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Himanthaliaceae (order Fucales) A family of brown seaweeds, which includes a single genus, Himanthalia. The plant lives for 2 years. During the first year a stalked, disc-like thallus is formed; this measures several centimetres across. In the second year 1–4 long, strap-like branches grow from the disc, and these bear the conceptacles within which gametes are formed. There is 1 British species, H. elongata (sea thong), found at or near the low-water mark on exposed shores.