Photo: Ulva intestinalis 2.

You can see why it's gut weed: it's just tubes, filled with air. Tammisaari archipelago, Finland. Wild. 2004-08-01.
Ulva intestinalis L. Engl.: gut weed. Deu.: Darmalge. Suom.: suolilevä. Sven.: enkel tarmtång. Jap.: bo-ao-nori. Bot. syn.: Enteromorpha intestinalis (L.) Link
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Photo: Ulva intestinalis 2.