BIOTIC Species Information for Ascophyllum nodosum
Researched byJacqueline Hill & Nicola White Data supplied byMarLIN
Refereed byDr Dagmar Stengel
Scientific nameAscophyllum nodosum Common nameKnotted wrack
MCS CodeZR375 Recent SynonymsNone

PhylumChromophycota Subphylum
Superclass ClassPhaeophyceae
Subclass OrderFucales
Suborder FamilyFucaceae
GenusAscophyllum Speciesnodosum

Additional InformationDetached 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).
Taxonomy References Chock & Mathieson, 1979, Chock & Mathieson, 1976, Gibb, 1957,
General Biology
Growth formShrub
Feeding methodPhotoautotroph
Mobility/MovementPermanent attachment
Environmental positionEpifloral
Typical food typesNot relevant HabitAttached
BioturbatorNot relevant FlexibilityHigh (>45 degrees)
FragilityFragile SizeLarge(>50cm)
Height Growth Rate5 - 15 cm/year
Adult dispersal potentialNone DependencyIndependent
General Biology Additional Information
  • The species is very long lived and has low recruitment. Growth rate is very slow in germlings but increases as the plant ages. During the first year growth takes place at 0.2 cm per year, rising to 1.5 cm per year in the second year (Sundene, 1973). The first air bladder is formed in the fifth year, after which they are produced annually. The holdfasts of Ascophyllum nodosum are thought to persist for several decades from which new fronds regenerate.
  • Growth is apical. 90% of the apical elongation takes place in the 0 to 5mm zone behind the apex. Growth rate is maximal in the morning, followed by a continuous decline throughout the day (Strömgren & Nielsen, 1986). In Strangford Lough in Northern Ireland, Stengel & Dring (1997) observed growth to be highly seasonal with low growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels.
  • Ascophyllum nodosum repeatedly sloughs its entire outer epidermis, a phenomenon not exhibited by other related seaweeds. (Filion-Myclebust & Norton, 1981). Despite its longevity Ascophyllum nodosum is remarkably free of epiphytes even when adjacent plants of other species of fucoid algae are heavily infested. Shedding activity appears to contribute to this difference. The authors frequently observed that when the outer layers are shed, potential epiphytes including spores and germlings of other algae that had settled on the surface were discarded with the epidermis. Only those epiphytes with deeply penetrating rhizoids, such as Polysiphonia lanosa (see below), are able to maintain a hold.
  • Polysiphona lanosa is an obligate epiphyte that occurs primarily on Ascophyllum nodosum. The rhizoids of Polysiphonia lanosa penetrate the host and obtain some nutrition from Ascophyllum nodosum. However, the quantity of carbon obtained is minimal and Polysiphonia is pigmented and can photosynthesize itself (Levin & Mathieson, 1991).
  • The thalli of Ascophyllum nodosum contain an endophytic fungus, the ascomycete Mycosphaerella ascophylli Cotton, that penetrates throughout the thallus (Fries, 1988). Garbary & MacDonald (1995) provided experimental evidence for an obligate mutualistic symbiosis where infected thalli were longer, had greater apical diameters and more apical hairs than non-infected thalli. Garbary & London (1995) also suggest that the fungus may protect Ascophyllum nodosum from desiccation.
Biology References Baardseth, 1970, Strömgren & Nielsen, 1986, Fish & Fish, 1996, Filion-Myclebust & Norton, 1981, Fries, 1988, Garbary & London, 1995, Garbary & MacDonald, 1995, Sundene, 1973, Chock & Mathieson, 1976, Gibb, 1957, Levin & Mathieson, 1991, Barton, 1892, Stengel & Dring, 1997,
Distribution and Habitat
Distribution in Britain & IrelandAll coasts of Britain and Ireland.
Global distributionGlobal distribution is restricted to the North Atlantic Ocean. Its northern limits are the White Sea in the east and Baffin Island in the west. Southern distributions extend to northern Portugal and New Jersey.
Biogeographic rangeNot researched Depth rangeNot relevant
MigratoryNon-migratory / Resident   
Distribution Additional InformationThe local distribution of Ascophyllum nodosum is largely determined by wave exposure. As exposure to wave action increases the number of plants becomes progressively less and they consist increasingly of stumps and short lived shoots. On sheltered shores Ascophyllum nodosum may competitively exclude %Fucus vesiculosus%. The species can tolerate salinities down to 15 psu and can tolerate constant immersion but thrives better when exposed to air at low tide.

Substratum preferencesSmall boulders
Large to very large boulders
Physiographic preferencesStrait / sound
Ria / Voe
Open coast
Biological zoneMid Eulittoral
Upper Eulittoral
Wave exposureModerately Exposed
Very Sheltered
Extremely Sheltered
Ultra Sheltered
Tidal stream strength/Water flowStrong (3-6 kn)
Moderately Strong (1-3 kn)
Weak (<1 kn)
Very Weak (negligible)
SalinityFull (30-40 psu)
Variable (18-40 psu)
Reduced (18-30 psu)
Habitat Preferences Additional Information
Distribution References Baardseth, 1970, Peckol, 1988, Hardy & Guiry, 2003,
Reproduction/Life History
Reproductive typeGonochoristic
Developmental mechanismSpores (sexual / asexual)
Reproductive SeasonInsufficient information Reproductive Location
Reproductive frequencyAnnual episodic Regeneration potential No
Life span11-20 years Age at reproductive maturity3-5 years
Generation time6-10 years FecundityInsufficient information
Egg/propagule size Fertilization type
Larval/Juvenile dispersal potentialInsufficient information Larval settlement period
Duration of larval stage2-10 days   
Reproduction Preferences Additional Information
  • Life span.
    Ascophyllum nodosum fronds can become up to 15 years old before breakage. The holdfasts, from which new fronds regenerate, are observed to exist for much longer so whole plants may live to be several decades old. Sundene (1973) found Ascophyllum nodosum needed five years to develop into fertile plants.
  • Time of first gametes.
    Mainly March and April, sometimes as early as January and February (D. Stengel pers. comm.).
  • Ascophyllum nodosum is dioecious and like all other species of fucoids has only a sexual generation. Receptacles are initiated in April and may take one year to become fertile. Thus, receptacles are present on the plant for 12-14 months and ripen in April to June of the following year. Gametes are released from April onwards and the release of gametes is triggered by the exposure of ripe receptacles to air overnight. Fertilization takes place externally and zygotes settle and form a rhizoid within ten days. The receptacles are then shed during June.
  • Recruitment.
    Recruitment in Ascophyllum nodosum is very poor with few germlings found on the shore. The reason for this poor recruitment is unclear, because the species invests the same high level of energy in reproduction as other fucoids and is extremely fertile every year (Printz, 1959). However, the reproductive period lasts about two months, much shorter than for other fucoids. Printz (1959) suggests that it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization of Ascophyllum nodosum. The slow growth rate of germlings, which increases the chance of their being covered by diatoms or grazed by Littorina, may also help to explain the scarcity of germlings (Baardseth, 1970).
  • Ascophyllum nodosum var. mackaii
    In Europe, direct reproduction of the ecad mackaii is vegetative and sexual reproduction gives rise to attached Ascophyllum nodosum. The formation of receptacles on intermediate ecad stages appears to be a frequent phenomenon, although abnormalities in receptacle shape and position usually accompany this (South & Hill, 1970). Sexual reproduction of intermediate and advanced forms of ecad mackaii in Newfoundland is relatively rare (South & Hill, 1970).
Reproduction References Baardseth, 1970, Printz, 1959, Bacon & Vadas, 1991, Sundene, 1973, South & Hill, 1970,
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