Ulva intestinalis, formerly referred to as Enteromorpha intestinalis (Linnaeus) Nees, is a green alga in the division Chlorophyta, 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.
Take a close look at the long strands of this seaweed, then there is no doubt where it got its name from: long, thin, inflated tubes. Its appearance can vary depending upon where it grows. There are lots of species of gutweed, often growing at the foot of a sea dike, on tidal flats and beaches. Some even grow in fresh water. You can find around ten different species in the Netherlands, which are only possible to identify with the help of a microscope.