(Linnaeus) S.F. Gray 
According to the World Register of Marine Species, Himanthalia elongata is the only member of its genus, Himanthalia Lyngbye, 1819  and the only member of its family, Himanthaliaceae (Kjellman) De Toni, 1891.
H. elongata is a common brown alga of the lower shore. The thallus is at first a small flattened or saucer-shaped disc up to three centimetres wide with a short stalk. In the autumn or winter, long thongs grows from the centre of this, branching dichotomously a number of times. They grow fast and can reach up to two metres by the following summer when they become mature. They bear the conceptacles, the reproductive organs, and begin to decay when the gametes have been released into the water. The discs live for two or three years.
Distribution and habitat
H. elongata is found in the Baltic Sea, the North Sea and the north east Atlantic Ocean from Scandinavia south to Portugal. It is found on gently shelving rocky shores in the lower littoral zone and the sublittoral zone particularly on shores with moderate wave exposure. It is sometimes abundant and forms a distinct zone just below the Fucus serratus zone.
- Himanthalia elongata (Linnaeus) S.F.Gray, 1821 World Register of Marine Species. Retrieved 2011-09-23.
- Himanthalia World Register of Marine Species. Retrieved 2011-09-23.
- Himanthaliaceae World Register of Marine Species. Retrieved 2011-09-23.
- Thongweed - Himanthalia elongata Marine Life Information Network. Retrieved 2011-09-23.
- Himanthalia elongata (Linnaeus) S.F. Gray The Seaweed Site. Retrieved 2011-09-23.
- Himanthalia elongata (Linnaeus) S.F.Gray AlgaeBase. Retrieved 2011-09-23.