BIOTIC Species Information for Halidrys siliquosa
Researched byDr Harvey Tyler-Walters & Paolo Pizzolla Data supplied byMarLIN
Refereed byDr Stefan Kraan
Scientific nameHalidrys siliquosa Common nameSea oak
MCS CodeZR372 Recent SynonymsNone

PhylumChromophycota Subphylum
Superclass ClassPhaeophyceae
Subclass OrderFucales
Suborder FamilyCystoceiraceae
GenusHalidrys Speciessiliquosa

Additional InformationNo text entered
Taxonomy References Fish & Fish, 1996, Dickinson, 1963, Hayward et al., 1996, Gibson et al., 2001, Hiscock, 1979, Hoek van den et al., 1995,
General Biology
Growth formFoliose
Feeding methodPhotoautotroph
Mobility/MovementPermanent attachment
Environmental positionEpilithic
Typical food typesNo text entered HabitAttached
BioturbatorNot relevant FlexibilityHigh (>45 degrees)
FragilityIntermediate SizeLarge(>50cm)
HeightOccasionally up to 2 m Growth RateUp to a maximum of 2 cm/month
Adult dispersal potentialNone DependencyIndependent
General Biology Additional InformationAlthough it is typically found in low abundances, Halidrys siliquosa can sometimes form beds (S. Kraan, pers. comm.). Growth rates
The growth rate of newly germinated Halidrys siliquosa (germlings) was found to be dependant on temperature, light intensity and day length. For example:
  • germlings grew up to ca 180 µm at 3 °C, up to ca 520 µm at 10 °C and up to ca 860 µm at 20 °C within 30 days of germination (Moss & Sheader, 1973);
  • increased light intensity or day length had little effect on slow growth at 4 °C but doubling day length doubled growth rates at 10 °C although doubling total light did not double growth, and
  • germlings grew faster but showed abnormal development at 20 °C (Moss & Sheader, 1973).
Moss & Lacey (1963) reported a maximum summer growth rate of 2 cm /month, although this figure was based on a single specimen.


  • The main axis develops its characteristic 'zigzag' form within 9 months in culture.
  • Young plants (up to 1 year old) composed of 'leafy' branches only, branching in one plane.
  • Air vesicles develop at the beginning of the second year of vegetative growth.
  • Fertile receptacles develop towards the end of the plants second year i.e. at the end of autumn / start of winter (Moss & Lacey, 1963).
In shallow rock pools or surf affected populations the plants are frequently damaged resulting in a turf-like growth form due to a proliferation of branches from the damaged main axis (Moss & Lacey, 1963).

Seasonal changes
Moss & Lacey (1963) studied Northumberland populations of Halidrys siliquosa and reported:
  • rapid growth and elongation of the axis between spring and the end of July;
  • proliferation of new 'leafy' branches in spring, reaching a maximum in June -July;
  • production of air bladders from Sept -November and again in Feb to peak in April that was highly variable, and
  • development of receptacles starting in July, becoming fertile in November and releasing gametes from December to March, after which the receptacles disintegrate.
In appears, therefore, that growth and development follows a seasonal cycle of allocation of energy towards growth in spring, followed by allocation to reproduction later in the year. However, Wernberg et al. (2001) did not detect any significant seasonal change in biomass in the Limfjord, Denmark, due to high monthly variation in biomass, although the specimens they examined were small. They did not detect any seasonal change in thallus height or percentage cover.

Halidrys siliquosa has been reported to support a number of epiphytic species, depending on location, including microflora (e.g. bacteria, blue green algae, diatoms and juvenile larger algae), Ulothrix and Ceramium sp., hydroids (e.g. Laomeda flexuosa and Obelia spp.), bryozoans (e.g. Scrupocellaria spp.), and ascidians (e.g. Apilidium spp. and Botrylloides leachi ). However, Halidrys siliquosa was considered to be relatively clear of epiphytes due to its ability to shed the outer layer of epidermal cell walls, together with adherent epiphytes (Moss, 1982; Lobban & Harrison, 1997).
Biology References Hayward et al., 1996, Gibson et al., 2001, Hiscock, 1979, Lüning, 1990, Lewis, 1964, Moss, 1982, Wernberg et al., 2001, Lüning, 1990, Lobban & Harrison, 1997, Hoek van den et al., 1995, Moss & Sheader, 1973, Moss & Lacey, 1963,
Distribution and Habitat
Distribution in Britain & IrelandWidely distributed and fairly common in the Britain and Ireland.
Global distributionRestricted to the north east Atlantic, and recorded from northern Norway, Scandinavia, the Baltic Sea, Helgoland and the Netherlands south to the Bay of Biscay, north Portugal and the Canary Islands (John et al., 2004).
Biogeographic rangeNot researched Depth rangeIntertidal to 4 m
MigratoryNon-migratory / Resident   
Distribution Additional InformationOn wave sheltered shores Halidrys siliquosa occur in the sublittoral and rock pools at low water. However, on wave exposed sites Halidrys siliquosa may also be found in deep high shore rock pools sheltered from the sun (Moss & Lacey, 1963). It is in such rock pools where the very weak water flow rate is likely to occur.

Substratum preferencesBedrock
Large to very large boulders
Small boulders
Physiographic preferencesOpen coast
Strait / sound
Ria / Voe
Enclosed coast / Embayment
Biological zoneMid Eulittoral
Lower Eulittoral
Sublittoral Fringe
Upper Infralittoral
Wave exposureExposed
Moderately Exposed
Very Sheltered
Tidal stream strength/Water flowModerately Strong (1-3 kn)
Weak (<1 kn)
Very Weak (negligible)
SalinityFull (30-40 psu)
Variable (18-40 psu)
Habitat Preferences Additional Information
Distribution References Fish & Fish, 1996, Dickinson, 1963, Norton, 1985, Hayward et al., 1996, Gibson et al., 2001, Lüning, 1990, JNCC, 1999, Picton & Costello, 1998, Lewis, 1964, Lüning, 1990, Guiry & Nic Dhonncha, 2002, Hardy & Guiry, 2003, Guiry & Nic Dhonncha, 2002, John et al., 2004,
Reproduction/Life History
Reproductive typePermanent hermaphrodite
Developmental mechanismSpores (sexual / asexual)
Reproductive SeasonDecember to March Reproductive LocationInsufficient information
Reproductive frequencyAnnual episodic Regeneration potential No
Life spanInsufficient information Age at reproductive maturity1-2 years
Generation time1-2 years FecundityInsufficient information
Egg/propagule sizeInsufficient information Fertilization typeInsufficient information
Larval/Juvenile dispersal potential100-1000m Larval settlement periodInsufficient information
Duration of larval stage<1 day   
Reproduction Preferences Additional InformationFucales, such as Halidrys siliquosa, have a single vegetative sporophyte stage, the diploid thallus that bears specialized reproductive bodies (meiosporangia) in the receptacles, in which the gametes are formed. Female gametes are large and immotile (oogonia) while the male gametes are small and motile (antheridia) (van den Hoek et al., 1995).
In Halidrys siliquosa, gametes are formed shortly before liberation from the receptacles. Female oogonia (80 -100µm in size) and male antheridia are shed simultaneously, so that fertilization may occur during or before liberation. Well developed zygotes were observed 12hrs after fertilization. Zygotes probably sink rapidly (especially if they cluster together), are covered in adhesive mucus and stick to the substratum. Further development is delayed for 5 or more days, after which 2-4 rhizoids develop and fix the zygote to the substratum. The early zygote wall is shed and the germling develops further (Moss & Sheader, 1973; Hardy & Moss, 1978).

In Northumberland, receptacles began to develop in July, became fertile in November and released gametes from December to March, after which the receptacles disintegrated. Fertile receptacles developed in the plants second year (Moss & Lacey, 1963).

Germlings are capable of growing in the dark for up to 40 days. In addition, germlings maintained in the dark for up to 120 days were able to resume growth when exposed to light, however, after 140 days of darkness germlings died (Moss & Sheader, 1973). The ability to survive darkness, and low light conditions, probably allows the germlings to survive under understory algae, ready to develop should the shading canopy be removed.
Zygotes are large and may form clusters (Hardy & Moss, 1978) and probably sink rapidly. Norton (1992) suggested that turbulent deposition by water flow (zygotes or spores being thrown against the substratum) was the most important force directing propagules to the substratum. Dispersal by spores is probably dependant on the hydrographic regime but is probably localized, e.g. in Sargassum muticum. Although some zygotes may settle 1km of more from the parent, most settle within 2m (Norton, 1992). The propagules of most fucales tend to settle near the parent plant (Norton, 1992; Holt et al., 1997). Halidrys siliquosa can float if detached, suggesting another potential route for dispersal. Floating plants remain fertile and spores may be released some distance from the point of detachment. However, although some long range dispersal must occur in macroalgae (resulting in colonization of oil rigs and similar structures), van den Hoek (1987) and Norton (1992) suggested that it is probably ineffective for most species of macroalgae. Wernberg et al. (2001) suggested that the lack of long range dispersal success in Halidrys siliquosa was responsible for its regional distribution in the north east Atlantic.
Sousa et al. (1981) reported that experimental removal of sea urchins significantly increased recruitment in long-lived brown algae. In experimental plots cleared of algae and sea urchins in December, Halidrys dioica colonized the plots, in small numbers, within 3-4 months. Plots cleared in August received few , if any recruits, suggesting that recolonization was dependant on zygote availability and therefore the season. Halidrys dioica did not colonize plots grazed by urchins in their experiments (Sousa et al., 1981). Svendsen (summary only, 1972) reported that Halidrys siliquosa became one of a few dominant algae 3 years after removal of Laminaria hyperborea by harvesting on the west coast of Norway. However, this observation may be explained by the growth of small germlings already present due to increased light and space freed by removal of the kelp canopy, as well as by recruitment.
Reproduction References Wernberg et al., 2001, Lobban & Harrison, 1997, Hoek van den et al., 1995, Moss & Sheader, 1973, Moss & Lacey, 1963, Hardy & Moss, 1978, Hoek van den, 1987, Norton, 1992, Svendsen, 1972, Holt et al., 1997, Vadas et al., 1992, Sousa et al., 1981,
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