Chris Mattison / Gutweed (Enteromorpha intestinalis) growing in shallow rock pool, Isle of Ulva, Mull, Scotland, June.

Solvin Zankl / Green algae (Ulva nematoidea) on white background, from Swakopmund, Namibia, Africa, South Atlantic, April. Preserved specimen.

Solvin Zankl / Intertidal rocks covered in seaweeds, including, brown algae: at top Toothed wrack (Fucus serratus) and on the left Bifurcaria bifurcata. Red algae: middle and bottom right Pepper dulse (Osmundea pinn...

Solvin Zankl / Dried Sea lettuce algae (Ulva sp.), Roscoff, France, April.

Philippe Clement / Gutweed / grass kelp (Enteromorpha intestinalis / Ulva intestinalis) washed ashore on Normandy coast, France, September

Christophe Courteau / Aerial view of Saint-Michel-en-Grve beach. The green color of water is due to algae (Ulva armoricana) growth caused by nitrogen pollution from agriculture. Cotes d'Armor, Brittany, France, September 2...

MYN / Carsten Krieger / Sea Lettuce (Ulva lactuca) on Common Limpet (Patella vulgata) from tide pool, County Clare, Republic of Ireland, December. Meetyourneighbours.net project

MYN / Carsten Krieger / Sea Lettuce (Ulva lactuca) on Common Limpet (Patella vulgata) from tide pool, County Clare, Republic of Ireland, December. Meetyourneighbours.net project

MYN / Carsten Krieger / Sea Lettuce (Ulva lactuca) from tide pool, Carageen, County Clare, Republic of Ireland, December. Meetyourneighbours.net project

MYN / Carsten Krieger / Sea Lettuce (Ulva lactuca) from tide pool, County Clare, Republic of Ireland, December. Meetyourneighbours.net project

Will Watson / Eutrophic ditch with bank erosion caused by cattle, with Spirogyra and Enteromorpha algae and floating leaves of Unbranched bur-reed (Sparganium erersum), Herefordshire Lowlands, England, UK. June, 20...

Lewis Jefferies / Diverse array of algae species including Sargassum weed (Sargassum horneri), Kelp (Laminariales), Sea lettuce (Ulva lactuca) on shallow reef with juvenile Pollock (Pollachius pollachius) and Ballan wr...