Littorella uniflora (L.) Asch., Shoreweed

Account Summary

Growth form and preferred habitats

L. uniflora is a type of plant called an 'isoetid', a category named after the Quillwort genus Isoetes although, of course, while there is a superficial similarity in general growth form, ie the plant is a small, slow-growing, evergreen, amphibious aquatic with thick, stiff leaves or stems that form basal rosettes and have a proportionally large below ground biomass. The plants of this grouping are not closely related in their taxonomy (eg Isoetes belongs to the spore-bearing Lycopsida). Incidentally, in relation to the mention of the genus Isoetes and the isoetids, the name is derived from the Greek 'isos', meaning 'equal' and 'etos', 'a year' or 'the year', combined to mean 'equalling one year', but really referring to 'evergreen' or 'green throughout' (Johnson & Smith 1946; Gilbert-Carter 1964; Gledhill 1985).

In the case of Littorella uniflora, the plant has a short, white, erect rootstock that sends off long white roots. The leaves are bright green, 2.0-10.0 cm long, nearly round in section and in cross-section show numerous lacunae. L. uniflora is readily distinguished from other similar-looking aquatics such as Isoetes lacustris (Quillwort), Lobelia dortmanna (Water Lobelia) and Subularia aquatica (Awlwort) by its developing stoloniferous, carpet-forming colonies, rather than possessing separate, discrete rosettes as do these other isoetid genera that occur in similar lakeshore and other aquatic habitats including rivers, streams, reservoirs, ponds and dune-slacks (Preston & Croft 1997).

The terrestrial form of Shoreweed is produced by germination from a large, long-persistent seed-bank whenever water bodies are drawn down sufficiently in suitably dry summers to expose a broad expanse of bare, often muddy shore available for colonisation. In addition to this annual form of growth, however, Shoreweed is a perennial species and has great powers of vegetative spread, its abundant stolons, rooting and producing rosettes of leaves at nodes enabling it to form dense, turf-like colonies, especially when it grows permanently submerged in relatively shallow, oligotrophic or mesotrophic waters. L. uniflora can grow on a very wide range of waterside or bottom substrates including stones, gravel, sand, peat, marsh and soft silty mud (Preston & Croft 1997; Sell & Murrell 2009). The established strategy of the species is categorised as 'SR/CSR', ie intermediate between a Stress tolerant Ruderal and a balance of all three strategies, Competitor, Stress tolerator and Ruderal (Grime et al. 1988, 2007).

In Britain, some of its sites, including winter-flooded depressions on heaths and sea-cliffs, dry out completely in summer. In deeper water, in sites that remain wet all year, L. uniflora grows in open swampy conditions dominated or characterised by Eleocharis palustris (Common Spike-rush) and Equisetum fluviatile (Water Horsetail). Here it appears both on exposed shorelines, and below this it forms a carpet on lake bottoms down to depths of around 3 to 4 m, where it is often accompanied by Isoetes lacustris and Lobelia dortmanna (Preston & Croft 1997).

There are interesting variations in tolerances across B & I. In Scotland and Ireland, L. uniflora is most frequent in regions with base-poor acidic geology and waters, but it can also grow in definite base-rich habitats such as clear limestone lakes, streams and turloughs (ie vanishing limestone lakes that drain vertically into subterranean rivers and streams). In the Burren, Co Clare (H9), it can even grow in pools perched on limestone pavement, as for example is the case on the island of Inishmore (Webb & Scannell 1983). In contrast, in SE England, L. uniflora is completely confined to acidic habitats (Preston & Croft 1997).

Flowering reproduction

Shoreweed flowers from June to August and usually does so only when in dry seasons it becomes emersed, ie the flowering stem protrudes above the water surface. Totally submerged plants always fail to flower. The species is monoecious (ie separate male and female flowers are borne on the same plant). Both kinds of flower are very simple, greatly reduced and are borne together on a flowering stem, c 10 cm, generally shorter than the leaves. The flowers are very definitely wind-pollinated, the solitary terminal male flower being stalked and the one, two or a few female subsessile ones, arranged below at the base of the male scape, hidden by the surrounding leaf bases. One or more bracts separate the male from the female flowers. Each plant can produce three, four or more flowering stems.

The flowers consist of four green sepals with scarious margins and four white petals, also scarious on their margins; the males have four stamens with very extended white filaments 10-20 mm long; the few female flowers are uni-ovulate and have one thread-like style, c 10 mm long, bearing a terminal, linear stigma. The fruit is a single-seeded, ovoid or cylindrical, indehiscent (non-splitting) capsule or nut, c 3 mm, its surface keeled, honeycombed and black (Hutchinson 1945; Melderis & Bangerter 1955; Butcher 1961; Sell & Murrell 2009).

There is convincing evidence from isolated ponds and marshes that fruit dispersal in L. uniflora involves transport overland by ducks (Stelfox 1922; Ridley 1930, p. 547). Most likely the fruit, which is not particularly large or bulky, has been conveyed attached in mud on the feet or feathers of ducks. L. uniflora has similarly reached remote oceanic island groups, such as the Azores (Ridley 1930, p. 550). Some seed is not or is hardly dispersed at all: Cook (1987 (b)) reported that in Littorella (no particular species stipulated, but read in context, this could refer to all species of the genus), "the seeds germinate while still attached to the mother plant".

Most published ecological treatments suggest that seed of L. uniflora is long persistent in the soil, ie more than five years and perhaps for decades (eg Grime et al. 1988, 2007; M.J. Wiggington, in: Preston et al. 2002), although some doubt on this was indicated in the latter reference. However, the published Survey of soil seed banks in NW Europe contained just one estimate of seed longevity, which concluded it was transient, ie seed persisting for less than one year (Thompson et al. 1997).

Vegetative reproduction

It is very likely that L. uniflora depends for local survival on its very effective vegetative reproduction more than on seed production, which only occurs whenever water levels drop and marginal ground becomes exposed for a sufficient length of time to allow anthesis, pollination and fertilisation. However, transport of seed and vegetative fragments are both likely to be very significant in achieving species dispersal to fresh habitats, with seed being especially so when jump dispersal to remote or unconnected water bodies is concerned (Ridley 1930).

Fossil record

The pollen of L. uniflora is as recognisable as the one-seeded, hard, indehiscent 'bony' fruits and fossils of both contribute substantially to the historical evidence. The fossil record in B & I goes right back to the Cromer Forest Bed series and there are records from every sub-stage of the subsequent Hoxnian interglacial. In the Ipswichian, it is only recorded from sub-stage IV, but in all the Flandrian zones there are pollen records with a thin scatter of fruit records. There are also abundant records from some of the glacial stages, many of the fruit records being from Ireland. It is evident that L. uniflora was prevalent in the shallow lakes of the Weichselian glacial period and that it has persisted in suitable situations ever since then (Godwin 1975).

Fermanagh occurrence

On the shores of many lakes, especially around mesotrophic to more eutrophic Lough Erne, this perennial grows abundantly in shallow water and produces its greatly reduced unisexual flowers when stretches of bare gravelly, sandy, muddy or peaty lakeshores become exposed in summer. Despite its frequency on lakeshores and especially so around the larger loughs in the VC, rather surprisingly L. uniflora does not appear to occur on any river sites in Fermanagh. It has been recorded in 132 tetrads, 25% of those in the VC. As the tetrad map shows, it is most frequently found around the larger lakes and is rather rare beyond them.

British and Irish occurrence

L. uniflora has a predominantly northern and western distribution in B & I, where it is often abundant in suitable wetland habitats (New Atlas). In SE England, it is rare and confined to acidic habitats, but in Ireland and Scotland, where it is much more frequent, widespread and locally abundant, it displays a much wider ecological range. The ability of L. uniflora to tolerate mesotrophic water, and even some degree of eutrophication, has allowed it to survive in areas where other isoetids have declined.

In S England and S Ireland, L. uniflora lost many suitable habitats due to drainage operations before 1930 and this progressive loss has continued to the present in S & E England where the species is now rare and widely scattered. Apart from drainage, some losses appear to have resulted from habitat change due to overgrowth by tall, rank vegetation (Preston & Croft 1997; M.J. Wigginton, in: Preston et al. 2002).

European and world occurrence

L. uniflora belongs to the suboceanic temperate phytogeographical element and occurs widely scattered in W, C & N Europe, with diminishing frequency eastwards and northwards, although it reaches 68°N in the Arctic. It is also present on Iceland, the Azores and on the tip of N Africa. Presence becomes more scattered in the south of Europe, although it reaches W Spain and Portugal. The species is sparse or absent from most of the Mediterranean basin, but is present in N Italy and in Sardinia (Godwin 1975; Clapham et al. 1987; Hultén & Fries 1986, Map 1729; Sell & Murrell 2009).

Names

The genus name 'Littorella' is derived from the Latin 'litus' or 'littus' meaning 'shore' or 'of the shore', referring to the habitat where it is most visible (Johnson & Smith 1946; Gilbert-Carter 1964). The Latin specific epithet 'uniflora' means 'one flowered', a reference to the solitary terminal male flower which is so conspicuous (Johnson & Smith 1946).

The English common name 'Shoreweed', or older and better 'Plantain Shoreweed' (Prior 1879) is another perfect example of an invented 'book-name', of no folk significance, but in this case, a useful reminder of the habitat (Britten & Holland 1886).

Threats

Experience in Fermanagh shows L. uniflora can tolerate a degree of eutrophication, provided it is not excessive. Acidification of upland lakes due to forestry run-off could see it decline locally.

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