Hippuris tetraphylla L. f.,  Fourleaf mare's-tail (Hippuridaceae)

Compiled by: Terhi Ryttäri, Finland

1. Description / Ecological demands / Autecology

Hippuris tetraphylla is a perennial aquatic plant, which is able to spread vegetatively by its horizontal rhizome. It can be confused with H. x lanceolata, which is a hybrid originated from H. vulgaris and H. tetraphylla. However, these taxa have some ecological differences. H. vulgaris grows barely in fresh water or in brackish water with extremely low salinity. The hybrid inhabits open patches among reed and sedge vegetation in slightly salty brackish water, and small ponds on grazed seashore meadows. The largest and most viable populations of H. tetraphylla are at the moment on wide and open shallow waters with at least partly soft bottoms, nearly always on more or less exposed islands or peninsulas. The plant reproduces by both seeds and vegetatively by pieces of its rhizome.

2. Distribution

H. tetraphylla has a circumpolar distribution and occurs on coastal areas of Finnmark and Iceland, Newfoundland, Hudson Bay, Alaska and on Russian coasts of  Pacific Ocean, Kola Peninsula and White Sea. In the Baltic Sea it has historically occured along the whole coastline of Finland. One population is known from Västerbottens län, Swedish coast. It has disappeared from the Swedish coasts of the Western Gotland Basin and the Northern Baltic Proper. It has been strongly declining on its previously most abundant areas along Finnish coasts, and at the moment it is known to exist only in the Bothnian Bay and the Bothnian Sea. From the Gulf of Finland it has disappeared.

3. Importance

H. tetraphylla is included in the Habitats Directive Annexes II and IV.

4. Status of threat

H. tetraphylla is classified as critically endangered in Sweden (Red List 2005) and endangered in Finland (Red List 2000).

5. Threat factors

Eutrophication of the sea, followed by overgowth with reeds and other strong competitors changes the habitats of H. tetraphylla. The cessation of cattle grazing has accelerated the overgrowth. Similarly to Alisma wahlenbergii reasons for local disappearances include road construction, harbours, marinas and summer cottages as well as dredging of waterways.

6. Conservation measures

The water quality of the Baltic Sea needs to be improved. The sites of H. tetraphylla need active management (e.g. preventing overgrowth by removing vegetation, e.g. by cattle grazing). The present status and distribution of H. tetraphylla and the hybrid H. x lanceolata needs to be clarified.

7. References

Ilmonen J., Ryttäri T. & Alanen A. (eds.) 2001: Finnish plants and invertebrate animals in the EU Habitats Directive. A scientific evaluation of the Finnish Natura 2000 –proposal. – The Finnish Environment 510: 1-177. (in Finnish)

Gärdenfors U. (ed.) 2005: Rödlistade arter i Sverige – The 2005 Red List of Swedish Species. Swedish Species Information Centre, Uppsala.

Rassi P., Alanen A., Kanerva T. & Mannerkoski I. (eds.) 2001: Suomen lajien uhanalaisuus 2000 (Threatened species in Finland 2000). – Ympäristöministeriö & Suomen ympäristökeskus, Helsinki.