Stoneworts used to be classified as members of the plant kingdom, but it is now agreed that they belong – along with other green algae – in the kingdom Protista. Put simply, the protistas are simple multi-celled or single celled organisms, descended from some of the earliest life-forms that appeared on Earth. Some of the Chlorophytes, specifically the stoneworts, are thought by scientists to have been the early ancestors of all plants. Stoneworts do indeed resemble plants, are frequently mistaken for them, and are often found as fossils. The main body of the plant consists of a series of "giant cells" up to several centimetres in length, which effectively makes up the stem, together with branches resembling leaves radiating out from 'nodes' that are made up of smaller cells. The stonewort anchors itself, not with roots like a plant, but with rhizoids, colourless, hair-like filaments. Like roots, these can absorb nutrients, but the organism can absorb through its entire surface. They live in fresh or brackish water, which is low in nutrients and many species require water that is high in calcium. Stoneworts are often encrusted with white lime deposits, giving a crusty texture (hence the name ‘stonewort’), and they often have an unpleasant smell, similar to stale garlic.
This species has often been confused with the similar species Nitella mucronata var. gracillima. There have been confirmed records of slender stonewort from twelve sites, most of which are from western Britain (Cornwall, Shropshire, Ceredigion, Arfon, Cumbria, Kyle & Carrick and Sutherland) with one record from West Sussex. Just six of these areas (Ceredigion, Arfon, Kyle & Carrick and Sutherland) support recent records. It is likely that it may have been overlooked at a number of sites when it occurs in deeper water. Elsewhere, the species has a very wide distribution around the world, including Europe.
In Britain, this stonewort typically occurs in acidic lakes and pools, either in shallow water amongst other vegetation or in deeper water in large lakes where other vegetation is sparse. In the other parts of Europe it has been found in a greater variety of habitats, including brackish and alkaline waters.
The threats facing this species at present are poorly understood, however pollution from agricultural run-off, and the growth of scrub and other vegetation are thought to be important factors that adversely affect this species. Fish-farming would be a cause for concern if it were proposed at sites supporting the species.
Slender stonewort is a priority species under the UK Biodiversity Action Plan (UK BAP). The Species Action Plan for this stonewort aims to maintain current populations, and to reintroduce the species to one site within the former range before 2005. It is clear that more must be learnt of this plant in order to conserve it effectively and diving surveys of the historic sites must be made in order to determine the current range of the species.
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