A rare member of an ancient group of aquatic plants.
Stoneworts are a very ancient group of aquatic plants: they are complex algae that look similar to higher plants. At first glance Stoneworts look more like vascular plants than algae, with what appear to be stems, roots and slender leaves. For many years they were in fact mistakenly classified with the horsetails Equisetum. However anatomical examination reveals their true kinship, they are algae with huge cells, in some species up to 20cm long. Stoneworts anchor themselves to the substrate, not with roots, but with rhizoids, hair-like filaments. They absorb nutrients, not only through their rhizoids, but along their entire lengths. Evolutionists suspect these remarkable plants may have played a part in the evolution of the earliest land plants.
Stoneworts are a difficult group to identify with confidence. Tassel Stonewort typically grows in dense clumps with multiple plants growing amongst each other. It is yellowish-green in colour, and is often encrusted with chalky deposits giving it a crusty feel. Tassel Stonewort, like other Tolypella, has a smooth and characteristically spine-free main stem. The fertile ‘tassels’ are small and curve back on themselves forming dense heads, ‘resembling an untidy bird nest’. It differs from the more common clustered stonewort Tolypella glomerata by its sharply tipped branchlets, and from great Tassel Stonewort T. prolifera by its smaller size (stem diameter is less than one millimetre).
Tassel Stonewort lives in seasonally wet ponds and ditches, usually in quite early successional habitat as it is a poor competitor and dislikes the build-up of silts. Furthermore, much like most Stoneworts, it has two key requirements:
- Clean water – Stoneworts thrive in clear, unpolluted water. One of the main reasons that many stoneworts are rare is the scarcity of clean water in the British countryside.
- A bare mineral substrates: bare sand, gravel or clay – Stoneworts like to root into bare mineral sediments like clay, sand or fine gravel. They do not like the organic-rich sediment that builds up on the bottom of maturing lakes and ponds, and this is why fallen tree leaves can be a particular problem along the margins.
Stoneworts reproduce sexually through oospores. These form within the female oogonia and are fertilised by the male antherodoids, which are released from the male antheridia. These reproductive organs are situated above (female) and below (male) the branchlet arm.
Reproduction is stimulated by both light and temperature. It generally occurs when the water temperature reaches 14°C. In ideal conditions, germination can take place within 24 hours of deposition. The burial of oospores in deep sediments inhibits germination and is linked to stonewort decline.
Distribution and threats
In recent years Tassel Stonewort, like many other stoneworts, have undergone a catastrophic decline as a result of nutrient enrichment of water bodies and the loss of traditional management practices – in particular grazing, which kept fenland ditches and small ponds open rather than overgrown.
Stoneworts are often easily out-competed by aggressive alien plants. Nuttall’s Waterweed and its relatives (Elodea species, Lagarosiphon etc.) can be a particular problem, both for stoneworts and other native aquatic plants.
Out of our 28 native species of stonewort in the UK, over half are listed in the Red Data Book or are Nationally Scarce.