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Thongweed (Himanthalia elongata)

Thongweed (Himanthalia elongata)

Small Thongweed buttons amongst red algae on lower shore

Thongweed is a brown alga which grows up to a metre in length. The first year of growth is just a small button, like a mushroom, 2 cm across. By the end of the year it has a depression in the middle of the button and from this the second year of growth emerges. This is a long thin, branching frond and the reproductive receptacles are on this.

Thongweed is characteristically found in the lower zone of rocky seashores where there is extreme wave action. Here it replaces the saw or serrated wrack in dominating this part of the shore. The two species compete for the same niche but Thongweed has a wider stress tolerance. However, Thongweed is not normally seen with the wrack. In theory, Thongweed should be found on all shores with its wide tolerance from exposure to extreme wave action to extreme shelter where there is only the gentle tidal movements. The reason is that the serrated wrack grows very quickly, in a matter of months. If the buttons of Thongweed develop beneath the serrated wrack they will be shaded out and killed. Serrated wrack is therefore the dominant competitor, only when conditions are too extreme for its survival will Thongweed thrive. Early in the year, as the Thongweed begins to grow on a shore, the buttons can be found on all rocky seashores. By the end of the year it is found growing only where there is great wave action or where there is none, but not in between.

In conclusion, therefore, Thongweed is a seaweed of extremes growing in the lower shore of rocky shores where there is extreme wave action or in very sheltered sea lochs/ fjords where there is no waves at all. Also in large, deep rock pools. It is a northerly species found in the Atlantic and English Channel.

Thongweed (Himanthalia elongata)

The second year fronds of Thongweed

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