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Apium nodiflorum × A. inundatum (A.
× moorei (Syme) Druce), Hybrid Fool's-water-cress

Account Summary

Native, very rare but probably regularly over-looked.

1857; Moore, D.; Lower Lough Erne.

July.

As explained in the A. inundatum (Lesser Marshwort) species account (below), the parent species of this perennial, sterile hybrid overlap ecologically in Ireland in an almost unique manner. As a result, this is a widespread but nevertheless still rather thinly recorded hybrid across the whole of Ireland, but as the 2002 New Atlas and 2015 Hybrid Flora hectad maps indicate, it is extremely rarely found in Britain. The hybrid is also totally unknown elsewhere in Europe. It is unfortunate that in Ireland, the hybrid is probably often overlooked and the current author (RSF) and RHN assume that we are among those guilty of under-recording it in Fermanagh.

A detailed account of this hybrid and its differentiation from the parent species is provided by O'Mahony in Stace et al. (2015) and will not be repeated here. The hybrid is recorded from places where the parent species overlap ecologically, typically in shallow water in swamps and at the edges of lakes, rivers, steams, canals and ditches and on damp ground near water. Well developed hybrid plants have been observed forming tangled, floating, felted mats with slightly submerged stems bearing aerial leaves and flowers (Praeger 1951). The F1 hybrid is absolutely sterile and reproduces and spreads entirely by vegetative growth and subdivision (T. O'Mahony, in: Stace et al. 2015).

Fermanagh occurrence

There are only four records of this hybrid in the Fermanagh Flora Database, all of which are confined to the shores of Lower Lough Erne. This very large body of water has a very much more open, rocky calcareous shoreline than the dissected Upper Lough Erne, which is shallower and is littered with a multitude of islands, both small and large. Both these large Fermanagh lakes are somewhat polluted with agricultural chemicals and sewage, leading to rather strong and steadily increasing eutrophication. The details of the remaining three local records are: Carrickreagh Bay, Lower Lough Erne, 1900 & 1934, R.Ll. Praeger; and Brockagh Point, Boa Island, Lower Lough Erne, 25 July 1976, Miss N. Dawson.

It is clear that only a small number of field botanists are able to distinguish this hybrid and RHN and the current author (RSF) have no personal finds of it ourselves as yet. Despite the three records above being confined to the Lower Lough, the most likely place to find the hybrid today would appear to be at the southern end of Upper Lough Erne, where both parent species are fairly common.

Apium inundatum (L.) Reichb. f., Lesser Marshwort

Native, locally frequent. Suboceanic temperate, including in NW Africa.

1806; Scott, Prof R.; Co Fermanagh.

May to November.

Growth form

This small, glabrous, heterophyllous, perennial umbellifer really is amphibious; it can survive completely submerged as a true aquatic with typical flaccid, hollow, branched stems, 100 cm or longer, bearing finely divided submerged leaves, plus broader, pinnate or trifoliate floating leaves. The creeping basal part of the stem, which is usually submerged, sometimes roots at the nodes touching the substratum (Jonsell & Karlsson 2010). However, should the shallow water bathing the plant significantly lower or temporarily disappear in the summer months, the plants of A. inundatum can assume a small-scale or even tiny terrestrial or semi-terrestrial mode, modifying their form and producing a crop of somewhat broader, singly-pinnate, flattened aerial leaves with up to seven sessile lobes c 5 mm long (Tutin 1980).

Identification from associated species and preferred habitats

In spring, the finely divided submerged leaves of Sium latifolium (Greater Water-parsnip), although differing in colour, are very similar in form to those of A. inundatum. In Fermanagh, both of these species are very frequent and they often coincide in sites around the sheltered, dissected, muddy lowland shores and nearby ditches of Upper Lough Erne in particular.

In Britain, A. inundatum is regarded as a species associated with moderately acid to weakly basic, nutrient-poor, moderately oligotrophic soft water conditions whereas, in comparison, Sium latifolium always occupies decidedly enriched, moderately to strongly eutrophic situations (Hill et al. 1999; M. Southam, in: Preston et al. 2002). The two species are thus ecologically well separated. However, in Fermanagh, and indeed throughout much of its widespread Irish occurrence, A. inundatum tolerates much more nutrient-enriched wetland habitats than it does elsewhere in its rather restricted western European range. This allows it to overlap ecologically not only with S. latifolium, but also with its near relative A. nodiflorum (Fool's-water-cress). Both Apium species flower freely and the outcome of this unusual juxtaposition is the sterile Apium hybrid, A. × moorei, which is widespread in Ireland, of very restricted occurrence in Britain and completely unknown in continental Europe (T.G. Tutin, in: Stace 1975, pp. 268-9; Preston & Croft 1997).

Part of the reason why these normally ecologically separate species are so very successful at co-existing in Upper Lough Erne in particular, may be the relatively high levels of calcium in the inflow waters of the lake. Perhaps this creates a base-rich but nutrient-poor environment, or an unbalanced supply of nutrients as far as plant growth is concerned (Gibson et al. 1980). Without being a limnological expert, the current author (RSF) suggests this potential explanation might be worth investigating. Another possibility, however, might lie in the restricted range of competitor species that exists in Ireland in comparison with British waters, and even more so when compared with Continental Europe.

Fermanagh occurrence

A. inundatum has been recorded in 64 Fermanagh tetrads (12.1%), 59 of which have post-1975 dates, so there is no pressing evidence of a decline locally.

Flowering reproduction

In B & I, A. inundatum flowers and fruits freely in both shallow water and in bare, recently exposed terrestrial ground around lakes, ponds, rivers, streams and ditches. It is believed that reproduction in this species is largely by seed (Preston & Croft 1997), rather than by vegetative fragmentation and re-establishment.

When flowering occurs, from June to September, umbels are reduced, very small and few flowered. The compound umbel is short-stalked or almost sessile, attached opposite the leaf supporting the leading lateral shoot. There are few umbellules (1-4), each bearing 3-7 very small, white flowers, stamens with purplish anthers, and 4-6 conspicuous, persistent bracteoles (Jonsell & Karlsson 2010). The fruits, 2.5-3.0 mm long, are elliptic to oblong in outline and laterally compressed with five thick ridges on each mericarp and a vitta in each groove (Tutin 1980; Stace 2019).

While there is some circumstantial evidence from disturbed sites which suggests seed may be long persistent in the soil seed bank, the survey of the literature on this topic in NW Europe by Thompson et al. (1997) uncovered no data.

British and Irish occurrence

The New Atlas map indicates A. inundatum is widespread and locally frequent in NI, and likewise in the RoI along the very long River Shannon, and in lakes in both Connemara and Co Cork. It is much more scattered and scarce elsewhere on the island.

The distribution remains widespread in Britain, showing a definite western bias and becoming more coastal and island-based further north and in Scotland. The New Atlas hectad map shows the species has declined considerably in Britain since the 1962 BSBI Atlas. This is particularly obvious in eastern and southern England and is said to reflect drainage and eutrophication associated with changes in land use including building and other development, including agricultural intensification (M. Southam, in: Preston et al. 2002).

European and world occurrence

A. inundatum has a restricted distribution in W Europe stretching from the Iberian peninsula eastwards to Sicily, northwards to SE Sweden and southwards to N Africa (Hultén & Fries 1986, Map 1411). A similar pattern of decline mirroring the situation in Britain appears to be happening in continental Europe, where the species is listed as 'vulnerable' in the Netherlands. It also appears to be declining in Germany (Preston & Croft 1997).

Threats

None really, but potentially drainage and excessive dredging of ditches could affect regeneration.