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Marine Animal Encyclopedia

Knotted wrack Ascophyllum nodosum

Knotted wrack belongs to a group of tough brown seaweeds that often dominate rocky seashores in cooler climates. It is firmly attached to the rocks by a disk-shaped holdfast, from which arise several narrow fronds that often grow to 3 ft (1 m) in length, and exceptionally to 9 ft (3 m) in very sheltered situations. Single oval bladders grow at intervals down the knotted wrack frond. The fronds produces about one bladder a year, so knotted wrack’s age can be roughly estimated by counting a series of bladders. The bladders hold the fronds up in the water so that they gain maximum light, which is an advantage in the often turbid waters where knotted wrack grows. This also makes it harder for grazing snails to reach the fronds when the tide is in.

The dark brown fronds of knotted wrack may be bleached almost to yellow in summer. Reproductive structures that look like swollen raisins are borne on short side-branches, and orange eggs can sometimes be seen oozing from them.

Knotted Wrack and Crofter’s Wig

In very sheltered bays and sea lochs, detached pieces of “normal” knotted wrack will continue to grow, lying loose on the seabed. In situations where the fronds are alternately covered by salt and fresh water, they divide repeatedly to form a dense ball that has no bladders or reproductive structures. This attached form of knotted wrack, which is known as crofter’s wig, appears very different from the attached form, even though it is genetically identical.

Knotted wrackzoom image
  • Kingdom Phaeophyta
  • Length Up to 10 ft (3 m)
  • Water_temperature 32–64°F (0–18°C)
  • Distribution Coasts of northwestern Europe, eastern North America, and north Atlantic islands
Knotted wrack habitat mapzoom image