Elatine hexandra (Lapierre) DC., Six-stamened Waterwort

Account Summary

Growth form and preferred habitats

This tiny, inconspicuous, prostrate or low-growing, opposite-leaved waterweed has its Irish strongholds in the far SW of the island and in Co Down (H38), with occasional other scattered stations, especially along the west coast. On intermittently exposed bare, soft sandy or peaty mud, silt, or fine gravel, Six-stamened Waterwort is an opportunistic colonising, minute, green, aquatic annual with fragile, creeping stems. It typically occurs in or on the margin of bodies of still or sheltered, shallow, mesotrophic water up to 50 cm deep. When exposed to the air or when just covered with water, the slender stems creep across the mud, rooting at every node. Individual plants, or several growing closely intermingled, can form temporarily dominant, but ephemeral, patches on the mud.

Much more rarely, E. hexandra grows permanently submerged in deeper, open water situations, often in sheltered bays of larger water bodies. In these deeper waters, the plant assumes a more straggly growth, as stems that are only barely anchored to the lake bed, reach up towards the water surface and light (Mitchell 1983). In the deep water situation, the species behaves as a more-or-less persistent perennial, although it usually fails to flower and is forced to rely on vegetative growth for any increase or spread (Preston & Croft 1997). When submerged in shallower water, E. hexandra flowers and fruits each year, although like Callitriche stagnalis (Common Water-starwort), individual plants probably are not long-lived (Salisbury 1967; Hawkins 1982).

While E. hexandra appears to prefer moderately nutrient-rich, mesotrophic conditions, inherent variation enables it to tolerate a much wider range of water chemistry than this implies. Occasionally it can occur on peat at the edges of moorland lakes (A.J. Silverside, in: Stewart et al. 1994). In reality, E. hexandra eschews only extremely calcareous hard water situations (Salisbury 1967; Preston & Croft 1997).

Elsewhere in Britain & Ireland, E. hexandra and its near relative E. hydropiper (Eight-stamened Waterwort), generally associate with Littorella uniflora (Shoreweed), Eleocharis acicularis (Needle Spike-rush) and Baldellia ranunculoides (Lesser Water-plantain), all of which occur quite frequently in Fermanagh. Like these other small species, both Elatine species cannot compete with tall perennial colonists of marginal muddy habitats, such as reeds and the larger sedges. Rather they are restricted to disturbed areas of exposed mud, where the larger, vigorous, secondary colonists are prevented from establishing by the instability of the habitat, due to, for instance, wave action, grazing stock animals or boating activity.

The physical extent of the water-marginal, bare mud habitat is determined by local rainfall and consequent water-levels in lakes and pools. Therefore population numbers of E. hexandra are known to fluctuate enormously from year to year (Salisbury 1967; A. J. Silverside, in: Stewart et al. 1994).

Although mainly a lowland species, Six-stamened Waterwort does occur in sites up to 440 m in N Kerry (H2), and to altitudes of almost 500 m in the Scottish Highlands (Preston & Croft 1997).

The plant is so small and so very easily overlooked (or mistaken for young Callitriche specimens), a careful eye needs to be kept out for it on any open, relatively bare, unvegetated areas of damp, recently exposed mud on the margins of lakes, ponds, or even around ditches and puddles.

Reproduction

The tiny flowers are only 3-4 mm across and they are produced from July to September. E. hexandra flowers have three delicate petals, while those of its close relative E. hydropiper have four. In both species, the petals are usually pale pink, although occasionally they are white. The trimerous flowers (ie with parts in threes) are habitually self-fertilized and, indeed, if submerged they remain unopened and are cleistogamous (ie selfed while in bud). However, occasionally insect visitors may achieve cross-pollination of aerially exposed flowers. Elatine flowers possess nectar glands, which suggests the flower family to which they belong was once terrestrial (Mitchell 1983). The fruit is a capsule with three (or rarely four) locules, each containing numerous minute seed. Hooker (1884) claimed the seed number generally lay between 24 and 36 and was a multiple of three (ie three locules each containing 8-12 seeds). A study by Salisbury (1967) examined the contents of 406 capsules and found the mean number present in each was 37.5 seeds. The number of seed per capsule varied between five and 72. Salisbury also showed that loculi of the same fruit could have varying numbers of seed in them, and that nutrition and the prevailing light climate probably were the major factors determining the number of seed produced.

Seed production is so rapid that under favourable growing conditions two fruiting generations can often occur in a single season. This creates an enormous dormant, overwintering seed population typical of pioneer species of intermittently exposed mud (Salisbury 1967).

Buried seed survival

According to Mitchell (1983), a persistent, long-lived soil seed bank is formed, yet the survey carried out in NW Europe does not include this species (Thompson et al. 1997). However, the behaviour of the species with its intermittent, almost explosive reappearance after a quasi-simultaneous germination, which in turn follows a drought period that provides a suitable open, muddy environment, strongly supports the notion that long-term dormant seed survival exists in E. hexandra. Buried seed remains dormant until water levels drop at the right time of year and the light intensity is sufficient to trigger germination (Mitchell 1983).

Seed dispersal

Dispersal of the minute seed is readily imagined by flotation and by current movement in linked water systems. Salisbury (1967) found that about 8% of freshly released seed in an experiment he conducted were still afloat after 24 hours in standing water. In moving flood water, the tiny, lightweight seed would probably remain floating longer than in still water and could certainly travel some distance. Since the seed coat possess mucilage and becomes very sticky when wet, the muddy feet and feathers of waterfowl are probably responsible for the spread of the species to new sites, such as those observed in freshly dug reservoirs or recently abandoned sand and gravel pits in S England (Salisbury 1967; Mitchell 1983; Preston & Croft 1997).

Fermanagh occurrence

A number of detailed, systematic surveys made of aquatic habitats in Fermanagh in recent years have now discovered four sites for this species in the VC. At the original station listed above the plant was confined to the summer exposed muddy shore of this large lake on the county boundary with Co Leitrim (H29). (NB Apparently there is no voucher.) The plant is so small and inconspicuous that at the time it was believed it could easily enough have been overlooked at other suitable sites in Fermanagh, although a great deal of time and effort had already been spent surveying aquatic habitats in the VC.

In 2006, a further survey of water quality at selected lakes in N Ireland was commissioned by the Environment & Heritage Service and this uncovered E. hexandra at three additional stations in NW Fermanagh, again near the county boundary but this time with East Donegal (H34): Lough Vearty, Lough Rushen and Lough A Waddy. RHN refound the plant at Lough A Waddy in October 2010: it was picked up amongst lakeshore plant debris consisting mainly of Isoetes lacustris (Quillwort) plus other isoetids and originally it was mistaken for a Callitriche (Water-starwort) species. This is a very difficult plant to identify by normal 'keying out' in a standard Flora, but Haslam et al. (1975) (British Water Plants) is recommended. The leaf venation is a good distinguishing character for separating vegetative material of Elatine from Callitriche (M. Jebb, pers. comm. 2010).

Irish occurrence

Elsewhere in N Ireland, since the early 19th century, E. hexandra has occurred rare and scattered around Cos Armagh, Down, Antrim and Londonderry (H37-H40), although in recent times it is principally found in a few lakes in Co Down (Flora of Lough Neagh, FNEI 3, NI Vascular Plant Database).

Considering how easily E. hexandra could be overlooked or mistaken for a Callitriche species, even with the modern identification guides available, it is all the more remarkable that H.C. Hart (1898) listed this minute aquatic species in his Flora of the Co Donegal in four of the eight subdivisions he made of the county (VCs H34, H35). One of the stations he listed is Bannus Lough, which is less than 500 m from the Fermanagh boundary and only 4 km from Lough A Waddy: Hart described E. hexandra as being, "plentiful at Bannus Lough, 2½ miles [4 km] SW of Pettigo, at the roots of Equisetum limosum [= E. fluviatile] and Carex ampullacea [= C. rostrata] in mossy mud."

Elsewhere in Ireland, the New Atlas hectad map shows E. hexandra thinly and widely scattered, mainly in coastal counties, and most frequent in the far south-west (Preston et al. 2002).

British occurrence

The New Atlas hectad map shows that E. hexandra is a decidedly scarce species, thinly but widely scattered across the whole latitudinal range of Britain from the Channel Isles to Shetland. Having said this, the distribution has a definite western preponderance, most especially displayed in Scotland and in Wales (Preston et al. 2002).

Reasons for rarity

Knowing the huge potential seed population of E. hexandra from its biology, its considerable dispersal ability and prolonged dormancy in soil, the comparative rarity the species displays across Britain & Ireland cannot be entirely due to the plant's small scale, prostrate habit and consequent reduced competitive ability, plus its avoidance of hard, limestone waters and soils. Rather, the observed scarcity must be attributed to the marked degree of intermittence of mud exposure in suitable sites, together with the rapidity with which such areas are colonised by taller, more vigorous plants if the exposure is prolonged (Salisbury 1967).

European occurrence

E. hexandra is very much confined to W & C Europe. On the W coast of Europe, it stretches from S Norway to Portugal, but is almost absent from Italy south of the Alps and entirely absent from the Balkans. It is also reported from the Azores and from Senegal, W Africa (Hultén & Fries 1986, Map 1347; Clapham et al. 1987; Preston & Croft 1997).

Names

The genus name 'Elatine' was an ancient Greek name used by Dioscorides for an unknown plant, but as he was a medic employed in the Roman army, it must have had herbal properties. The name translates as 'little fir tree' and is applied to the genus because of the appearance of one of the species unique in the Family Elatinaceae, E. alsinastrum L. The latter is a continental European aquatic that resembles a seedling conifer since it possesses an unbranched stem with whorled leaves (Gilbert-Carter 1964; Mabberley 1987). The Latin specific epithet 'hexandra' means 'six stamened', which generally applies to this species (Gilbert-Carter 1964).

Threats

Cultural eutrophication, due for instance to runoff of agricultural fertilisers or slurry, causing nutrient enrichment that leads to massive algal blooms that can shade out and destroy suitable growing conditions for waterworts.

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