Biodiversity & Conservation

Knotted wrack - Ascophyllum nodosum

Ascophyllum nodosum

Image Keith Hiscock - Isolated growth of Ascophyllum nodosum on rock. Image width ca 60 cm.
Image copyright information

  • #
  • #
  • #
  • #
  • #
  • #
  • #
  • #
  • #
  • #
  • #
Distribution map

Ascophyllum nodosum recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

Why do the maps differ?

Sightings Have you seen Ascophyllum nodosum?
If so please submit your record.

  • WC_Act_1981
  • UK_BAP
Taxonomy icon Taxonomy Taxon English term
Phylum Ochrophyta Brown and yellow-green seaweeds
Class Phaeophyceae
Authority Le Jolis, 1863
Recent synonyms None
Map icon Recorded Distribution in Britain and Ireland All coasts of Britain and Ireland.
Habitat information icon Habitat information The species attaches to rocks and boulders on the middle shore in a range of habitats, from estuaries to relatively exposed coasts. It occupies a similar shore height as Fucus vesiculosus. Subtidal populations have been reported, for example in the very clear waters of Rhode Island, USA. However, an intertidal habit is more usual.
Text page icon Description A common large brown seaweed, dominant on sheltered rocky shores. The species has long strap like fronds with large egg-shaped air bladders at regular intervals. The fronds of Ascophyllum nodosum are typically between 0.5 and 2m in length. The species often bears tufts of the small reddish-brown filamentous epiphytic algae Polysiphonia lanosa. Ascophyllum nodosum occurs on the middle of the shore, often with Fucus vesiculosus. The species grows slowly and plants can live to be several decades old. Individual fronds can become up to 15 years old before breakage.
Identifying features
  • Frond narrow without midrib.
  • Large swollen egg shaped air bladders at intervals along middle of the frond.
  • Reproductive bodies rounded on short stalks.
  • Dichotomously branched.
Additional information icon Additional information Detached forms of Ascophyllum nodosum are known from several habitats. Ascophyllum nodosum var. mackaii is found on very sheltered shores, in sea lochs and is sometimes common on the west coasts of Ireland and Scotland. The frond has extensive dichotomous branching and bears few air bladders. The plants drift in large, spherical masses in sheltered waters. Ascophyllum nodosum var. scorpioides, which is abundant in New Hampshire (U.S.A.), is often associated with the marsh grass Spartina alterniflora. According to Gibb (1957) the major difference between the ecads mackaii and scorpioides is the proportion of apical to lateral branching. If branching is both 'apical and lateral' the algae would be designated as mackaii while if it is 'almost entirely lateral' it would be designated as scorpioides. Unattached forms arise when detached fragments of Ascophyllum nodosum are deposited onto the shore where they continue to multiply and branch independently of the original fragment (Chock & Mathieson, 1976).

Chock & Mathieson (1979) demonstrated the physiological responses of Ascophyllum nodosum and its detached ecad scorpioides were similar under varying conditions of light intensity, temperature and salinity.

Ascophyllum nodosum var. mackaii:
The presence of the ecad in any particular situation depends on the combination of a number of conditions applying at a tide level between high and low water neaps:
  • frequent alternation of high and low salinities so a supply of freshwater is of primary importance;
  • good shelter from wave action because of the unattached state of the ecad;
  • absence of fast moving water, whether caused by freshwater streams or tidal conditions;
  • flat, undulating or slightly sloping shore profile where stability is high, and
  • substratum type, the porosity of which affects the conditions of salinity and also influences, to some extent, the development of the ecad.
Very sheltered conditions are often found at loch heads on the west coast of Scotland and in these situations the ecad is sometimes present in great abundance. Sheltered or land-locked bays or situations in the lee of small islands are other favourable positions (Gibb, 1957).

Want to know more? more

This review can be cited as follows:

Jacqueline Hill & Nicola White 2008. Ascophyllum nodosum. Knotted wrack. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 09/02/2010]. Available from: <http://www.marlin.ac.uk/speciesinformation.php?speciesID=2632>