information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Hornwrack (Flustra foliacea)

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



Flustra foliacea forms a stiff but flexible bushy clump 6 -10 cm high, occasionally up to 20cm high. Flustra foliacea is much divided into fronds that are usually broadly lobed, occasionally strap-like, and made up of zooids (individuals) on both sides (bilaminar). Fronds are light grey to brown in colour. Zooids are tongue shaped, 0.4 mm long and 0.2 - 0.28 mm wide. They bear 4 to 5 marginal club-like spines at the broad (distal) end of each zooid. The fronds have a distinct smell of lemons when freshly collected. Hornwrack is sometimes found washed ashore after storms.

Recorded distribution in Britain and Ireland

Common on all rocky coasts of Britain and Ireland.

Global distribution

Flustra foliacea occurs in the Kara Sea, White Sea and Barents Sea in the Arctic circle, the North Sea, and extends south as far as Bay of Biscay. Also found on the east coast of Greenland.


Found on coarse sediment and rocky substrate in the shallow sublittoral, where it favours current-swept rocky grounds.

Depth range

Sublittoral 1 -200m

Identifying features

  • Bushy clumps of palmate, flattened fronds, 6 -10 cm but occasionally 20 cm high.
  • Distinct smell of lemons when fresh.
  • Zooids are tongue shaped and bear 4 to 5 marginal club-like spines on their broad, distal margin and lacks an ascus (anascan).
  • Fronds composed of a double layered (bilaminar) sheet of zooids.
  • Zooids arranged in a quincunx pattern, i.e. four zooids surrounding a central one.
  • The polypide of each zooid bears 13-14 tentacles.
  • Avicularia about half the size of the autozooids (feeding zooids) and arise at the bifurcation between rows of autozooids.
  • Ovicell immersed, endozooidal, embedded in the base of the distal zooid.

Additional information

Flustra foliacea forms only a flat incrustation during its first year of growth, erect growth occurs in subsequent years. Fronds can often be encrusted by other bryozoans, hydroids and sedentary polychaetes.

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Further information sources

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Biology review


Authority(Linnaeus, 1758)
Recent Synonyms


Typical abundanceModerate density
Male size range
Male size at maturity
Female size rangeMedium(11-20 cm)
Female size at maturity
Growth formModular, Turf
Growth rate1.6-3cm/year
Body flexibilityHigh (greater than 45 degrees)
Characteristic feeding methodActive suspension feeder
Diet/food sourceOmnivore
Typically feeds onPhytoplankton, detritus and dissolved organic matter.
Environmental positionEpibenthic

Small green algae, bryozoans, hydroids, sessile polychaetes, barnacles, lamellibranchs and tunicates (see additional information below)

Is the species harmful?See additional information

Biology information

Detailed diagrams of the autozooid and avicularium of Flustra foliacea are provide by Silén (1977).

Growth form
The newly metamorphosed coronate larvae develops into the first zooid of the new colony, the 'ancestrula'. In its first year of growth, Flustra foliacea forms a flat incrustation on the substratum and commences erect growth during the second year. This is achieved simply by the opposition of actively growing lobes of a colony; on contact two growing edges are deflected vertically (J. Porter, pers. comm.).
The two layers of zooids grow, synchronously 'back to back' forming a bilaminar, erect frond at 90 ° to the original encrusting mat. Branching of the erect fronds varies between branches and colonies (Stebbing, 1971a; Silén, 1981).
Ryland (1976) suggested that erect growth avoids the spatial constraints (availability of substratum and competition) suffered by encrusting forms. Repair of grazing damage, i.e. removal of one bilaminar layer, may result in generation of an new bilateral branch (Stebbing, 1971a).

Growth rates
Stebbing (1971a) reported that growth began in late February/early March but stopped in November in specimens off the Gower Peninsula , with a slight check in growth in August, and no growth occurred over winter. Growth stopped in October in Isle of Man specimens (Eggleston, 1963; cited in Stebbing, 1971a). The winter growth check results in visible annual growth lines, which have been used to age colonies (Stebbing, 1971a; Eggleston, 1972; Menon, 1978). Stebbing (1971a) suggested that the growth line formed a line of weakness, which gave the frond flexibility.

Stebbing (1971a) stated that the length of time spent as an encrusting form was unclear but assumed the first growth line at the base of the frond represented the first winter, 1 years growth. Flustra foliacea colonies regularly reached 6 years of age, although 12 year old specimens were reported off the Gower Peninsula (Stebbing, 1971a; Ryland, 1976). Furthermore, O'Dea & Okamura (2000) demonstrated seasonal fluctuations in zooid size synchronous with temperature regimes, the largest zooid zooids occurring with the lowest temperatures.

Stebbing (1971a) reported that growth rates were reasonably consistent between samples, age classes and years. Stebbing (1971a) reported a mean increment in frond height of 16.8mm/yr, whereas Eggleston (1972) reported that annual lines were usually between 2-3cm apart in Isle of Man specimens, and Menon (1978) reported that Helgoland specimens reached an average of 21.2 mm in height at 2 years old and an average of 79.3 mm after 8 years. Silén (1981) reported that erect fronds grew in zooid number about 10-20 times that of the encrusting base. Menon (1978) reported that growth rates varied in specimens over 5 years old.

At the base of fronds, in the holdfast area, the zooids give rise to layers of non-feeding frontal buds after 3 years of age, which strengthen the base of the frond. The number of layers increases with frond height up to 145mm in height, and up to 20 layers deep (Stebbing, 1971a).

Growth rates probably vary between locations. O'Dea & Okumara (2000) noted that colonies of Flustra foliacea from the Bay of Fundy showed reduced growth compared to colonies in the Menai Straits and the Skagerrak. Low primary productivity, genetic variation and parasitism were cited as possible explanations for the difference.

Regeneration and repair
Silén (1981) reported that experimental removal of a notch in the frond was repaired within 5 -10 days. The newly formed margin grew at normal rates (4-5 zooid lengths per month). Removal of one layer of the bilaminar frond, experimentally (Silén, 1981) or by predators (Stebbing, 1971a) was repaired with similar rapidity, the un-damaged layer, halting growth while the damaged area was repaired (Silén, 1981).

The epizoic fauna of Flustra foliacea was described by Stebbing (1971b) and consisted of 25 species of bryozoan, 5 hydroid species, some sessile polychaetes, barnacles, lamellibranchs and tunicates. The bryozoans Bugulina flabellata, Crisia spp. and Scrupocellaria spp. were major epizoites. The stolons of Bugulina flabellata penetrate the zooids of Flustra foliacea. Scrupocellaria spp. settled preferentially on the youngest, distal, portions of the frond, possibly to elevate their branches into faster flowing water (Stebbing, 1971b). A small green alga Epicladia flustrae was reported to be a specific epiphyte (Nielsen, 1984). Stebbing (1971a) reported that the growth rates of Flustra foliacea were reduced by ca 50% when encrusted by epizoites. Peters et al, (2003) reported the presence of chemical compounds in Flustra foliacea that demonstrated antagonistic effects against the growth of some associated bacterial species, and electron microscopic examination of the distal end of the zooid revealed no microbial settlement. Dyrynda (1985, cited in Peters et al., 1985) reported the toxicity of extracts of Flustra foliacea on larvae of other modular invertebrates, fish and bacteria.

Some people can react to Flustra sp. and some fishermen have reported allergic reactions to it although this is anecdotal information (J. Porter, pers. comm.). Research into biomedical compounds from marine organisms has revealed that a sample of Flustra foliacea from the southern North Sea yielded deformylflustrabromine, which was moderately cytotoxic to the human colon cancer cell-line HCT-116 (Lysek et al., 2002; Jha & Zi-rong, 2004).

Habitat preferences

Physiographic preferencesOpen coast, Offshore seabed, Strait / sound, Sea loch / Sea lough, Ria / Voe
Biological zone preferencesLower circalittoral, Lower infralittoral, Upper circalittoral, Upper infralittoral
Substratum / habitat preferencesBedrock, Cobbles, Large to very large boulders, Mixed, Small boulders
Tidal strength preferencesModerately Strong 1 to 3 knots (0.5-1.5 m/sec.), Strong 3 to 6 knots (1.5-3 m/sec.), Very Strong > 6 knots (>3 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferencesExposed, Moderately exposed, Sheltered, Very exposed
Salinity preferencesFull (30-40 psu)
Depth rangeSublittoral 1 -200m
Other preferencesNo text entered
Migration PatternNon-migratory / resident

Habitat Information

Flustra foliacea may colonize any hard substratum, such as shells, stones, or cobbles but forms dense aggregations particularly in otherwise barren, current swept rocky bottoms. Although present in a wide range of tidal streams and wave exposure, Flustra foliacea is abundant in moderately strong to strong tidal streams (Hiscock, 1983). Dyer et al. (1982) reported between <10 to >200 colonies per m² in trawls in the North Sea. Flustra foliacea is associated with strong currents and areas subject to sediment abrasion (Stebbing, 1971a; Knight-Jones & Nelson-Smith, 1977; Hartnoll, 1983; Holme & Wilson, 1985) and requires stable hard substrata (Eggleston, 1972b; Ryland, 1976; Dyrynda, 1994). The abundance of bryozoans is positively correlated with supply of stable hard substrata and hence with current strength (Eggleston, 1972b; Ryland, 1976).

Life history

Adult characteristics

Reproductive typePermanent (synchronous) hermaphrodite
Reproductive frequency Annual episodic
Fecundity (number of eggs)100,000-1,000,000
Generation time1-2 years
Age at maturitySee additional text
SeasonAugust - April
Life span5-10 years

Larval characteristics

Larval/propagule typeCyphonautes
Larval/juvenile development Lecithotrophic
Duration of larval stage< 1 day
Larval dispersal potential See additional information
Larval settlement periodInsufficient information

Life history information

Bryozoan colonies are hermaphrodite, however, zooids may be monoecious, dioecious, protandrous or protogynous, depending on species (Hayward & Ryland, 1998). Flustra foliacea bears both male and female zooids and is presumably hermaphrodite.
  • Male zooids of Flustra foliacea in the Isle of Man were reported to be full of sperm in September, giving the entire colony a white appearance. Sperm were absent by October. Orange eggs were visible in August and the yellow coloured embryos had entered the ooecia (ovicells) by October (Eggleston, 1970; 1972a). Sperm are shed from pores in the polypide tentacles of male zooids.
  • Fertilization in brooding species such as Flustra foliacea is probably internal (Hayward & Ryland, 1998). In bryozoans, released sperm are entrained by the tentacles of feeding polypides and may not disperse far, resulting in self-fertilization. However, genetic cross-fertilization is assumed in oviparous and brooding bryozoans, although there is evidence of self fertilization (Hayward & Ryland, 1998).
  • Eggleston (1972a) reported that about one third of zooids produced a single embryo in their first and second years, but that older zooids were infertile. Embryos were brooded over winter and larvae released between February and April.
Dalyell (cited in Hincks, 1880) stated that ca 10,000 larvae were released from a specimen of Flustra foliacea within 3 hrs. Eggleston (1972a) reported that each zooid produced a single embryo, so that fecundity is probably related to the number of sexual zooids and hence size of the colony.

Flustra foliacea colonies regularly reached 6 years of age, although 12 year old specimens were reported off the Gower Peninsula (Stebbing, 1971a; Ryland, 1976).

  • Larvae are positively phototactic on release, and swim for only short periods, although in species in which light stimulus is un-important, larvae may delay metamorphosis for 12 hrs of more (Hayward & Ryland, 1998). Daylength is an important cue for larval release in some species of bryozoa, and Flustra foliacea releases larvae in spring (February- April) (Eggleston, 1972a; Hayward & Ryland, 1998), however, at the depths Flustra foliacea can occur light may not be important.
  • Larvae are probably sensitive to surface contour, chemistry and the proximity of conspecific colonies. However, Hayward & Ryland (1998) suggested that larval behaviour at settlement is only of prime importance to species occupying ephemeral habitats. Eggleston (1972b) demonstrated that the number and abundance of species of bryozoan increased with increased current strength, primarily due to a resultant increase in the availability of stable, hard substrata (Eggleston, 1972b; Ryland, 1976). Dyrynda (1994) noted that the abundance of Flustra foliacea was greatest in the deepest, and most current scoured, mouth of Poole Harbour due to the presence of circalittoral boulders not commonly found in other parts of the harbour, although reduced salinity probably also restricted its distribution within the harbour. Therefore, recruitment is probably dependant on the availability of suitable substratum.
  • The short larval life probably results in good local but poor long-range dispersal. Ryland (1976) reported that significant settlement in bryozoans was only found near a reservoir of breeding colonies. However, the hydrographic regime probably strongly influences potential dispersal, and in the strong currents tolerated by Flustra foliacea, larvae may be transported some distance in a short time, unless constrained within eddies between faunal turf forming species. The sand abrasion tolerated by Flustra foliacea may remove other species, providing Flustra foliacea with space to colonize.

Sensitivity reviewHow is sensitivity assessed?

Physical pressures

High High Moderate Moderate
Removal of the substratum would result in the removal of the Flustra foliacea population and its associated fauna. Recovery will depend on recruitment from other populations and is assessed as high (see additional information below).
Tolerant Not relevant Not sensitive High
Holme & Wilson (1985) examined the bottom fauna in a tide-swept region of the central English Channel. Flustra foliacea dominated communities were reported to form in, and hence tolerate, areas subject to sediment transport (mainly sand) and periodic, temporary, submergence by thin layers of sand (ca <5 cm). In some cases, Flustra foliacea was seen to be partially buried by sand. It is likely that Flustra foliacea would withstand smothering by 5cm of sediment for a month. Large colonies are likely to be >6cm in height and exposed autozooids, will be able to feed, providing food for the rest of the colony. Therefore, not sensitive has been recorded. However, smothering for protracted periods or by impermeable materials may have a greater impact.
Tolerant* Not relevant Not sensitive* Moderate
Suspended sediment is likely to cause abrasion and effect suspension feeding physically. But Flustra foliacea dominated communities have been reported from areas subject to sediment abrasion due to strong tidal streams, either by mainly sand (Holme & Wilson, 1985) or by gravel (Knight-Jones & Nelson-Smith, 1977; Hartnoll, 1983). Their toughness and erect form, coupled with their flexibility probably confers tolerance (Knight-Jones & Nelson-Smith, 1977; Holme & Wilson, 1985). Hyman (1959) noted that erect forms were more calcified than encrusting forms, e.g. Flustra foliacea was reported to have a calcareous content of 97-99%. In addition, Flustra foliacea was reported to be abundant in turbid, fast flowing waters of the Menai Straits (Moore, 1977). Therefore, together with evidence of periodic partial burial by sediment (see smothering above) Flustra foliacea is likely to tolerate increased suspended sediment. Bryozoan larvae are reported to avoid areas affected by siltation (Eggleston, 1972b; Ryland, 1976), however, the abundance of Flustra foliacea in areas subject to sediment abrasion and suspended sediment loads subjects that the some of its larvae are also able to settle and survive. An increase in siltation and associated scour may remove competitors and provide additional space for colonization, therefore, 'tolerant*' has been recorded.
Tolerant Not relevant Not sensitive Moderate
A significant reduction in suspended sediment load for a month (see benchmark) is unlikely to adversely affect Flustra foliacea. Therefore, a rank of tolerant has been recorded. However, the longer term reduction may allow other species to colonize available substratum and increase competition for space with Flustra foliacea.
Not relevant Not relevant Not relevant Not relevant
Flustra foliacea is probably highly intolerant of desiccation, and specimens washed ashore are usually dead. However, it is a subtidal species unlikely to be exposed to the air.
Not relevant Not relevant Not relevant Not relevant
Flustra foliacea is a subtidal species, and unlikely to be affected by a change in emergence regime at the benchmark level.
Not relevant Not relevant Not relevant Not relevant
Flustra foliacea is a subtidal species, and unlikely to be affected by a change in emergence regime at the benchmark level.
Tolerant High Not sensitive High
Flustra foliacea colonies are flexible, robust and reach high abundances in areas subject to strong currents and tidal streams (see distribution; Stebbing, 1971; Eggleston, 1972b; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1983, 1985; Holme & Wilson, 1985). Dyrynda (1994) suggested that mature fronded colonies do not occur on unstable substratum due to the drag caused by their fronds, resulting in rafting of colonies on shells or the rolling of pebbles and cobbles, resulting in destruction of the colony. Dyrynda (1994) reported that the distribution of Flustra foliacea in the current swept entrance to Poole Harbour was restricted to circalittoral boulders, on which it dominated as nearly mono-specific stands. Therefore, an increase in water flow from moderately strong to very strong may not adversely affect colonies on stable substrata. However, colonies, especially if large, on coarse grounds, shells, cobble and pebbles are likely to be more intolerant of increase in water flow, and a proportion of the population may be displaced, although the colony may survive as long as it is not crushed in the process. Therefore, tolerant has been recorded.
Intermediate High Low Low
Flustra foliacea reaches high abundances in strong currents (see above) but decreases in abundance in weak currents. Decreased water flow may result in increased siltation and accumulation of fine sediments, increased competition from other space occupying species such as sponges, hydroids and other bryozoans, and a decrease in food availability. While, the pumping activity of the lophophores provide the greatest proportion of the colonies food requirements (Hayward & Ryland, 1998), the current generated is probably very localized and the colonies are dependant on water currents to carry food particles to them. Therefore, a decrease in water flow from, e.g. moderately strong to weak, is likely to result in a decrease in the abundance of Flustra foliacea and an intolerance of intermediate has been recorded.
Tolerant Not relevant Not sensitive Low
No information concerning temperature tolerance was found. However, Flustra foliacea is an amphiboreal species found in the Arctic Circle and south to the Bay of Biscay, and from the shallow sublittoral to deep circalittoral. Therefore, it is unlikely to be adversely affected by long term changes in temperature within British waters, and a rank of tolerant has been recorded.
Tolerant Not relevant Not sensitive Low
No information concerning temperature tolerance was found. However, Flustra foliacea is an amphiboreal species found in the Arctic Circle and south to the Bay of Biscay, and from the shallow sublittoral to deep circalittoral. Therefore, it is unlikely to be adversely affected by long term changes in temperature within British waters, and a rank of tolerant has been recorded.
Tolerant Not relevant Not sensitive Low
Increased turbidity will reduce light penetration and hence phytoplankton productivity. Small phytoplankton are probably an important food source in the shallow subtidal, although, Flustra foliacea is also found at greater depths, where organic particulates (detritus) are probably more important. Therefore, an increase in turbidity is unlikely to adversely affect Flustra foliacea.
Tolerant* Not relevant Not sensitive* Low
Decreased turbidity is likely to result in increased primary productivity, and therefore indirectly increase the availability of food (phytoplankton or detritus) for Flustra foliacea, which may, therefore, benefit. Therefore, a rank of tolerant* has been recorded.
Intermediate High Low Moderate
Flustra foliacea occurs from very wave exposed to sheltered waters, although probably limited to deeper waters in very wave exposed conditions. The oscillatory water flow generated by wave action may be more damaging than constant strong currents, e.g. strong wave action may generate an oscillatory flow of 2m/sec at 20m (Hiscock, 1983, 1985). Flustra foliacea is a common member of the flotsam, having been removed from its substratum by storms. Colonies on unstable substrata or subject to extreme wave exposure and storms are likely to be more intolerant. An increase in wave action from exposed to extremely exposed, or storms (especially in more wave sheltered environments or shallow water populations) are likely to remove a proportion of the population. Therefore, Flustra foliacea is probably of intermediate intolerance to increases in wave action. Recoverability is likely to be high (see additional information below).
Intermediate High Low Low
Flustra foliacea occurs from very wave exposed to sheltered waters. In wave sheltered habitats water flow rates are probably more important than wave action in determining the occurrence and abundance of Flustra foliacea. Water movement (wave or water flow induced) is important for suspension feeding invertebrates such as Flustra foliacea, to prevent siltation and provide adequate food supplies, oxygenation, nutrients and remove waste products. A decrease in wave action to very sheltered or ultra sheltered, in the absence of adequate water flow, may decrease water movement, leading to an increase in fine sediments, loss of suitable substratum and a reduction in food supply and oxygenation. Predation and competition from space occupying species such as hydroids, and macroalgae are likely to increase.
In very wave exposed sites, however, a decrease in wave exposure may be beneficial by reducing the populations vulnerability to storm damage (see above).

Overall, Flustra foliacea populations are most abundant in areas of high water movement (e.g. Menai Straits), and a decrease in wave action, in the absence of tidal flow or currents, is likely to result in a reduction of the abundance of Flustra foliacea. Therefore, an intolerance of intermediate has been recorded. Recoverability has been assessed as high (see additional information below).
Tolerant Not relevant Not sensitive High
Bryozoa probably react to local vibrations, that may herald the attach of predators, interrupting feeding. But they are unlikely to be aware of noise at the benchmark level.
Tolerant Not relevant Not sensitive High
Bryozoa probably react to very local shading effects, interrupting feeding. But their visual acuity is probably extremely poor, and they are unlikely to be affected by visual disturbance.
Intermediate High Low Moderate
Flustra foliacea is tolerant of sediment abrasion (see smothering and suspended sediment above) but physical disturbance by fishing gear has been shown to adversely affect emergent epifaunal communities. For example, emergent epifauna were indicative of scallop dredge damage on Modiolus modiolus beds (see species review), and hydroid and bryozoan matrices were reported to be greatly reduced in fished areas (Jennings & Kaiser, 1998 and references therein). Mobile gears also result in modification of the substratum, including removal of shell debris, cobbles and rocks, and the movement of boulders (Bullimore, 1985; Jennings & Kaiser, 1998).
Although, Flustra foliacea is flexible physical disturbance by a passing scallop dredge (see benchmark) is likely to damage fronds and remove some colonies, suggesting an intolerance of intermediate. Colonies on hard substrata are probably less vulnerable to fishing activity but would probably be damaged or partially removed. Colonies growing on rocks, cobbles and shells on coarse grounds, may be removed by a scallop dredge (see substratum loss above) and therefore, be highly intolerant. Recovery will depend on local recruitment from surviving colonies (depending on where the nearest undamaged colonies are situated) and a recoverability of high has been recorded (see additional information below).
High High Moderate Moderate
Colonies of Flustra foliacea that are displaced while attached to their substratum, e.g. shell debris or rocks will probably survive if moved to a suitable habitat and not crushed in the process. But if removed from its substratum, Flustra foliacea colonies cannot reattach and will probably be washed to deep water or be deposited on the strand line and die. Therefore, an intolerance of high has been recorded, with a recoverability of high (see additional information below).

Chemical pressures

High High Moderate Very low
Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1979; Holt et al., 1995). Bryan & Gibbs (1991) reported that there was little evidence regarding TBT toxicity in bryozoa with the exception of the encrusting Schizoporella errata, which suffered 50% mortality when exposed for 63 days to 100ng/l TBT. Rees et al. (2001) reported that the abundance of epifauna (including bryozoans) had increased in the Crouch estuary in the five years since TBT was banned from use on small vessels. This last report suggests that bryozoans may be at least inhibited by the presence of TBT. Hoare & Hiscock (1974) suggested that polyzoa (bryozoa) were amongst the most intolerant species to acidified halogenated effluents in Amlwch Bay, Anglesey and reported that Flustra foliacea did not occur less than 165m from the effluent source.
Therefore, an intolerance of high has been recorded. A recoverability of high has been recorded (see additional information).
Heavy metal contamination
Low Very high Very Low Very low
Bryozoans are common members of the fouling community, and amongst those organisms most resistant to antifouling measures, such as copper containing anti-fouling paints (Soule & Soule, 1977; Holt et al., 1995). Bryozoans were shown to bioaccumulate heavy metals to a certain extent (Holt et al., 1995). For example, Bowerbankia gracialis and Nolella pusilla accumulated Cd, exhibiting sublethal effects (reduced sexual reproduction and inhibited resting spore formation) between 10-100 µg Cd /l and fatality above 500 µg Cd/l (Kayser, 1990). However, given the tolerance of bryozoans to copper based anti-fouling treatments, as assuming similar physiology between species, an intolerance of low has been recorded albeit with very low confidence.
Hydrocarbon contamination
No information Not relevant No information Not relevant
Little information on the effects of hydrocarbons on bryozoans was found. Flustra foliacea is likely to be protected from the direct effects of oil spills by its subtidal habit but may be exposed to water soluble fractions of oils, PAHs or oil adsorbed onto particulates. In addition, Ryland & Putron (1998) did not detect adverse effects of oil contamination on the bryozoan Alcyonidium spp. in Milford Haven or St. Catherine's Island, south Pembrokeshire although it did alter the breeding period. However, it is difficult to extrapolate between species, and no assessment has been made.
Radionuclide contamination
No information Not relevant No information Not relevant
Changes in nutrient levels
No information Not relevant No information Not relevant
A moderate increase in nutrient levels may increase the food available to Flustra foliacea, either in the form of phytoplankton or detritus. However, no effects of nutrients enrichment on bryozoans were found.
Not relevant Not relevant Not relevant Not relevant
Flustra foliacea is unlikely to encounter hypersaline conditions in current swept, subtidal habitats.
High High Moderate Low
Ryland (1970) stated that, with a few exceptions, the Gymnolaemata were fairly stenohaline and restricted to full salinity (ca 35 psu) and noted that reduced salinities result in an impoverished bryozoan fauna. Similarly, Dyrynda (1994) noted that Flustra foliacea and Alcyonidium diaphanum were probably restricted to the vicinity of the Poole Harbour entrance by their intolerance to reduced salinity. Although, protected from extreme changes in salinity due to their subtidal habitat, the introduction of freshwater, or hyposaline effluents may adversely affect Flustra foliacea colonies. Therefore, an intolerance of high has been recorded, and recoverability assessed as high (see additional information below).
No information Not relevant No information Not relevant
No information on the tolerance of Flustra foliacea to changes in oxygenation was found.

Biological pressures

Low Immediate Not sensitive High
No information on diseases was found. Stebbing (1971a) reported that encrusting epizoites reduced the growth rate of Flustra foliacea by ca 50% and Stebbing (1971b) described the epizoic fauna of hornwrack in detail. The bryozoan Bugulina flabellata produces stolons that grow in and through the zooids of Flustra foliacea, causing "irreversible degeneration of the enclosed polypide" (Stebbing, 1971b). Therefore, given the reduction in growth caused by epizoic infestation an intolerance of low has been recorded. Recovery and repair would probably be rapid (see additional information below).
No information Not relevant No information Not relevant
No information found.
Not relevant Not relevant Not relevant Not relevant
Flustra foliacea is not presently known to be subject to extraction. However, many bryozoans have been recently found to contain pharmacologically active substances (Hayward & Ryland, 1998; Lysek et al., 2002; Peters et al., 2003). Therefore, Flustra foliacea may be subject to harvesting in the future.
High High Moderate Low
Flustra foliacea may occur on coarse grounds used for fishing of other marine species, e.g. scallops. Mobile fishing gear, such as scallop dredges and beam trawls result in physical disturbance to the sediment surface. Emergent epifauna have been shown to be particularly intolerant of physical disturbance, depending on intensity (see abrasion) and emergent fauna, together with shells, and rocks form part of by-catch. Therefore, an intolerance of high and a recoverably of high have been recorded (see additional information below).

Additional information

Silén (1981) reported that Flustra foliacea could repair physical damage to its fronds with 5-10 days. Presumably, as long as the holdfast remains intact, Flustra foliacea will survive and grow back.
The brooded, lecithotrophic larvae of bryozoans have a short pelagic life time of several hours to about 12 hours (Ryland, 1976). Recruitment is dependant on the supply of suitable, stable, hard substrata (Eggleston, 1972b; Ryland, 1976; Dyrynda, 1994). Even in the presence of available substratum, Ryland (1976) noted that significant recruitment in bryozoans only occurred in the proximity of breeding colonies. For example, Keough & Chernoff (1987) reported that population of Bugula nerita demonstrated spatial variation over very small scales, and populations were sometimes absent even when substantial populations were <100m away.

Flustra foliacea colonies are perennial, and potentially highly fecund when large. In the strong currents occupied by Flustra foliacea populations many larvae are probably swept away, either to colonize other substrata or lost. Recruitment may be enhanced in areas subject to sediment abrasion, where less tolerant species are removed, making more substratum available for colonization, especially if larvae release in spring coincides with the end of winter storms. Once settled, new colonies take at least 1 year to develop erect growth and 1-2 years to reach maturity , depending on environmental conditions. Four years after sinking, the wreck of a small coaster, the M.V. Robert, off Lundy was found to be colonized by erect bryozoans and hydroids, including occasional Flustra foliacea (Hiscock, 1981). The wreck was several hundreds of metres from any significant hard substrata, and hence a considerable distance from potentially parent colonies (Hiscock, 1981 and pers comm.)

Overall, local recruitment is probably good and a damaged or reduced population may recover its numbers and percentage cover in less than 5 years. Where the population was removed, recruitment would depend on the proximity of other populations or individuals and the hydrographic regime, and is likely to be more protracted, taking up to 5 years. In areas isolated by either by distance or hydrographic regime, Flustra foliacea may take longer to recolonize.

Importance review


- no data -



Importance information

Aggregations of Flustra foliacea provide a habitat for various species of sponges, hydroids, caprellid amphipods and the suspension feeding crab Pisidia longicornis (as Porcellana), as well as a number of epizoic species (see MCR.Flu and Stebbing, 1971b)

Flustra foliacea is the preferred prey of the pycnogonid Achelia echinata, whose feeding behaviour is described by Wyer & King, 1973 and Ryland, 1976. In addition, Flustra foliacea is preyed on by sea urchins such as Echinus esculentus and Psammechinus miliaris (Ryland, 1976) and nudibranchs, especially the dorid Crimora papillata (Picton & Morrow, 1994).


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This review can be cited as:

Tyler-Walters, H. & Ballerstedt, S., 2007. Flustra foliacea Hornwrack. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from:

Last Updated: 11/09/2007