Ulva intestinalis, formerly referred to as Enteromorpha intestinalis (Linnaeus) Nees, is a green alga in the family Ulvaceae, of the genus Ulva (sea lettuce), also known by the common names gutweed  and grass kelp. Until they were reclassified by genetic work completed in the early 2000s, the tubular members of the genus Ulva were in the genus Enteromorpha.
Generally world-wide. It can be found in Bering Sea near Alaska, Aleutian islands, Puget Sound, Japan, Korea, Mexico, Philippines,and Russia. Besides this, places it can be found in Israel, and in such European countries as Azores, Belgium, Denmark, Ireland, Norway, Poland, and in such seas as the Baltic, and Mediterranean Sea. It is also found in the shores of the Pacific Ocean.
The fronds have branches and are completely tubular expanding in width to mid-thallus. reaching 15 cm long or more. The cells are irregularly arranged and the chloroplast is hood-shaped and placed to one side, generally with only one pyrenoid. The species may be 10–30 centimetres (3.9–11.8 in) long and 6–18 millimetres (0.24–0.71 in) wide. They have rounded tips as well. The algae may be reproductive at all times of the year. The life-history shows an alteration of generations, isomorphic - gametophytic and sporophytic. In some references the species (Enteromorpha intestinalis) is treated as two subspecies: ssp. intestinalis (L.) Link and ssp. compressa (L.) Link.
In other languages
- Danish: Tarm-rørhinde
- Dutch: Echt darmwier
- French: Entéromorphe
- German: Darmtang or German: gemeiner Darmtang
- Norwegian: Bokmål tarmgrønske or Norwegian: Nynorsk tarmgrønske
- Polish: Sałata, Polish: Taśma or Polish: Watka
- Swedish: Tarmalg
- Thai: สาหร่ายไส้ไก่
- Portuguese: Erva-Patinha
- Guiry, M.D., John, D.M., Rindi, F. and McCarthy, T.K. (Eds) 2007. New Survey of Clare Island. Volume 6: The Freshwater and Terrestrial Algae. p. 23. Royal Irish Academy. ISBN 978-1904890-31-7
- Gutweed - Enteromorpha intestinalis
- Grass-kelp, Gutweed
- Burrows, E.M. 1991. Seaweeds of the British Isles. Volume 2 Chlorophyta. British Museum (Natural History). ISBN 0-565-00981-8
- "Ulva intestinalis". Seaweeed of Alaska. Retrieved March 24, 2013.
- Guiry, M.D. (2012). "Ulva intestinalis Linnaeus, 1753". World Register of Marine Species. Retrieved March 24, 2013.
- "Gut weed - Ulva intestinalis". Retrieved March 24, 2013.
- Morton, O. 1994. Marine Algae of Northern Ireland. Ulster Museum ISBN 0 900761 28 8
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