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 make 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 them through its entire surface. They live in fresh or brackish water, which is low in nutrients. Many species also 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 stonewort is usually a summer annual, but in some years, if the winter is mild, it may not fully die back. This species very rarely produces spores. It is thought that spore production is controlled by light levels, and tends to take place from July to September. This species spreads mainly by means of the star-shaped bulbils that occur on the lower stem nodes. These bulbils stay viable for several years.
In Britain, this stonewort has always occurred mainly in the Norfolk Broads, where six sites are known to have supported the species. There are recent records from five broads and from two sites in interconnecting rivers. It has recently been discovered at a gravel pit in Gloucestershire. Old records for this species occurred in Surrey, Hampshire and Devon, but it is thought that it has been lost from these areas. Elsewhere, starry stonewort is found in Europe, where it is scarce, as well as southern Asia and the Caribbean.
Starry stonewort tends to occur at depths of 1-6 m in lakes or sluggish rivers. It is typically found in calcareous water, often close to the sea, hinting at a preference for saline conditions. It can withstand low light conditions.
The threats facing this species are thought to include water pollution, particularly from sewage treatment plants and agricultural run-off. In some areas, boat traffic and large fluctuations in the salinity of the water may also be problems.
The starry stonewort is a priority for conservation under the UK Biodiversity Action Plan (UK BAP). The Species Action Plan produced to coordinate the conservation of the species aims to maintain all current populations. Plantlife, the wild plant conservation charity is the ‘lead partner’ responsible for this species under the UK BAP. At present, all of the areas supporting starry stonewort are Sites of Special Scientific Interest (SSSIs), one is a National Nature Reserve and another is a reserve owned by the Wildlife Trusts.
The UK Biodiversity Action Plan for this species is available at UK BAP.
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