ecology

Vertebrata lanosa is ubiquitous on both sides of the North Atlantic. It has been reported to also occur in the Red Sea, Persian Gulf, and along the Indian coast, but those reports need confirmation using molecular approaches. As a rockweed epiphyte, V. lanosa grows only where rockweed grows, and both are considered to be cold water species that tolerate anything between freezing temperatures to as warm as 25°C (77°F). V. lanosa has a narrower salinity range than A. nodosum and is rarely found in estuarine environments, instead preferring full salinity seawater.

Interactions between V. lanosa and A. nodosum have been extensively studied for their unique and biologically interesting features. The two species appear to have co-evolved, with evidence that chemical signaling and nutrient transfer may occur between the two. There is some debate as to whether V. lanosa is a harmless rockweed epiphyte or is actually a harmful parasite. Ascophyllum normally deters epiphytes from attaching via epidermal shedding, a process of continuously shedding and regenerating epidermal cells to maintain a smooth layer. V. lanosa rhizoids are able to penetrate deep through the rockweed epidermal layer and thus avoid detachment. The rhizoids may do considerable damage to rockweed cells, and phlorotannins have been observed to accumulate in the host cells adjacent to the rhizoids, possibly as a defense or repair mechanism. However, the damage may have occurred to the rockweed before V. lanosa colonization, with the damage itself providing a route of entry for V. lanosa to initially attach. Rockweed growing in highly wave-exposed sites, where it is more likely to be damaged by wave action, supports maximum biomass of V. lanosa. One study found that V. lanosa spores preferentially settle on branching axils and damaged areas of rockweed thallus and had higher survival rates there as opposed to other areas of the thallus.

Complicating this relationship is that a third species is usually found associated with V. lanosa and A. nodosum, the obligate and mutualistic (beneficial to both species) fungus Mycophycias ascophylli. The fungus grows in A. nodosum tissue adjacent to V. lanosa rhizoids, and it is hypothesized that it may facilitate nutrient transfer to V. lanosa, protect A. nodosum from being damaged by V. lanosa rhizoids, or both. In addition, V. lanosa itself is often a host to the parasitic red alga Choreocolax polysiphoniae, which forms transparent to opaque cushion-like structures on V. lanosa thalli and receives nutrients from its host.

V. lanosa is a perennial alga with a triphasic life cycle alternating between two diploid spore-producing generations and one haploid gamete-producing generation made up of males and females. The reproductive cycle can be said to start in the spring, when the gametophytes produce spermatia (males) and carpogonia (females). The spermatia drift in the water until they encounter carpogonia, which are attached to female gametophytes. After fertilization these develop into carposporophytes, the first diploid generation. These form cystocarps with reproductive carposporangia, which then release diploid carpospores from July until October. The carpospores attach to new host rockweed fronds, germinate, and develop into tetrasporophytes, which are the second diploid generation. The tetrasporophytes then produce haploid tetraspores the following summer to establish a new generation of haploid gametophytes. In the field, tetrasporophytes and gametophytes are morphologically similar and they often co-exist at equal abundance within a population.

History of Use

V. lanosa was first described by Carl Linnaeus, the father of taxonomic nomenclature, in 1767, though at the time he named it Fucus lanosus. This shows that V. lanosa was recognized as a unique species by at least the mid-1700’s, but otherwise little is known regarding its history of human use. V. lanosa has a distinctive aroma that more than likely piqued the culinary interest of coastal people who encountered it while harvesting rockweed. The aroma is similar to that of white truffles, which have been consumed by humans for at least 4,000 years. Dr. Paul Hass is credited with determining in 1931 that methyl sulfide, which is also found in white truffles, was the primary compound responsible for truffle seaweed’s complex aromatic profile, which has savory, earthy, musky, and sulfurous aspects. However, it may not be until recently (about 2015) that Norwegian and Icelandic researchers first marketed the culinary potential of V. lanosa as sea truffle. Prior to 2015 there appears to be no written references of V. lanosa (or Polysiphonia lanosa) being known as sea truffle, and nothing indicating it was used as food, medicine, or for any other purpose. If so, this makes truffle seaweed a good example of how we are still discovering the benefits of the 10,000 or so seaweed species, most of which have been little studied.

Baked sweet potato fries with truffle aioli dipping sauce and truffle & salt sprinkled on top!

Culinary Attributes

Truffle fungi have been coveted for thousands of years for their distinctive earthy, woodsy, and umami flavor and aroma. Sumerian texts written on clay tablets 4,000 years ago provide the first written records of truffles, and they retain their mystique in modern times as one of the world’s most expensive and legendary ingredients. Like many edible seaweeds, truffles are umami-rich, but they are more aromatic and have more complex flavor profiles than most seaweeds. Truffles are used as a seasoning, often for otherwise mildly flavored dishes such as pastas, risotto, eggs, creamy soups, or potatoes, and they can be stirred into all kinds of sauces. They are best when added just before serving because heat destroys the aromatic flavor compounds. Black truffles are milder than the more pungent white truffle.

Like white truffles, truffle seaweed is umami-rich and pungently aromatic. Truffle seaweed can substitute for white truffles in most recipes and should be used with the same understanding that heat will dull or destroy the flavor. Less than a teaspoon of dried truffle seaweed, or even just a pinch, will suffice for most recipes, but entire clumps can be used as garnish for scallop or mussel dishes. When using truffle seaweed as a garnish, it should be rinsed, blanched, and shocked in cold water to reconstitute. We’ve found lightly roasting and crumbling as garnish changes the flavor but is also delicious. Check out the MSCV Recipe page for more ideas.

Consumers may conflate truffle seaweed (V. lanosa) with the red alga species Osmundea pinnatifida, which is known as pepper dulse or truffle of the sea. Other than both being small red algae, they are not closely related and do not closely resemble one another. Pepper dulse, like truffle seaweed, is gaining a culinary reputation and both species are available from on-line specialty retailers. Pepper dulse has not been reported to occur in the Northwest Atlantic (Maine and Canada) and it’s mostly known in Europe, especially Ireland, France, and Spain. Another marine species known as sea truffle is the clam Venus verrucosa. Sea truffle clams are a tasty European delicacy, but like all things with the name truffle attached to them they are expensive.

Nutritional & Medicinal Attributes

Truffle seaweed is highly nutritious and it contains the same types of polysaccharides and bioactive compounds with potential medicinal properties found in other red seaweeds. V. lanosa is especially high in organosulfur compounds, responsible for its unique flavor and aroma, and bromophenols. Both groups of compounds are of interest to researchers for pharmaceutical applications. Like other edible seaweeds, truffle seaweed has high mineral and iodine content, and it tends to contain more potassium than other red seaweeds. It is also relatively rich in zinc and selenium (81-185 µg zinc per gram and 0.53 – 0.85 µg selenium per g). Compared to other seaweeds, truffle seaweed is also relatively high in n-3 and n-6 polyunsaturated fatty acids, which are essential for human health. Truffle seaweed has enough complete protein to theoretically meet ones’ daily protein requirement, though that would require eating about 350 grams of dried seaweed, which is too much & too expensive. Truffle seaweed is best used sparingly and most people consume it for culinary reasons rather than for its nutritional or medicinal properties.

Wild Harvest & Processing

Truffle seaweed is hand-harvested from rockweed fronds, a laborious, time consuming, and even somewhat dangerous endeavor due to the high potential for slips and falls while negotiating rocky and slippery rockweed beds. Small-scale artisanal harvesting of this species for commercial sale has been happening in Iceland and Norway since about 2015, and since about 2020 in Maine and Canada. So far, just small amounts are harvested, and this will probably remain true going forward. In this respect truffle seaweed is similar to truffle fungi, which are also mostly wild-harvested in scarce amounts. To date, rockweed and truffle seaweed harvesting is not integrated, i.e., rockweed harvesters do not selectively remove truffle seaweed. This approach could potentially help rockweed harvesters earn more income provided the additional labor did not cancel out the additional income. A study published in 2019 examined V. lanosa abundance in an area in southwestern Norway in relation to environmental factors, with the aim of identifying best harvest areas. The study concluded that V. lanosa was most
abundant in areas of high wave exposure, in the mid to lower parts of the Ascophyllum zone (i.e., at the lower tidal range), and in areas where Ascophyllum growth was less dense. Another study examining harvest impacts in the Bay of Fundy estimated that monthly hand-harvesting V. lanosa from rockweed in the summer months impacted about 10% of the standing crop and allowed for full recovery of the population. The authors concluded that truffle seaweed could be a new, economically-viable, and ecologically sustainable fishery in southwestern Nova Scotia.

After harvesting, truffle seaweed is minimally processed with solar or greenhouse drying. Care is taken to maintain drying temperatures below 90°F to preserve flavor compounds. The dried truffle seaweed is then examined for quality and packaged for sale in small quantities to chefs and other culinary enthusiasts.

 

 

Aquaculture

Its status as a semi-obligate epiphyte means truffle seaweed could theoretically be cultured, but not without challenges, and attempts to do so are still in early stages. A Norwegian graduate student published a 2022 thesis on growing V. lanosa in the lab. Small branched tetrasporophyte fragments were collected from the wild and grown in flasks in suspension with seawater and nutrients. Over the course of 45 days the student measured growth and condition of the fragments. Although growth was observed and the student was able to maintain stock cultures for over a year, growth rates were relatively slow. However, the lab grown tetrasporophytes did produce tetraspores, indicating the potential to fully close the life cycle under culture. Efforts to grow this intriguing and valuable species in the lab will undoubtedly proceed, but until it can be grown at scale, truffle seaweed will have to be hand-harvested from the wild.