Himanthalia elongata (Linnaeus) S.F. Gray

Aglaothamnion sepositum

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Common names: Ríseach (Irish); Thongweed, Buttonweed, Sea Spaghetti (English).
Description: Thallus consisting of a button-shaped vegetative thallus to 30 mm wide and 25 mm high, and a long, narrow, strap-like, sparingly branched, light yellow-brown reproductive receptacle to 2 m in length and up to 10 mm in width, on which the conceptacles are borne.

Aglaothamnion sepositum

Buttons, initially club-shaped (top) but later mushroom-like (above), develop from zygotes in late summer, mature in winter, and begin to form the reproductive receptacles in January/February. Some 4-6 dichotomies are produced at this stage, and the fronds then elongate and thicken without developing any further branches, and become reproductively mature in July-September.

Himanthalia elongata

Himanthalia elongataHabitat: on gently sloping rocks at MLWN, particularly on semi-wave-exposed shore (below), on which they may form a distinct zone at low water. Sparse populations sometimes develop in sheltered lagoons and the plants are more yellow and less flattened.
Aglaothamnion sepositumSimilar species: none, except perhaps Chorda filum, which is unbranched, darker and generally the plants are twisted together.
Key characteristics: long thong-like fronds, basal mushroom-like buttons.
Usage: sold in France and Ireland as 'Sea Spaghetti' for human consumption. The receptacles are air-dried, cut into finger-like lengths and packaged. The are soaked in water and used in salads. It has a very mild taste.

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Photographs © M.D. Guiry