Two-headed wrack (Fucus distichus)

Fucus distichus on rocky shore.

Photographer: Dr Keith Hiscock Copyright: Dr Keith Hiscock

Fucus distichus

Photographer: Robert Beharie Copyright: Robert Beharie

Fucus distichus

Photographer: Robert Beharie Copyright: Robert Beharie

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help

Researched by	Nicola White	Refereed by	Dr Graham Scott
Researched by Nicola White	Refereed by Dr Graham Scott
Authority	Linnaeus, 1767
Authority Linnaeus, 1767
Other common names	-	Synonyms	Fucus distichus distichus Powell 1957a, Fucus distichus anceps Linnaeus, 1767
Other common names -	Synonyms Fucus distichus distichus Powell 1957a, Fucus distichus anceps Linnaeus, 1767

Summary

Description

A small tufted brown alga. It has narrow fronds without airbladders and short receptacles. The species has a lifespan of 3 years and grows up to 30 cm long.

Recorded distribution in Britain and Ireland

Restricted to northern shores of Scotland and north and west Ireland.

Global distribution

Norway, northern Scotland, Iceland, Greenland, eastern North America from Labrador to Maine and the Pacific coast of America discontinuously from Alaska to California

Habitat

Occurs in rock pools and on rock faces in the upper eulittoral at wave exposed locations in Scotland & Ireland.

Depth range

Not relevant

Identifying features

Narrow frond without airbladders.
Caecostomata rare.
Receptacles short, typically 18 mm, max. 40 mm.
Plants small, typically 10 cm long at maturity (max. 30 cm).

Additional information

No text entered

Listed by

Biology review

Taxonomy

Level	Scientific name	Common name
Phylum	Ochrophyta	Brown and yellow-green seaweeds
Brown and yellow-green seaweeds
Class	Phaeophyceae

Order	Fucales

Family	Fucaceae
Genus	Fucus
Authority	Linnaeus, 1767
Recent Synonyms	Fucus distichus distichus Powell 1957aFucus distichus anceps Linnaeus, 1767

Biology

Parameter	Data
Typical abundance	Moderate density
Male size range	Up to 30cm
Male size at maturity	10cm
Female size range	10cm
Female size at maturity
Growth form	Foliose
Growth rate	10cm/year
Body flexibility
Mobility
Characteristic feeding method	Autotroph
Diet/food source
Typically feeds on
Sociability
Environmental position	Epifloral
Dependency	Independent.
Supports	No information
Is the species harmful?	Data deficient

Biology information

The morphology of Fucus distichus is remarkably varied. Powell (1957a) recognised four subspecies, which were later separated into two distinct species (Fucus distichus and Fucus evanescens) by Rice and Chapman (1985). In its rock pool habit on the east coast of North America Fucus distichus is considered by Pearson & Davison (1994) to be slow growing, a poor competitor and stress intolerant. Adults of Fucus distichus are very tolerant of grazing due to a high content of phlorotannin, but germlings do not have any protection and are susceptible to grazing by littorinids.

Habitat preferences

Parameter	Data
Physiographic preferences	Open coast
Biological zone preferences	Upper eulittoral
Substratum / habitat preferences	Bedrock, Large to very large boulders
Tidal strength preferences	Moderately strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Very weak (negligible), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferences	Extremely exposed, Very exposed
Salinity preferences	Full (30-40 psu), Variable (18-40 psu)
Depth range	Not relevant
Other preferences	No text entered
Migration Pattern	Non-migratory or resident

Habitat Information

In Britain and Ireland, Fucus distichus has only been recorded attached to bedrock in the mid to upper eulittoral zone on exposed rocky shores in northern Scotland and Ireland. It is thought to be prevented from growing further south due to its poor tolerance of desiccation and inability to compete with plants growing further down the shore. However, on the east coast of North America, Fucus distichus is only found in rock pools and is incapable of growing on emergent rock surfaces in the mid to upper eulittoral.

Life history

Adult characteristics

Parameter	Data
Reproductive type	Permanent (synchronous) hermaphrodite
Reproductive frequency	Annual protracted
Fecundity (number of eggs)
Generation time	1-2 years
Age at maturity	2
Season	April - August
Life span	2-5 years

Larval characteristics

Parameter	Data
Larval/propagule type	-
Larval/juvenile development	Not relevant
Duration of larval stage	No information
Larval dispersal potential	No information
Larval settlement period	Not relevant

Life history information

Receptacles of Fucus distichus are initiated in December, they become ripe in April and gametes are released from April to August. Plants usually become mature when 100mm long. The whole plant dies back after reproducing and is removed from the rock by wave action during its third winter. Most plants live for 2 to 3 years (Powell, 1957b).
Fucus distichus produces gametes of both sexes within each conceptacle. When released, ova can survive and disperse for several days. Antherozoids can only live for several hours. Self-fertilisation is thought to be high in the species and once a zygote is formed it can only be dispersed over limited distances (Rice et al., 1985).

Sensitivity reviewHow is sensitivity assessed?

The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.

Physical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Substratum loss [Show more] Substratum loss Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details Evidence Fucus distichus lives permanently attached to rock so would be removed upon substratum loss. The species has been observed to readily recruit to cleared areas (Ang, 1991) and has fast growth rates, so recovery rates should be high.	High	High	Moderate	Moderate
Smothering [Show more] Smothering Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details. Evidence The effect of smothering depends on the state of the tide when the factor occurred. If smothering happened while the plant was emersed the whole plant would be buried underneath the sediment preventing photosynthesis. If smothering occurred while the plant was immersed the impact of smothering would be lessened because some of the fronds would escape burial. The species has been observed to readily recruit to cleared areas (Ang, 1991) and has fast growth rates, so recovery rates should be high.	High	High	Moderate	Moderate
Increase in suspended sediment [Show more] Increase in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence Silt may cover some of the frond surfaces reducing light available for photosynthesis and lowering growth rates. On return to normal siltation levels the normal growth rate would be quickly restored.	Low	Very high	Very Low	Moderate
Decrease in suspended sediment [Show more] Decrease in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence			No information
Desiccation [Show more] Desiccation A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details. Evidence Fucus distichus is intolerant of desiccation, but the critical water content is not known. Increases in desiccation would cause the upper limit of the species distribution to become depressed. Decreases in desiccation may allow the upper limit of the species to extend up the shore. The species has been observed to readily recruit to cleared areas of the shore (Ang, 1991) and has fast growth rates, so recovery rates should be high.	High	High	Moderate	Moderate
Increase in emergence regime [Show more] Increase in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence Fucus distichus would suffer desiccation, nutrient stress and extremes of temperature when exposed to air. Increases in the period of emergence would cause the upper limit of the species distribution to become depressed. Decreases in the period of emergence may cause the species to extend further up the shore. The species has been observed to readily recruit to cleared areas (Ang, 1991) and has fast growth rates, so recovery rates should be high.	High	High	Moderate	Moderate
Decrease in emergence regime [Show more] Decrease in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence			No information
Increase in water flow rate [Show more] Increase in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence Increased water flow rate may cause plants to be torn off the substratum. However, Fucus distichus appears to attach very strongly to the substratum because it lives in areas exposed to very high wave action and strong water currents.	Low	High	Low	Low
Decrease in water flow rate [Show more] Decrease in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence			No information
Increase in temperature [Show more] Increase in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence Fucus distichus reaches the southern limit of its distribution in the British Isles, so would probably be very intolerant of increases in temperature. Decreases in temperature would probably have little effect and may allow the species to colonize further south. The species has been found to tolerate freezing in small rock pools in Maine (Pearson & Davison, 1994).	High	High	Moderate	Moderate
Decrease in temperature [Show more] Decrease in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence			No information
Increase in turbidity [Show more] Increase in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence Turbidity would reduce light available for photosynthesis and so lower growth rates. On return to normal turbidity levels the normal growth rate would be restored.	Low	Very high	Very Low	Moderate
Decrease in turbidity [Show more] Decrease in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence			No information
Increase in wave exposure [Show more] Increase in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence Fucus distichus has been recorded worldwide from very sheltered to very exposed conditions, but in Britain & Ireland it is mainly found on very exposed shores. A shift to more sheltered conditions may allow other fucoid species to inhabit the shore which are faster growing and would out-compete Fucus distichus. An increase in the level of wave exposure may result in plants being torn off the substratum. Provided nearby Fucus distichus populations are maintained, recovery rates should be high because the species has been observed to rapidly recruit to cleared areas (Ang, 1991).	Intermediate	High	Low	Low
Decrease in wave exposure [Show more] Decrease in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence			No information
Noise [Show more] Noise Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details Evidence Seaweeds have no known mechanism for sound perception.	Tolerant	Not relevant	Not sensitive	Not relevant
Visual presence [Show more] Visual presence Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details Evidence Seaweeds have no known mechanism for visual perception.	Tolerant	Not relevant	Not sensitive	Not relevant
Abrasion & physical disturbance [Show more] Abrasion & physical disturbance Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details. Evidence Abrasion may damage the fronds of established seaweeds and kill germlings of Fucus distichus. Human trampling has been shown to significantly reduce the cover of fucoids on a shore (Holt et al., 1997) but trampling is unlikely to occur on the very exposed shores on which Fucus distichus lives. The species has been observed to readily recruit to cleared areas (Ang, 1991) and has fast growth rates, so recovery rates should be high.	Intermediate	High	Low	Low
Displacement [Show more] Displacement Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details Evidence Fucus distichus lives permanently attached to the substratum and upon removal it cannot re-establish an attachment. The species has been observed to readily recruit to cleared areas (Ang, 1991) and has fast growth rates, so recovery rates should be high.	High	High	Moderate	Moderate

Chemical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Synthetic compound contamination [Show more] Synthetic compound contamination Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example: evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity; evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity; evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity. The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details. Evidence Fucoids are generally robust in the face of chemical pollution (Holt et al., 1997), but no studies have been carried out on this particular species.	No information	Not relevant	No information	Not relevant
Heavy metal contamination [Show more] Heavy metal contamination Evidence No studies have been carried out on this particular species. However, fucoids are generally robust in the face of chemical pollution and do not appear to be harmed by heavy metals (Holt et al., 1997). Intolerance is therefore, reported to be low.	Low	Very high	Very Low	Very low
Hydrocarbon contamination [Show more] Hydrocarbon contamination Evidence Fucoids are generally robust in the face of chemical pollution (Holt et al., 1997), but no studies have been carried out on this particular species.	No information	Not relevant	No information	Not relevant
Radionuclide contamination [Show more] Radionuclide contamination Evidence Insufficient information	No information	Not relevant	No information	Not relevant
Changes in nutrient levels [Show more] Changes in nutrient levels Evidence Insufficient information	No information	Not relevant	No information	Not relevant
Increase in salinity [Show more] Increase in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence Fucus distichus lives on shores where it is frequently drenched in rain water, so it must be able to withstand variations in salinity. The species also extends into estuaries on the coast of North America.	Low	High	Low	Low
Decrease in salinity [Show more] Decrease in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence			No information
Changes in oxygenation [Show more] Changes in oxygenation Benchmark. Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details. Evidence Fucus distichus is unlikely to be affected by a reduction in oxygen levels because it can generate its own oxygen by photosynthesis. However, no studies have been found to support this.	No information	Not relevant	No information	Not relevant

Biological pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Introduction of microbial pathogens/parasites [Show more] Introduction of microbial pathogens/parasites Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details. Evidence Insufficient information	No information	Not relevant	No information	Not relevant
Introduction of non-native species [Show more] Introduction of non-native species Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details. Evidence Insufficient information	No information	Not relevant	No information	Not relevant
Extraction of this species [Show more] Extraction of this species Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence Fucus distichus would be able to recover within five years if 50 percent of the area was cleared. The species has been observed to recruit readily to cleared areas (Ang, 1991) and has a reasonably fast growth rate.	Intermediate	High	Low	Moderate
Extraction of other species [Show more] Extraction of other species Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence Insufficient information	No information	Not relevant	No information	Not relevant

Additional information

Importance review

Policy/legislation

Designation	Support
UK Biodiversity Action Plan Priority	Yes

Status

National (GB) importance	Not rare or scarce	Global red list (IUCN) category	-
National (GB) importance Not rare or scarce
Global red list (IUCN) category -

Non-native

Parameter	Data
Native	Native
Origin	Eastern Canada, Northeastern U.S.A., Northern Europe
Date Arrived	1951

Importance information

-none-

Bibliography

Ang, P.O., Jr., 1991. Natural dynamics of a Fucus distichus (Phaeophyta, Fucales) population: reproduction and recruitment. Marine Ecology Progress Series, 78, 71-85.
Fish, J.D. & Fish, S., 1996. A student's guide to the seashore. Cambridge: Cambridge University Press.
Hardy, F.G. & Guiry, M.D., 2003. A check-list and atlas of the seaweeds of Britain and Ireland. London: British Phycological Society
Holt, T.J., Hartnoll, R.G. & Hawkins, S.J., 1997. The sensitivity and vulnerability to man-induced change of selected communities: intertidal brown algal shrubs, Zostera beds and Sabellaria spinulosa reefs. English Nature, Peterborough, English Nature Research Report No. 234.
Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]
JNCC (Joint Nature Conservation Committee), 1999. Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line] http://www.jncc.gov.uk/mermaid
Norton, T.A. (ed.), 1985. Provisional Atlas of the Marine Algae of Britain and Ireland. Huntingdon: Biological Records Centre, Institute of Terrestrial Ecology.
Pearson, G.A. & Davison, I.R., 1994. Freezing stress and osmotic dehydration in Fucus distichus (Phaeophyta): evidence for physiological similarity. Journal of Phycology, 30, 257-267.
Powell, H.T., 1957. Studies in the genus FucusL. , I. Fucus distichus L. emend. Powell Journal of the Marine Biological Association of the United Kingdom, 36, 407-431.
Powell, H.T., 1957b. Studies on the genus Fucus L. II. Distribution and ecology of Fucus distichus L. emend Powell in Britain and Ireland. Journal of the Marine Biological Association of the United Kingdom, 36, 663-693.
Rice, E.L. & Chapman, A.R.O., 1985. A numerical taxonomic study of Fucus distichus (Phaeophyta). Journal of the Marine Biological Association of the United Kingdom, 65, 433-459.
Rice, E.L., Kenchington, T.J. & Chapman, A.R.O., 1985. Intraspecific geographic-morphological variation patterns in Fucus distichus and F. evanescens. Marine Biology, 88, 207-215.

Datasets

NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.
OBIS (Ocean Biodiversity Information System), 2025. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2025-05-11
Royal Botanic Garden Edinburgh, 2018. Royal Botanic Garden Edinburgh Herbarium (E). Occurrence dataset: https://doi.org/10.15468/ypoair accessed via GBIF.org on 2018-10-02.

Citation

This review can be cited as:

White, N. 2007. Fucus distichus Two-headed wrack. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 11-05-2025]. Available from: https://marlin.ac.uk/species/detail/1350

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Last Updated: 03/09/2007

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