Ranunculus penicillatus (Dumort.) Bab., Stream Water-crowfoot
Account Summary
Native, very rare. European temperate.
1892; Praeger, R.Ll.; Ardy More, Ballycassidy River.
May to July.
Identification difficulties and under-recording of subgenus Batrachium
Although there has been a huge amount of survey work on the aquatic flora of Fermanagh since 1986, almost all of it has concentrated on the lakes and comparatively little focus and attention has been given to rivers and streams in the county. The resultant plant records could give the impression that field workers appear to have avoided Ranunculus subgenus Batrachium. If this really is the case and the group is under-recorded, it will largely be on account of the difficult taxonomic nature of the subgenus, the range of variation and the confusion engendered by frequently changing names within the complex. Another contributory factor is the fact that the Irish field Flora in use for much of the period 1975-97 (ie An Irish Flora 1977) did not handle this plant group at all well. For example, it failed to mention R. penicillatus and completely ignored the detailed monographic treatment of the subgenus carried out by Cook (1966a).
Growth form and habitat preferences
R. penicillatus s.l. is by far the most common and one of the most robust perennial species of water-crowfoot in the flora of Britain and Ireland. It frequently forms large, conspicuous rafts of dense branching stems rooted in the bed of moderately flowing rivers and streams, but it can also occur in waterways that are liable to occasional, torrential spates (Haslam 1978, p. 31; Holmes 1980; Webster 1988). The waters that subsp. penicillatus frequents range from oligotrophic to mesotrophic nutrient status (ie they are capable of supporting poor to medium levels of plant and animal growth).
Under favourable water conditions with stable bottom substrates of well-cemented rounded gravel and silt, R. penicillatus subsp. penicillatus can become the dominant aquatic vascular plant, its dense floating mats representing a significant structural feature of the water channels it occupies. It has been observed covering around 70% of the stream bed and modifying water flow, promoting silt deposition, and providing both shelter for other plants and animals, and food for invertebrate animals and fish (Cook 1966a; Haslam 1978, p. 40; Ham et al. 1982; Preston et al. 2001).
In an Irish study of water quality indicator species, Caffrey (1985) placed all of the Batrachian Ranunculi in his group of macrophytes that are most sensitive to increased nutrient forms of pollution, ie especially high levels of nitrogen and phosphates. We believe that all or most of the previously luxuriant rafts of R. penicillatus in the Colebrooke River have disappeared in recent times, almost certainly due to increased nutrient levels in the water originating from run-off of agricultural chemical fertilisers, slurry and manure applied to adjacent field swards.
Heterophylly and identification features
Throughout the year, plants of R. pencillatus bear dissected, thread-like submerged leaves and, during the summer months, in response to longer day-length, they develop additional floating leaves that are 3-5 lobed and either distinctly or bluntly toothed. Thus the plants become heterophyllous, ie bearing leaves of two quite different types. The teeth on the laminar floating leaves may end in long slender filaments, which actually are the leaf veins extending beyond the margin. The submerged leaves of R. penicillatus subsp. penicillatus are always limp or flaccid, and when removed from the water they collapse to form a very slender tassel. When fully developed these leaves are generally longer than the adjacent stem internode (Webster 1991; Webb et al. 1996). This feature, used with care, helps separate the species from R. peltatus (Webster 1991).
Fermanagh occurrence
Whatever is or was the situation regarding the difficulty of identifying Batrachian Ranunculi, relatively few records have been made of the water-crowfoot group as a whole in the Fermanagh survey (Cook 1966a; Webster 1991). There are a total of just eleven records for R. penicillatus in the Fermanagh Flora Database, the species being present in eight post-1975 tetrads in the VC. Ten of the records need checking since only one voucher specimen has been made by a field worker (R.M. Barrington). A few finds were originally determined as R. peltatus subsp. pseudofluitans using An Irish Flora (1977), but we can reassign them to R. penicillatus since they carried with them information that floating leaves were absent (which could be a seasonal effect), and that the submerged leaves were longer than the internodes.
Webster (1991) looked at a wide range of subgenus Batrachium material throughout Ireland and, with the exception of one site on the River Roe in Co Londonderry (H40), she recognised all the R. penicillatus found in Ireland as belonging to subsp. penicillatus. Our Fermanagh records undoubtedly are of this subspecies and therefore we have transferred all our R. penicillatus records to this taxon. The Fermanagh tetrad distribution map shows the subspecies well represented along the Colebrooke River and scattered in several other rivers feeding into Lough Erne, most notably the Swanlinbar River, also referred to as the Claddagh River discharging into Upper Lough Erne, and the Ballycassidy River entering Lower Lough Erne.
Flowering
The white or very pale pink flowers are frequently produced in large numbers and R. penicillatus is arguably the most conspicuously beautiful of all the Batrachian Ranunculi. Although the flowers of subsp. penicillatus are usually slightly larger than those of R. peltatus, like many other characters in this subgenus, the flower size varies considerably with the environmental conditions, and in this case they do not allow reliable distinction between the two species (Cook 1966a).
The flowers are self-compatible and there is a tendency towards pollination in the bud (ie cleistogamy), so that seed is regularly set and can be independent of external pollinating agents (Cook 1966a). Flowering material of subsp. penicillatus from Ireland and Wales is reported to be very fertile, but it is usually far less so in the rivers of the English Lake District and SW England (Holmes 1980).
Anchorage in swift-flowing waters
Like other Batrachian Ranunculi species living in more or less rapidly flowing water, R. penicillatus anchors itself firmly in stream bed shingle or lightly silted gravel by means of profuse clusters of fine branching roots produced at the lower stem nodes (Sculthorpe 1967, p. 155). Unlike R. fluitans (River Water-crowfoot), which also frequents moderate- to fast-flowing waters (although this latter species is only very rarely recorded in Ireland), R. penicillatus does not form a prostrate compact overwintering stem firmly rooted to the bottom substrate. Despite lacking this specialised structure, R. penicillatus is capable of developing fresh roots from numerous lower stem internodes throughout the year, and thus anchors and maintains itself very satisfactorily, even on somewhat unstable calcareous substrata (Cook 1966a).
Finding a 'safe site' and surviving long enough to become established in a suitable habitat is generally the most hazardous phase in the life history of any plant species, terrestrial or otherwise. It is almost impossible to imagine just how minute the odds of achieving successful anchorage must be for a plant propagule in the bed of a moderate or swift-flowing stream. It is similar to the stretch in imagination required to comprehend the improbability of long-distance jump dispersal to remote oceanic islands (Carlquist 1974). Nevertheless, in both instances, somehow the extremely unlikely event does happen! In the case of aquatic macrophytes, downstream dispersal is never a problem since the water flow supplies the transporting vector. Rather the difficulty occurs in the plant propagule settling out from rapid or even moderately swift flowing water, making the initial anchorage, and rooting securely without being dislodged again by the current.
Reproductive strategy
In contrast to the fruits and seeds of emergent waterside plants, the propagules of submerged and floating-leaved flowering plants generally have no great powers of flotation. They either sink immediately after release from the parent plant, or after a few hours, or a day (Sculthorpe 1967). Clearly there must be some method of establishment, since aquatic Ranunculus species do occur and survive in relatively fast-flowing streams. Do propagules find sheltered sites in eddies behind boulders, or do they temporarily lodge in the slower flow amongst established plants or other debris lodged in the riverbed? Or do they grow out from the relative shelter of the riverbank?
Perhaps for aquatic species found in faster flowing waters, the major or maybe the only role for seed has to do with long-range or jump-dispersal from one water system to another. Most probably this involves animal vectors and, in particular, water birds (Sculthorpe 1967, pp. 331-2; Cook 1988). In swift or even moderately flowing waters, colonisation and establishment within the same (ie original) river system or water body by vegetative means involving fragmentation of an existing clone, appears very much more likely, straightforward and achievable, than from seed. The stems of R. penicillatus break at quite low forces (Haslam 1978, p. 49) and it is easy to imagine detached rafts snagging downstream on rocks, firmly 'planted' sticks or detached branches of trees, thus re-establishing the macrophyte in a fresh site. According to Haslam (1978, p. 163), such trailing fragments must be held sufficiently close to the stream bed for a month or two in order for them to root and become anchored.
In comparison with this vegetative process, it is impossible to conceive of a suitable site for seed germination occurring in a moderate or swift flow of water. Indeed, except in almost perfectly still water, it is difficult to imagine how a Ranunculus or any other seedling might produce sufficient roots to achieve anchorage.
Clonal longevity
Well established aquatic clones probably survive for many years, but they undergo vegetative cycling, the clump increasing and decreasing on an annual basis with the seasons, a process referred to by ecologists as 'wash out' (Haslam 1978; Ham et al. 1982).
Really basic research is still required on the reproductive biology and ecology of aquatic Ranunculus species in order to definitively answer the sort of questions posed here. Let us hope that airing the topics here, will help stimulate the necessary work.
British and Irish occurrence
Subspecies penicillatus is rather local and has a decidedly western distribution in Britain and Ireland. It generally occurs in Britain in rivers over base-poor, acidic igneous rocks, but in Ireland it tolerates a much wider range of alkalinity, both base-poor, pH 6.1, and base-rich, up to pH 8.5 (Webster 1988). In Britain, on the other hand, in base-rich waters subsp. penicillatus is replaced by subsp. pseudofluitans and the latter is much more widespread and abundant than the former (Webster 1991; Preston et al. 2002).
European occurrence
The distribution of R. penicillatus in Europe is even less well known than is the case in Britain and Ireland for exactly the same taxonomic and identification reasons. However, it appears to be quite widely scattered in middle European latitudes, the distribution thinning out considerably to both north and south. For instance, apart from two provinces in Denmark, it is entirely absent from the Nordic countries (Cook 1966a, Fig. 29; Jalas & Suominen 1989, Map 1892; Jonsell et al. 2001).
Names
The Latin specific epithet 'penicillatus' means, 'furnished with a tuft of hairs, like a paintbrush' (Gilbert-Carter 1964), presumably a reference to the tassel-like submerged leaves.
Population control measures
Where aquatic macrophytes including Batrachian Ranunculi become too productive and begin to form extensive mats obstructing river and stream drainage, the best method of achieving control is not to physically cut the vegetation, but to shade the banks by planting trees. If cutting does become essential, then where aquatic Ranunculus species are dominant, the most successful modification of subsequent growth is achieved when the mats are cut in full flower. This usually means cutting and dredging in June, but the timing of flowering is dependent on both site altitude and latitude (Dawson 1980; Ham et al. 1982).
Threats
Pollution of rivers and streams and over-zealous drain clearance operations.