Sea lettuce

Sea lettuce

Latin name: Ulva lactuca
Global Distribution: Europe, North America (east and west), Central America, Caribbean, South America, Africa, Indian Ocean Islands, southwest Asia, China, Pacific Islands, Australia and New Zealand
UK Distribution: Absent only on the most exposed rocky shores in the UK, possibly under-recorded in Ireland
Size: Up to 30cm across and 18cm in length
Diet: Photosynthesis
Habitat: Intertidal, brackish water (e.g. estuaries) and shallow subtidal

Sea lettuce, as the name suggests, is a small green to dark green alga with a flat, crumpled body that is edible for humans. Like many seaweeds, they attach to the rocky shore, seabed, or other algae with a disc shaped holdfast, which resembles the roots of land plants. They are common where suitable surfaces are available for them to settle on and can tolerate brackish conditions, such as those you would find in estuaries. They are particularly abundant when there are high levels of nutrients in the water, such as sites where there is runoff from intense farming. In very sheltered waters they can also live and grow in floating clumps following detachment.

The life cycle of this species is a bit complicated, consisting of two similar but separate adult stages, which differ in the number of copies of each gene they possess in their cells. The diploid adults (which contain 2 copies of each gene) release haploid spores (which contain 1 copy of each gene) which then settle and grow into adults before releasing gametes (i.e. sperm and egg cells) which fuse to form diploid algae.

Fun facts

  • As spores, they rely on the presence of certain groups of bacteria to develop properly, in antibiotic conditions this seaweed grows into barely organised clumps of cells.
  • These seaweeds are eaten as part of the cuisines of many countries around the world, such as China, Scandinavia and the UK. They are a good source of proteins, minerals and vitamins.
  • Decaying sea lettuce can be hazardous to human health given the hydrogen sulphide fumes released. There have been infamous cases of this in Brittany (northern France) where these seaweeds have been washed up in massive number and have been linked to the deaths of two people and a horse.