Knotted Wrack

Rockweed – Knotted Wrack
Ascophyllum nodosum

Phylum: Phaeophyta
Family: Fucaceaeknotted wrack

Knotted wrack is a perennial, olive green, seaweed with swollen unpaired airbladders2 called knots1,2 that help it stay afloat, optimizing light exposure for photosynthesis.  These airbladders can be counted to determine the age of the specimen, as they are produced yearly1.  Knotted Wrack is one of the dominant rockweeds of the intertidal zone, with a lifespan of 8-23 years1.   A wrong step on this slippery seaweed could land your shin on a rock, so be careful when stepping over rocks covered in this species.

Tubed weed (Polysiphonia lanosa) is a common epiphyte found on knotted wrack, and bears resemblance to a cheerleader’s maroon pompoms clinging to the fronds.  Knotted wrack can be distinguished from Fucus sp. because the blade is not flat, but instead looks like a bunch of giant noodles2.  Knotted wrack does branch dichotomously1, but does not have the flat Y shape that Fucus sp. show. Knotted wrack may also have small paired branches 2.5-5 cm (1-2”) in length with swollen tips that resemble rabbit ears2.

Some sources report that swollen, warty, yellowish structures called receptacles appear in the spring (May-June), while others claim reproductive structures appear from Jan-April3 from short branches.  These receptacles release motile gametes and subsequently detach from the organism1,2.

Knotted wrack more recently has been used for organic fertilizer, and commercially exploited as animal food, an emulsifier, and as a thickening agent.1.

Substrate: rocks, piers
Size: usually 0.6m (2’)2 but may grow to be 3 m (10 ft)1
Range: Arctic to Long Island Sound2


1. Watling, L., Fegley, J., Moring, J., White, S., Sulzer, A. (2003) Life Between the Tides. Tilbury House Pub

2. Gosner, Keneth L. (1978), Peterson Field Guides: Atlantic Seashore – A field guide to sponges, jellyfish, sea urchins, and more

3. Alden, P., Cassie, B. (1998). National Audubon Society Field Guide to New England. New York: Alfred A. Knopf, inc..

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