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