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Dieren en planten

Brown seaweed   Knotted wrack   Marine flora   Flora of the sea floor   Seaweeds   

Water en land

Mens en Milieu

Knotted wrack


30-150 centimeters long


sturdy stems, round at the base, further flattened and irregularly branched

  • Dut: Knotswier
  • Eng: Knotted wrack (Norwegian kelp, knotted Kelp, egg wrack)
  • Ger: Knotentang
  • Lat: Ascophyllum nodosum
Knotted wrack, foto fitis, sytske dijksen

Knotted wrack

It's not hard to see where this seaweed got its name. The large, sturdy single bladders filled with gas in the middle of the fronds look like large knots. These 'airbags' help the plant to stand up straight under water. Knotted wrack is found in the North Sea and along the Atlantic coasts. The species attaches itself to rocks and stones in the middle of the tidal region. It can survive for a long time after breaking off from its base. All sorts of other seaweed species grow underneath knotted wrack, species that without protection would normally dry out during ebbing water. This seaweed also provides shelter for small marine animals.

On Texel

, Sytske Dijksen,

Sometimes you find knotted wrack along the wadden dike and in the harbours. There's almost always knotted wrack washed upon the beach, but this is usually from seaweed that broke loose from elsewhere, coming from as far away as France or southern England.

  • Reproduction

    Besides the large, sturdy bladders, knotted wrack has softy slimy bulges that contain the reproductive organs. Both bladders and bulges can be seen in the photo. The bright orange-colored male cells are released in early spring. Clumps of knotted wrack are known to grow very old, as old as 400 years!

  • Usefulness

    Knotted wrack is used for packing shellfish that are shipped to restaurants.