Mat-forming herb with extended, and much branched, horizontal rhizome system and erect shoots. Shoots 5–20 cm with leaves in 10–20 or more whorls, (5)6(7) leaves in each whorl. The 4–6 lowermost whorls with reduced leaves (prophylls).

Leaves 5–10 ×1–2 mm, entire, without petiole, linear or slightly tapering, obtuse or subacute, green to dark green.

Formally a spike with sessile flowers in each leaf axil in the middle and upper parts of the shoot.

Flowers bisexual. Perianth reduced to a calyx as a lobed rim on the top of the ovary or fruit (epigynous), petals absent. One stamen adnate to the ovary, with a filament shorter than the anther. The anther protrudes just above the rim of the ovary. Ovary one-fruited with a single, long style appearing next to the anther and usually protruding above it. Flower (ovary and stamen) dark purple, contrasting the green leaves.

The fruit is a one-seeded nut.

Sexual reproduction by seeds, potentially; vegetative reproduction by rhizome and rhizome fragments. Whether seed reproduction occurs in Svalbard is unknown; all studied plants are non-flowering but we assume that some flowering and seed-set may take place in good seasons.

The fruits float and may be dispersed with water along brooks and with birds between watercourses. However, the main or perhaps the only means of dispersal on Bjørnøya is by rhizome fragments transported between small lakes and brooks with birds.

There is no similar plant in Svalbard. For comparison with the two other species present in the North Atlantic regions, see Comments.

An aquatic plant confined to shallow waters in small lakes and slow-flowing brooks, usually on mud with rhizomes permanently submerged or soaked, whereas the shoots emerge above water. The plant survives well on shores and may flower more regularly when shoots are above water for longer periods. There are no preferences as to water chemistry except that the species is rarely found in acidic waters, but often in slightly saline waters. Farther south, this species is confined to sites with brackish water.

In Svalbard confined to the island of Bjørnøya where it is rather common at the margins of ponds and streams from 10 to 30 m a.s.l. mainly on the northern and western parts of the island. Very large numbers of shoots were seen at the lakes of N Flakmyrvatna, were it appears to be promoted by some disturbance and manuring by polar gulls and skuas of the floating moss carpet (Engelskjøn 1986).

The global range is uncertain as there has been much confusion concerning the species in the genus Hippuris. Recent studies show that this is the most common species in the Western Arctic (Elven et al. 2019). It is the only species documented from Greenland and the Canadian Arctic Archipelago; its two relatives (H. tetraphylla L. f. and H. vulgaris L.) appear together with H. lanceolata on the American mainland. Our field investigations in Siberia and the Russian Far East (R. Elven & H. Solstad observ.) suggest that it also is the only or most frequent species in the arctic parts there; we have found only H. lanceolata in sites and regions where the two others (but not H. lanceolata) have been reported in the literature. In N Scandinavia all three occur, but H. lanceolata and H. tetraphylla are restricted to Finnmark in N Norway and the Bothnian Bay, whereas H. vulgaris is ubiquitous, present in all other northern temperate regions and also in the southern hemisphere.

Hippuris is a very small genus with only 4 species recognized, all present in northern regions. The rather different and unisexual H. montana Ledeb. ex Rchb. is restricted to the Pacific coast of North America north to Alaska, with a single dubious record from the Russian side of the Pacific (see Elven et al. 2019). The three others are circumpolar: H. lanceolata has more or less continuous distribution in the arctic and northern boreal zones, H. tetraphylla is very disjunct in the southern arctic and northern boreal zones, reaching temperate regions on the east coast of North America and along the Pacific, and H. vulgaris occurs almost everywhere except for the Arctic, both in the lowlands and in the mountains, in the temperate zones and also in many other places throughout the world (Elven et al. 2019).

The three species differ in few characters. Hippuris tetraphylla regularly has only (3)4(5) obovate to oblanceolate, obtuse leaves in each whorl, H. lanceolata (5)6(7) mostly linear, obtuse to subacute leaves, and H. vulgaris (7)8–9(12) linear, acute leaves. Hippuris vulgaris has stamens with long filaments, usually longer than the anthers, and the anthers are thereby lifted up and well exposed from the ovary. The two others have short filaments so that the anthers are seen as just emerging from the calyx rim at the top of the ovary.

Many authors have assumed the features of both Hippuris tetraphylla and H. lanceolata to be only modifications due to their preference, in some regions, for saline and brackish waters (see, e.g., McCulley & Dale 1961). Numerous field studies (R. Elven, D.F. Murray & H. Solstad observ.) of H. lanceolata together with H. tetraphylla or H. vulgaris in the same ponds or drainages suggest that this view is based on ‘Schreibtisch’ considerations rather than field experience.The majority of authors have considered H. lanceolata to be the hybrid between H. tetraphylla and H. vulgaris, including Elven in Lid & Lid (2005). Also this opinion we have rejected (see also the Hippuris treatment in Elven et al. 2013). There are several features counting against the hybrid theory. Hippuris lanceolata has a wide range well outside those of both of its assumed parents, in the Arctic including all of Greenland and the Canadian Arctic Archipelago where both its assumed parents are unknown, and also a range north of and independent of its assumed parents in arctic Russia and N Alaska. It flowers and produces pollen and fruits regularly (see Elven et al. 2019), just as regularly as its proposed parents (e.g., observed by R. Elven & H. Solstad in W Greenland in 2013, thousands of kilometres away from any H. tetraphylla or H. vulgaris). If it has any hybrid origin at all (which we doubt), it must be a long time ago, probably before the last glacial maximum and perhaps before the Pleistocene.

Consulting previous literature, you will see that the Hippuris of Bjørnøya was considered to be H. vulgaris, e.g., by Engelskjøn & Schweitzer (1970) and in all editions of Rønning's Svalbard flora (Rønning 1964, 1979, 1996). The illustration in Rønning's flora, purporting to be a H. vulgaris from Bjørnøya, is drawn from a specimen of H. lanceolata from the Varanger Peninsula in NE Norway (so even though the name is erroneous, the illustration shows the right species).

As to how Hippuris reached the isolated dot of Bjørnøya in the middle of the Barents Sea, there is only one feasible hypothesis: bird dispersal of either fruits or shoot fragments. The closest occurrences are found in N Fennoscandia and Russian Novaya Zemlya. As it has attained a certain range on the island, the only hypothesis is, again, bird dispersal. There is no way a diaspore (whether fruit or shoot fragment) can get from one lake or brook to another unless some animal physically transports it.

Elven, R., Fremstad, E. & Pedersen, O. (eds.) 2013. Distribution maps of Norwegian vascular plants. IV The eastern and northeastern elements. – Akademika Publishing, Trondheim.

Elven, R., Murray, D.F. & Solstad, H. 2019. Hippuris Linnaeus. – In: Flora of North America 17. Magnoliophyta: Tetrachondraceae to Orobanchaceae. http://floranorthamerica.org/Hippuris

Elven, R., Murray, D.F. & Solstad, H. In press. Hippuris Linnaeus. – In: Flora of North America 17. Magnoliophyta: Tetrachondraceae to Orobanchaceae.

Engelskjøn, T. 1986. Eco-geographical relations of the Bjørnøya vascular flora. – Polar Research, n.s., 5.: 79–127.

Engelskjøn, T. & Schweitzer, H.J. 1970. Studies on the flora of Bear Island (Bjørnøya). I. Vascular plants. – Astarte 3: 1–36.

Lid, J. & Lid, D.T. 2005. Norsk Flora. Ed. 7 by R. Elven. – Det Norske Samlaget, Oslo.

McCully, M.E. & Dale, H.M. 1961. Heterophylly in Hippuris, a problem in identification. – Canadian Journal of Botany 39: 1099–1116.

Rønning, O.I. 1964. Svalbards flora. – Norsk Polarinstitutt, Oslo.

Rønning, O.I. 1979. Svalbards flora. Ed. 2. – Norsk Polarinstitutt, Oslo.

Rønning, O.I. 1996. Svalbards flora. Ed. 3. – Norsk Polarinstitutt, Oslo.