Ulva: Sea lettuce

This is a small genus of marine and brackish water green algae. It is edible and is often called 'Sea Lettuce'. Species with hollow, one-layered thalli were formerly included in Enteromorpha, but it is widely accepted now that such species should be included in Ulva.

The thallus of ulvoid species is flat and blade-like and is composed of two layers of cells. There is no differentiation into tissues; all the cells of the plant are more or less alike except for the basal cells, which are elongated to form attachment rhizoids. Each cell contains one nucleus and has a cup-shaped choroplast with a single pyrenoid.

Ulva undergoes a very definite alternation of generations. Biflagellate isogametes are formed by certain cells of the haploid, gametangial plant. These are liberated and fuse in pairs to form a diploid zygote which germinates to form a separate diploid plant called the sporophyte; this resembles the haploid gametangial plant in outward appearance. Certain cells of the sporophyte undergo meiosis and form zoospores in sporangia; these zoospores are quite different to the gametes in that they form quadriflagellate zoospores (with 4 flagella). These are released, swim around for a time, settle and germinate to form the haploid gametangial thallus. Note that the haploid gametes are capable of settling and germinating without fusion to form a haploid thallus directly; most Ulva populations reproduce by this form of parthenogenesis and sexual reproduction is not very common.

Ulva can be quite a nuisance in areas that are nutrient enriched from sewage outfalls e.g. Cork Harbour and Belfast Lough, where populations of Ulva may cover large areas of mudflats in the summer.