Chorda filum is a true kelp disguised as a slippery brown string. Where the bootlace kelp grows, how a hollow cord survives a year, and its edible end.
Snorkel over a sheltered northern bay in late summer and you may part something unexpected: great drifting tangles of brown string, metres long, sliding over the eelgrass like a spilled crate of bootlaces. Grab one (it's harmless — the ominous name refers entirely to its looks) and you'll find a slippery, hollow cord the width of a shoelace, anchored nowhere in particular anymore if it's drifted in. This is Chorda filum — dead man's bootlace, bootlace weed, or, in a more romantic Scandinavian register, mermaid's fishing line. It is one of the strangest things the kelp order produces, and proof of how far the body plan can stretch.
| Scientific name | Chorda filum |
| Common names | Dead man's bootlace, bootlace weed, sea lace, mermaid's fishing line |
| Group | Brown algae (Phaeophyceae), order Laminariales — a true kelp |
| Form | A single unbranched cord, 3–8 m long and only 3–6 mm thick; hollow with mucilage-filled chambers |
| Habitat | Very sheltered, often brackish water: lagoons, bays, estuary mouths, seagrass beds; low intertidal to about 5 m |
| Range | Cold-temperate to Arctic North Atlantic and North Pacific, including the Baltic and White seas |
| Lifespan | Annual — sporeling to eight-metre cord to spore release in under a year |
Look at any familiar kelp — tangle kelp, oarweed, sugar kelp — and you see the standard laminarian design: root-like holdfast, stiff stipe, broad blade. Chorda scrapped nearly all of it. The holdfast remains but is tiny, a small disc of a few millimetres, because it barely needs to grip: this is a plant of calm water, where wave load is negligible. The stipe-blade distinction is gone entirely. The whole visible plant is one cord, and that cord is the blade, the stipe and eventually the reproductive surface all at once.
Internally the cord is a marvel of economy. It's hollow, divided into chambers lined with a slippery mucilage — sorbitol-rich and slick to the touch — that helps the plant maintain buoyancy and keeps the cord supple. Textbooks sometimes call it a "false" or honorary kelp, but phylogenetically it sits squarely in the Laminariales; it's simply what kelp architecture looks like when a lineage specialises for quiet water.
Chorda filum is strictly annual, and that single fact governs everything. Spores released in autumn germinate into microscopic gametophytes — the same alternation of generations as every kelp, described in our kelp life cycle explainer — and the tiny sporophytes that emerge in late winter have perhaps eight months to do an entire life. They respond phenomenally: through spring the young cords elongate at centimetres per day, racing toward eight metres by midsummer, while thickening only modestly.
By comparison with the perennial canopy kelps, which invest for years in holdfast and stipe, Chorda lives like a weed made good — a strategy explored in annual versus perennial kelps. Growth, not defence or structure, is where the budget goes. Then, in late summer, patches of the cord's surface darken slightly: zoosporangia, releasing the spores of the next generation. By autumn the cords weaken, snap, and wash ashore in ropy tangles that beach-walkers know well — a significant rain of drift kelp and wrack into the shore's food web.
Almost everything about Chorda reads as an adaptation to calm, often brackish water:
The species' limits are equally instructive. Too much wave energy snaps the cords; too much exposure and there's nothing to anchor to. It is northern cold-water in taste and does poorly where summers warm strongly, which is why marine heatwaves in sheltered, poorly flushed bays — the sort documented in our marine heatwave overview — hit Chorda habitat especially hard: the water that shelters the plants also traps the heat.
If you want to see live Chorda, think calm: a landlocked salt lagoon, a harbour's edge, a broad tidal pool in an estuary mouth, midsummer. Wade in and look down. The cords are slicker than they look (that mucilage earns its keep against grazers and fouling) and they'll slide through your fingers. Autumn shorelines after a blow will show you the spent generation piled in wrack lines — ropy, dark, and already coming apart. In northern Japan and Korea, closely managed harvests of young spring Chorda end up dried and reconstituted into a molasses-sweet kelp confection — one of the more obscure entries in the long catalogue of edible kelps, and a reminder that nearly every cold-water brown alga has found its way into someone's kitchen.
Globally the species is common and secure; locally it is more delicate than it appears. The sheltered lagoons and brackish bays it loves are precisely the habitats most dredged, infilled, and polluted by coastal development — a textbook case of coastal squeeze. Where the Baltic's Chorda beds thin, it's usually water quality, not the alga, that has changed. Restoration here mostly means protecting water quality and soft sediment habitat — the seaweed will do the rest, as annuals tend to.
Not remotely. The name is pure appearance — tangles of long dark cords do look macabre draped across a lagoon. The alga contains no toxins and mild mucilage. Getting briefly wrapped in it while swimming is messy, not hazardous; the cords snap easily.
A kelp, properly. Chorda belongs to the order Laminariales alongside giant kelp and kombu, and runs the same gametophyte-sporophyte life cycle. Its string form is a specialisation within the order, not evidence of an outsider.
Cold-temperate northern shores: sheltered bays, tidal lagoons and estuary mouths from Scandinavia and Britain around to Iceland, Atlantic Canada, the Baltic and White seas, and the North Pacific from Hokkaido to Alaska. Midsummer, at low water, in the calmest corner you can find.
The cord's chambers hold slick mucilage rich in the sugar-alcohol sorbitol. The slipperiness plausibly deters grazers and epiphyte settlement, and keeps the long cords supple and tangle-sliding rather than knotting tight in currents.
Yes — young Chorda is eaten in parts of northeast Asia, usually dried and candied or simmered. Flavour is mild and the texture slippery-tender. As with any foraged seaweed, harvest the cords, leave the substrate, and check local rules first.
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