Convergent Stonewort

This stonewort is usually found in water up to 3 metres deep, and most of its known sites are near the sea. It is usually a summer annual, but may overwinter in deeper water.

Convergent stonewort is listed as a priority species under the UK Biodiversity Action Plan (UK BAP), is included in English Nature's Species Recovery Programme (SRP), and Plantlife's 'Back from the Brink' project. The pollution threat to this species certainly seems to have resulted in the loss of the stonewort from two of the Devon sites. The story from the Norfolk Broads is better, with improved water quality resulting in the stonewort's re-appearance at several former sites. However, the problems associated with pollution and enrichment of rivers, lakes and other water bodies through agricultural run-off affect many different species of both plants and animals, as well as algae. Long-term improvements to water quality would be of considerable conservation benefit.

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 called 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 and breathe 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. Convergent stonewort is a slender species, yellowish-green in colour. The characteristic little branches, or branchlets, positioned regularly up the stem, are often strongly curved backwards, although this is not always the case in British specimens. This stonewort also smells stronger than other related species. The male and female sexual reproductive structures grow at the nodes and, in this species, on separate plants; the male organs are spherical and often orange in colour.

This is an aquatic species, which grows in lakes, occasionally in ponds and ditches, but always in alkaline water with a clay or sandy bottom. In Britain, many sites are near the coast, and the species can tolerate mildly salty conditions.

Convergent stonewort has a patchy distribution across southern Europe from the Mediterranean to Britain, Ireland and Germany. The only UK records are from England, including the Norfolk Broads, East Sussex and Devon, and some old sites in Suffolk and Hampshire. It is now thought to be extinct at one of the East Anglian sites, the Sussex site and one at Slapton Ley in Devon.

Classified as Endangered in the UK, and protected under the Wildlife and Countryside Act, 1981 as amended.

The main threats to this species seem to be from pollution and increasing levels of nutrient enrichment, some of this from sewage treatment. This is certainly believed to have affected the populations in the Norfolk Broads. Agricultural run-off is also known to be affecting two of its Devon sites.