Ceratophyllum demersum L., Rigid Hornwort

Account Summary

Fermanagh Occurence

Growth form and history of discovery in Fermanagh

This submerged, floating, truly aquatic species typical of eutrophic, still or slow-flowing water, was found in quantity on 6 October 2010 by Robert Northridge at a jetty near the National Trust's Crom Castle estate Visitor Centre on the shore of Upper Lough Erne. The Rigid Hornwort was floating in a tangled mat of aquatic species including Elodea nuttallii (Nuttall's Waterweed). A specimen of the hornwort was collected and sent to DBN for confirmation. Dr Matthew Jebb confirmed the identification and we were confident that this was a new County Record.

On checking this supposition with the CEDaR Database for N Ireland, Robert Northridge and the current author were very surprised to discover a previous Fermanagh record had been listed. The record details were given as: 8/9 August 1968 at Mill Lough near Bellanaleck, by staff of the Department of Agriculture & Rural Development NI, Fisheries Division. We presume the record was made during survey work on water quality or fish stocks. In checking this Mill Lough record, we found that no voucher exists, nor any note of the recorder's name (or names). CEDaR supplied three other plant records for the same date and lake: Littorella uniflora (Shoreweed), Phragmites australis (Common Reed) and Schoenoplectus lacustris (Common Club-rush). All three are common widespread species in Fermanagh and, together with five other common wetland species on a total list of 18 records across several lakes, they give us no real indication of the recorder's identification skill.

This particular Mill Lough (there are three others in Fermanagh) has been visited by RHN on three occasions (1979, 1985 and 2000) and also by the NI Lakes Survey 1988-90, without occurrence of C. demersum. RHN revisited Mill Lough on 13 October 2010 and made a thorough search of floating plants at seven jetties around the lake. C. demersum was not found, but masses of Water Crowfoot were present, either Ranunculus circinatus (Fan-leaved Water-crowfoot) or R. trichophyllus (Thread-leaved Water-crowfoot), with very short submerged leaves that might easily be mistaken for Rigid Hornwort by inexperienced botanists or non-botanical field workers. In view of all this, we have discounted the 1968 Fisheries record entirely and have asked for it to be removed from the CEDaR Database.

On mentioning the above error to CEDaR staff, Robert and I were disconcerted to be then given additional lake data for Fermanagh collected at two sites for NIEA by the English consultant company, ENSIS in 2006 and 2007, and from another four sites on Upper Lough Erne discovered by the NIEA's own Lake Ecology Team in August 2010. Since the timing of these October 2010 revelations coincided with evidence given to the current authors regarding the recent arrival of Elodea nuttallii in Upper Lough Erne, RHN carried out a speedy late season survey of Upper Lough Erne shores (3-5 November 2010) to search for these two species. This led to the discovery of C. demersum at a further eight sites in the southern part of Upper Lough Erne. Thus Rigid Hornwort is now known to have first appeared in the VC in 2006, and during the last four years it has spread to a total of 11 Fermanagh tetrads as shown in the map.

Irish occurrence

Although occasionally abundant in lakes, pools and canals, C. demersum remains a relatively rare species in Ireland. The 1987 Cen Cat Ir Fl 2 listed 17 VCs where it had been recorded at least once. Inspection of the New Atlas hectad map using a transparent overlay of the 40 Irish VCs (not as accurate or as straight forward an operation as it sounds) increases the number of VCs where it has occurred up to the year 2000 to 23. The VCs featured in these two sources do not completely agree, however, so while a wider distribution is indicated, we cannot conclude that the species has spread to six additional VCs. The major change in C. demersum distribution in NI has been its arrival in Lough Neagh in 1988 and its subsequent spread around the lake basin. Nevertheless, this was preceded in Co Tyrone (H36) by a find in 1982 at Ballagh Lough, 5.5 km NE of Fivemiletown (McNeill 2010). This site is only 4 km from the boundary of Fermanagh.

Although it flowers regularly and freely in B & I, Rigid Hornwort very rarely sets seed, reproducing almost entirely by vegetative fragmentation. While it is considered native in some parts of Ireland, the species is commonly used to oxygenate garden pools and indoor aquaria. We therefore suspect that the Crom colony at least is an introduction originating from recently discarded cultivated material. Crom jetty is a heavily visited public amenity.

British occurrence

C. demersum is widespread in lowland aquatic habitats across the length and breadth of Great Britain. However, it is very unevenly spread, being much more frequently recorded in the English Midlands and the South East of the country (Preston et al. 2002). This pattern of distribution, taken together with the fact that fruiting in Britain and Ireland is rare at present, suggests that temperature may be limiting both the occurrence of the species and its sexual reproduction.

Rigid Hornwort is not confined to lakes, but occupies a wide variety of still and slow-moving eutrophic habitats including ponds, rivers, canals and ditches. In smaller water bodies such as ponds and ditches, it often forms large, dense mats of growth that can rise above the water surface. Comparison of the New Atlas hectad map with the 1962 BSBI Atlas shows it is now more frequently recorded, but this is probably a reflection of better co-ordinated plant recording, rather than a real increase in presence. In any event, at least the distribution in Britain appears stable, if not definitely increasing (C.D. Preston, in Preston et al. 2002).

World occurrence

Rigid Hornwort is an extremely widespread circumpolar species, not only in Eurasia and N America, but also in C and S America, C and S Africa, S India and S Australia (Hultén & Fries 1986, Map 817).

Fossil record

The long-spined fruits of C. demersum are very easily recognised fossils and the sedimentary record shows that this species has persisted in Britain as far back as the Cromer Forest Bed series and through all the subsequent interglacial periods including the present Flandrian. The record shows that it was also present in the less severe terminal parts of the glacial stages. Godwin (1975) quotes Samuelsson (1934) regarding the post-glacial immigration of C. demersum back into northern Finland in the early Post-glacial period as glaciers retreated, where fossil records show the species established a considerable distribution. However, the record in Finland also demonstrated a subsequent contraction and readjustment of the species distribution, possibly due either to changes in water chemistry or to subsequent climate deterioration.

A study of lake deposits on the Scottish isle of Skye by Birks (1969) suggests that C. demersum suffered a restriction of range southwards since the Late Weichselian glacial period, a notion which supports the case for a present day temperature limitation on the species.

Names

The genus name 'Ceratophyllum' is from the Greek 'keras', meaning a horn, and 'phyllon', a leaf, the many divisions of the leaves suggesting the shape of horns. The Latin specific epithet 'demersum' means 'down under the water' or 'submerged'. The English common name 'Rigid Hornwort', is a so-called book name and the plant does not appear to be associated with any folk traditions. Despite having 'wort' as part of the name, the species does not have any use in herbal medicine that the present author can detect.

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