Limosella aquatica L., Mudwort

Account Summary

Growth form and preferred habitats

Mudwort (or Common Mudwort) is a rather inconspicuous, semi-aquatic, loosely tufted, emergent dwarf annual with slender, creeping stems 10-15 cm long and leaf rosettes only 2-7 cm high, composed of roots, leaves and numerous very reduced flowers produced at each node. It does not have an upright stem, but its slender leafless runners or stolons begin their growth erect before becoming horizontal and spreading. The stolons are short-lived and are produced during optimal vegetative activity, their primary function undoubtedly being as organs of reproduction, rather than perennation (Sculthorpe 1967).

The leaves are radical (ie all basal) and can be either emergent, floating or submerged. The floating leaves have blades entire, elliptic, obtuse, spoon-shaped, glabrous, apex obtuse to rounded with a slender petiole that is often reddish and nearly twice as long as the blade (ie 2-7 cm long). In submerged plants, the leaf petiole can be elongated to as much as ten times that of the emergent leaves and the leaf blade is almost non-existent, the leaf being subulate or ± filiform (Sculthorpe 1967; Meikle 1985).

Mudwort, occasionally also referred to as 'Water mudwort', is found on exposed mud of temporary pools and stream banks, from 0-300 m altitude. As a semi-aquatic plant, it grows in moist or wet ground habitats including damp hollows in meadows, in mud and wet sand next to water and partly submersed or floating in shallow depths around pools, lakes, streams and ditches.

As Salisbury (1970) pointed out, the great majority of the 50 or so B & I species characteristic of exposed muds are actually of local, sometimes extremely local, occurrence, which suggests their need for an infrequent combination of environmental conditions.

In B & I, L. aquatica is a typical member of the flora of drying mud, but it is very local and generally is regarded as a rare annual (Salisbury 1967). It grows on and is confined to the stretch of mud exposed on the wet margins of mainly lowland lakes, ponds, reservoirs, rivers, streams and ditches and other periodically flooded places as water levels shrink in hot, dry summer weather. As Salisbury (1970) points out, "Where mud borders the larger, more permanent, areas of inundation a gradual lowering of the water-table may only enhance the vigour and centrifugally extend the zone of the 'reed-swamp' species, or other shallow-water vegetation, so that, normally, a severe summer drought (or artificially abrupt lowering of the water-level in a lake or reservoir) will alone provide an expanse of bare mud where pioneer species can colonize and flourish."

Plants may be partly or completely submerged as a result of renewed rainfall and a rising water table. L. aquatica is a very rapidly germinating ephemeral that colonises bare mud when it first becomes exposed to the air in summer, or later in the season and the small plant completes its short life cycle before autumn or early winter rains re-flood the occupied ground.

The species is described as erratic as well as ephemeral, L. aquatica often reappearing at sites where it has been absent, or has not been seen, for many years. Population sizes can vary enormously from year to year, depending on the extent of the muddy shore exposed; the number of individuals in a locality, when it sporadically reappears, may be astonishingly large (Salisbury 1967). The explanation for this appears to be that seed can remain dormant and viable in submerged or regularly wetted mud for many years.

L. aquatica does not appear to be a very competitive species, so that winter submergence of its seed, as well as summer desiccation providing abruptly available expanses of bare mud, may have their main significance in diminishing competitive pressure from other species, rather than through conferring a direct benefit on Mudwort (Salisbury 1970).

While in Fermanagh and its few other sites in Ireland, including in the karst limestones of the Burren, Co Clare, L. aquatica appears calcicole. Elsewhere in B & I, there is a feeling that the species is somewhat (maybe only slightly) calcifuge, preferring mildly acidic soils, which are nutrient-enriched by manure from grazing animals and, when sufficient water is present, by the droppings of flocks of feeding water fowl.

Flowering reproduction

Plants flower from June onwards, although the normal season generally stretches from July to September (Salisbury 1967). The numerous, tiny, bisexual, regular (actinomorphic) flowers, 2-3 mm in diameter are solitary on rather short peduncles, 1-2 cm long (shorter even than the leaf petioles) in the axils of bracts. The peduncles become spreading (decurved) when in fruit. Flowers have a 5-lobed calyx that persists and becomes membranous in fruit. The corolla consists of five white or pale pink petals fused together into a short basal tube. The flowers are pollinated by insects and, if this fails, they self-pollinate (Fitter 1987). In a number of mud species, cleistogamy and self-fertilization is the norm, at least in submerged plants, so seed set is automatic and very large soil seed bank numbers have been reported in some instances.

After fertilisation a bluntly ellipsoid capsule, usually c 3.0 x 1.5 mm, the largest fruits rarely exceeding 4.0 × 3.0 mm, develop, each containing a surprisingly large number of very small seeds (Salisbury 1967).

Seed production

Salisbury (1942, 1967) examined a total of 178 plants over three seasons and discovered the mean number of capsules per plant was 31, and the mean number of seeds per capsule was 117, a high figure for such a small fruit. The mean seed productivity is therefore calculated to be around 3,600 per plant (Salisbury 1967).

It has to be emphasized that the size attained by individual L. aquatica plants varies enormously owing to the marked diversity in the density of the Mudwort sward on the one hand and, on the other, to the seasonal timing of the plants' development associated with the onset of drought conditions and mud exposure. If a sunny summer occurs after mud is exposed, seeds germinate rapidly and large plants may develop producing secondary fruiting rosettes. If the seedling sward is very dense, however, rosette formation will be reduced or even prevented, resulting in fewer fruits per plant and possibly smaller capsules containing fewer seed.

Having said this, clearly potential seed production of L. aquatica is remarkably large for a species with such a small photosynthetic assimilatory surface. A large seed output in relation to size appears characteristic of a number of species that specialise in the colonisation of bare mud. A strong parallel is afforded by Elatine hexandra (Six-stamened Waterwort), an even smaller plant which also produces thousands of seeds and which regularly co-habits with Mudwort. Environmental conditions for successful reproduction by these ephemeral species recur at infrequent intervals making them sporadic in their occurrence and natural selection has made them opportunistic, giving them rapid germination and colonising ability when conditions are favourable, enabling them to generate enormous populations on bare mud (Salisbury 1967).

Fully developed specimens of L. aquatica are quite easy to identify by the shape of the leaves and the small white flower, although often plants fail to develop fully.

Seed dispersal

Seeds are often released underwater when the capsules open and may thus be readily dispersed within the water catchment in flowing streams. While the seeds are very small and lightweight and could easily be carried by wind blowing across open capsules, wetland birds probably are significant vectors also, carrying seed of the plant in attached mud on their feet and feathers to fresh sites of a more remote nature (C.D. Preston, in: Stewart et al. 1994).

Variation

A second very rare species of the genus occurs in Britain only very locally in Wales, and is otherwise totally absent in Europe. Elsewhere it occurs in eastern N America, and is more widely distributed in the southern hemisphere and the N Pacific. It is L. australis R.Br. (syn. L. subulata E. Ives), Welsh Mudwort, which differs in having all its leaves needle-like (ie subulate), and its calyx is shorter than the corolla tube. A plant of wet sandy mud at the margins of pools where water has stood for some time, in Wales L. australis is very local in Glamorganshire (VC 41), Merionethshire (VC 48) and Caernarvonshire (VC 49). This usually annual species produces stolons or rhizomes both above and below the soil surface, and these organs can occasionally perennate (Hutchinson 1972; Sell & Murrell 2007).

Fermanagh occurrence

The mouth of the Swanlinbar (or Claddagh) River as it enters Upper Lough Erne and Ross Lough (including the mouth of the Sillees River) are the only two sites where L. aquatica has ever been seen in NI. When Meikle and co-workers discovered the plant in 1957 it was abundant and some was growing up to 10 cm tall on soft muddy ground under 5–30 cm of water along with Eleocharis acicularis (Needle Spike-rush). The L. aquatica plants varied greatly in size and luxuriance, some with floating leaves closely resembling Baldellia ranunculoides (Lesser Water-plantain), while others plants were minute (Carrothers et al. 1958).

From 1991 onwards this little, ephemeral, sporadically appearing annual has been seen at Ross Lough growing on mud exposed after periods of summer drought. In September 1991, it covered a strip of shoreline 100 m long × 5 m wide. In August 1994, it formed an elongated mat 60 m long × 3 m wide here, positioned several metres out from the normal shoreline. On these two occasions, the plants were so densely packed they resembled a grassy sward. Accompanying species on the exposed mud were Alisma plantago-aquatica (Water-plantain), Bidens cernua (Nodding Bur-marigold), Callitriche agg. (Water-starworts), Persicaria hydropiper (Wall-pepper), Rorippa amphibia (Great Yellow-cress) and R. palustris (Marsh Yellow-cress).

At the entrance of the Sillees River to Ross Lough in 1992, 1994 and 2002, the species grew very sparingly on exposed mud, but only as scattered individual plants. In September 2002, very scattered L. aquatica grew along an extremely muddy track trampled by cattle about a metre above the foreshore of Ross Lough, rather than on the shore itself where it had been recorded on earlier occasions. In October 2005 and 2006, it was very scattered along approximately 150 m of muddy shore, which had been trampled by cattle. The plant had to be searched for on the latter occasion. In July 2010, it was found growing in its usual place, just above the normal high water mark, scattered along 200 m of the muddy lakeshore.

Ross Lough is next to the limestone hills at Boho and the local habitat for the plants here has similarities with turlough sites, ie vanishing lakes sited above limestone cave systems in Cos Clare and Galway, where drainage is essentially vertical and where the habitat is grazed by cattle during the summer months (Curtis et al. 1985). At Fisherstreet, a coastal area of the Burren, Co Clare (H9), the plant has rarely and very exceptionally been found growing in small, extremely shallow solution cups etched into the horizontal surface of clints, the flat surface slabs of limestone pavement (Curtis et al. 1987). This could hardly be a more ephemeral habitat for a semi-aquatic plant.

British and Irish occurrence

Most of the 50 or so 'mud-species' that occur in Britain have greatly diminished in their frequency, the average loss in the period around the late 1960s being probably about 40% of their former locations (Salisbury 1970). Since then, the decline has continued: an analysis of the total number of hectads in B & I with records in the period 1987-99 displayed in the New Atlas, showed just 80 squares with L. aquatica present. A further 20 hectad squares had records dating from 1970-1986 and there were pre-1970 records known from another 136 squares, making the rate of decline on both islands very obvious.

In all or almost all its stations in B & I, L. aquatica is sporadic and very variable in abundance from year to year. As has just been discussed, it has been recorded from more than one hundred stations scattered over England, in a considerable proportion of which it no longer occurs, and it is also known from only a very small number of sites in Scotland and Ireland (Perring & Walters 1962; Preston et al. 2002). The species is very strikingly intermittent in its occurrence, sometimes reappearing after many years of apparent absence (missing for up to eight decades), yet the number of individuals in a locality when it reappears may be astonishingly large (Salisbury 1967; C.D. Preston, in: Stewart et al. 1994). Mudwort is thus scarce or rare, extremely local and thinly scattered across Britain from Hampshire northwards to NE Aberdeenshire.

There has been a remarkable increase in records of this species in Scotland since the 1950s (Leach et al. 1984), probably due to better recording of an easily overlooked, inconspicuous plant, but perhaps indicating some degree of northward spread. Losses in England and Wales appear to be associated with pools and a range of smaller water bodies that previously represented the most characteristic habitat of the plant, while by comparison the species has survived better around larger waters (C.D. Preston, in: Stewart et al. 1994).

In Ireland, sites again are very rare, widely scattered and confined to a few sites in Co Clare (H9), Co Cork (H3 & H4) along the River Lee and its reservoirs, Co Fermanagh (H33) and SE Co Galway (H15) (Curtis & Ryan 1985; O'Mahony 1986; Curtis & McGough 1988).

The fact that Mudwort has become such a very rare and rather widely disjunct species in Ireland and a scarce, erratic and declining local one in Britain, suggests that losses of suitable, very specific muddy habitats may have reached a critical threshold. This is an especially serious situation for a sporadic species which only erratically produces a very large, but perhaps a not always very long-lived soil seed bank.

European and world occurrence

L. aquatica is widespread in most of Europe except the Mediterranean region, although it is present in N Italy and Cyprus and has scattered colonies in the Alps. It is often rare and local. It is also present in Egypt and in arctic and temperate Asia eastwards to India and Japan. The species has a circumpolar boreo-temperate distribution that includes Greenland and N America from Labrador to the North West Territory and south in the mountains to Colorado and California (Meikle 1985; Hultén & Fries 1986, Map 1626; Sell & Murrell 2007).

Names

The genus name 'Limosella' is a diminutive, derived from 'limosus', meaning 'muddy'. The Latin specific epithet 'aquatica', translates as 'aquatic'.

The English common name 'Mudwort' or 'Water Mudwort' delimits the habitat concisely.

Threats

On account of its rarity in NI and the vulnerability of its habitat, L. aquatica has been listed under the Wildlife (NI) Order, 1985 as one of 56 plant species given special conservation protection in Schedule 8, part 1.

• Find out more about the format of the species accounts
• View abbreviations
• View references for all species accounts
• View a gazetteer of all the sites