Halidrys siliquosa

Researched By

Dr Harvey Tyler-Walters & Paolo Pizzolla

Data Supplied By

MarLIN

Refereed by

Dr Stefan Kraan

Taxonomy

Scientific name

Halidrys siliquosa

Common name

Sea oak

MCS Code

ZR372

Recent Synonyms

None

Phylum

Chromophycota

Subphylum

Superclass

Class

Phaeophyceae

Subclass

Order

Fucales

Suborder

Family

Cystoceiraceae

Genus

Halidrys

Species

siliquosa

Subspecies

Additional Information

No 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 form

Foliose

Feeding method

Photoautotroph

Mobility/Movement

Permanent attachment

Environmental position

Epilithic, Epifloral

Typical food types

No text entered

Habit

Attached

Bioturbator

Not relevant

Flexibility

High (>45 degrees)

Fragility

Intermediate

Size

Large(>50cm)

Height

Occasionally up to 2 m

Growth Rate

Up to a maximum of 2 cm/month

Adult dispersal potential

None

Dependency

Independent

Sociability

Solitary

Toxic/Poisonous?

Additional Information

Although 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.

Development

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.

Epiphytes
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 & Ireland

Widely distributed and fairly common in the Britain and Ireland.

Global distribution

Restricted 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 range

Not researched

Depth range

Intertidal to 4 m

Migratory

Non-migratory / Resident

Distribution Additional Information

On 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 preferences

Bedrock, Large to very large boulders, Small boulders, Rockpools, Cobbles

Physiographic preferences

Open coast, Strait / sound, Sealoch, Ria / Voe, Enclosed coast / Embayment

Biological zone

Mid Eulittoral, Lower Eulittoral, Sublittoral Fringe, Upper Infralittoral

Wave exposure

Exposed, Moderately Exposed, Sheltered, Very Sheltered

Tidal stream strength/Water flow

Moderately Strong (1-3 kn), Weak (<1 kn), Very Weak (negligible)

Salinity

Full (30-40 psu), Variable (18-40 psu)

Habitat 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 type

Permanent hermaphrodite

Developmental mechanism

Spores (sexual / asexual)

Reproductive Season

December to March

Reproductive Location

Insufficient information

Reproductive frequency

Annual episodic

Regeneration potential

Life span

Insufficient information

Age at reproductive maturity

1-2 years

Generation time

1-2 years

Fecundity

Insufficient information

Egg/propagule size

Insufficient information

Fertilization type

Insufficient information

Larvae/Juveniles
Larval/Juvenile dispersal potential	100-1000m	Larval settlement period	Insufficient information
Duration of larval stage	<1 day

Additional Information

Fucales, 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.
Dispersal
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.
Recruitment
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