BIOTIC Species Information for Halidrys siliquosa
Researched byDr Harvey Tyler-Walters & Paolo Pizzolla Data supplied byMarLIN
Refereed byDr Stefan Kraan
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,
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