Halidrys siliquosa (Linnaeus) Lyngbye
Common names: Pod-weed. Sea Oak (English).
Crúba Préacháin,
Fraoch Freangach,Rupán (Irish).
Description: Thallus 30-130 cm long, tawney to
yellow-brown ochre, tough and leathery; attached by a large,
discoid holdfast, giving rise to compressed, irregularly
alternately branched fronds, with several orders of close
branching in the same plane. Pod-shaped, segmented, air
bladders, are produced replacing some lateral branches.
Reproductive conceptacles forming in swollen conceptacles at the
apices of the branches.
Habitat: Found most commonly in large,
mid-intertidal pools, often dominating in the very large, sunny
pools, but more often forming occasional stands. Occasionally
forming extensive forests in the shallow subtidal to about 10 m,
generally in current-exposed situations. Widespread and common.
Similar species:
Sargassum muticum, which has single, teardrop-shaped bladders that are not
segmented, and is branched spirally in more than one plane.
Cystoseira species, which are generally more or less
terete and branched in more than one plane.
Bifurcaria bifurcata, which occupied the same habitat, but is regularly
dichotomously branched, cylindrical and arises from branched
rhizoids; it also occurs only rarely (south-west Britain,
western Ireland, mainly on limestone shores).
Key characteristics: alternate branching,
segmented pod-like air-bladders.
Ecology: Halidrys produces
meroditerpenoids that seemingly act as antifouling agents
preventing other organisms adhering to the surface of the plant.
The large brown pom-poms of
Sphacelaria cirrosa,
another brown alga, occur frequently on larger plants.
Halidrys also regularly sheds the upper epidermal layer
to rid itself of surface organisms; Sphacelaria manages
to penetrate beyond this layer allowing it to remain in place.
Photographs © M.D. Guiry
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