The unique charophytes are considered to be green algae

Charophytes, also known as stoneworts are large-sized green algae, which can be found in many soft bottom habitats along the entire Finnish coastline. Charophytes are an evolutionary link between simple green algae and more highly specialised plants.

Charophytes are similar in appearance to the higher plants in that they have an actual stem-like structure with distinguishable leaf-like projections. They even attach to the bottom substrate with root-like structures called rhizoids. However, the cells of stoneworts remain non-specialised, and like the algae, they are considered to have a thallus or frond rather than a true stem.

Charophyte structures are not specialised to perform different functions so the cells of both the stem and the leafy branches can photosynthesize and absorb the needed nutrients directly from the water.

There are 21 species of charophytes in Finland

According to the latest information, there are 21 species of charophytes in Finland, occurring in both brackish and freshwater areas. The are several genera in the family, of which the two largest are the rough- and the smooth stoneworts.

It can be difficult to identify stonewort species from one another because of the large variation between individuals. The help of a microscope is often needed to examine identifying features, such as spikes on the surface of the bark cells or shape of the reproductive structures.

A close-up photo of the rough stonewort. The reproductive organs can be distinguished at the base of the upward projecting branches.
In charophytes, the reproductive organs are used to identify species. In the case of the rough stonewort (Chara aspera) pictured here, the reproductive organs appear as orange balls.

Stoneworts provide habitats for animals and filter the water

Stoneworts often thrive in shallow, soft-bottomed bays, where they may form dense and extensive meadows. Such charophyte meadows form valuable habitats for aquatic animals. These plants are also efficient at filtering the nutrients they need from the water, which makes the water clear. These underwater meadows have been classified as a critically endangered natural habitat due to their importance and rarity of occurrence.

The rough-, coral-, and bird’s-nest stoneworts all live in the Baltic Sea

The most common stonewort species in Finland is the vivid green rough stonewort, i.e. Chara aspera. It is easier to identify than other stonewort species due to the presence of white, pearl-like overwintering tubers called bulbils on the root-like rhizoids that are used to attach to the substrate.

The next easily recognisable species is the sturdily-built coral stonewort, i.e. Chara tomentosa, which differs from its other relatives due to its coarse appearance. As the name suggests, the surfaces of the branches and branchlets of this species are shiny and glisten like coral and plants growing close to the water’s surface are also rusty red. Deeper-growing individuals are generally a greyish dark green.

Taking the form of an odd-looking, green-coloured crow’s nest, the so-called bird’s-nest
stonewort, i.e. Tolypella nidifica, is the only representative of this genus in the Baltic Sea. It too is easy to recognise due to its tangled clusters. By contrast, stonewort species belonging to the genus Nitella, such as the smooth- (N. flexilis) and the dark stonewort (N. opaca) are mainly found in freshwater and are quite difficult to tell apart. Separating these species usually requires a microscope and a practised eye.

A close-up photo of a coral stonewort. The branchlet tips are bright orange.
The coral stonewort is easy to distinguish from its relatives due to its sturdy appearance and the rusty-red colour of its shoots.

Charophyte or stonewort species:

  • Rough stoneworts (Chara spp.)
  • Smooth stoneworts (Nitella spp.)
  • The rough stonewort (Chara aspera)
  • Coral stonewort (Chara tomentosa)
  • Bird’s-nest stonewort (Tolypella nidifica)
  • The smooth stonewort (Nitella flexilis)
  • Dark stonewort (Nitella opaca)