Ulothrix flacca

Ulothrix flacca is a green alga from the family Ulotrichaceae. It lives in marine or brackish water, typically in the intertidal zone. The alga forms small, woolly tufts of soft, unbranched filaments, each no more than 10 cm long, attached to rocks. The filaments are usually one cell thick, though a cell may rarely split lengthwise to create a double row. At the base, cells are elongated, or in older filaments they develop into a complex system of branching rhizoids. The cylindrical cells measure 14.4–32.6 μm wide and 4.8–9.6 μm long. Each cell contains a single, parietal chloroplast that may be lobed; its shape changes with age, being shorter in young cells and covering less of the cell circumference. The chloroplast usually holds one to three pyrenoids. Some biologists consider *Ulothrix pseudoflacca* and *Ulothrix consociata* to be forms of this species. It can be hard to tell apart from *Urospora*.

For reproduction, *Ulothrix flacca* can spread vegetatively when filaments break apart. It also reproduces asexually or sexually. In both cases, vegetative cells (except those forming rhizoids) turn into zoosporangia or gametangia. Zoosporangia are yellow-green and slightly longer than vegetative cells, each producing 8 to 32 zoospores. These zoospores are spindle-shaped, 6.6–10.6 μm long, with two flagella, a cup-shaped chloroplast, a pyrenoid, and a small stigma. They exit through an opening in the zoosporangium wall, swim, then attach to a hard surface, lose their flagella, and grow into new filaments. Sometimes aplanospores form instead of zoospores, and these may start germinating while still inside the zoosporangium. Gametangia are olive-brown and look similar to zoospores but are smaller. Gamete fusion is monoecious and isogamous, producing zygotes with four flagella. These zygotes develop into a tiny, single-celled sporophyte, which then undergoes meiosis to create zoospores or aplanospores.

This alga is common all around the British Isles and has been recorded from Scandinavia south to Portugal, in the Mediterranean and Black Seas, and across the Pacific worldwide. It grows on rocks, stones, and as an epiphyte on other algae. It occupies a wide range of habitats in the littoral zone but is typically absent from beaches.