Ceratophyllum submersum L., Soft Hornwort

Account Summary

Growth form and history of possible Fermanagh discovery

Although this free floating or lightly anchored aquatic has a similar ecology and reproductive capacity to C. demersum (Rigid Hornwort), in Ireland it is extremely rare, having been mapped in the New Atlas in a total of just three hectads in two widely separated regions near the east coast in Cos Wexford and Down (H12 and H38). A water quality survey carried out by the Water Management Unit (WMU) of the Northern Ireland Environment Agency (NIEA) at 15 sites around Upper Lough Erne in August 2010 produced two unexpected records that were communicated to the current authors in late October, 2010. The two sites were the one listed above and another at Knockninny Quay. Plant material was located at depths of 0.7-1.6 m at Cornaleck, and a small quantity only was found at a depth of 1.6 m at Knockninny Quay.

C. submersum was first recorded in Ireland in August 1989 when the NI Lakes Survey discovered it in two lakes on the Lecale Peninsula near Downpatrick (Smith & Wolfe-Murphy 1991). To date, these are the only known sites for Soft Hornwort in Northern Ireland. We therefore made enquiries to ascertain if vouchers had been collected at the Upper Lough Erne sites, and whether or not the identification of the species material had been checked by a known botanical authority. Plant material had indeed been collected and was identified in the WMU laboratory in Lisburn. Unfortunately, specimen samples were not retained. The determination of C. submersum from C. demersum was based entirely on the dividing of the leaves, "into threes, rather than dichotomously" (B. Walker, pers. comm. 18 February 2011).

The significant leaf distinction between these two Ceratophyllum species refers to the number of times the leaf segments divide dichotomously: once or twice forked for C. demersum, three or four times forked for C. submersum. The leaf segments are obviously toothed in C. demersum and much less so in C. submersum, and the leaves are more rigid and darker in colour in C. demersum in comparison with C. submersum (Stace 1997).

Without vouchers these two C. submersum records are very doubtful and cannot stand as First and Second County Records. We believe the plants were incorrectly identified. However, the possibility remains that Soft Hornwort just might be present in eutrophic, base-rich Fermanagh waters growing along with C. demersum and it should certainly be looked for. The point has been well made by the recently retired Curator of the BEL herbarium, that, "it is a regrettable tendency these days for agencies and consultants not to collect or retain vouchers. It undermines all the time and expense spent in doing surveys." (P. Hackney, pers. comm. 23 February 2011).

C. submersum is reputed to flower more freely in Britain and Ireland than C. demersum, and although vegetative reproduction by fragmentation is certainly the norm in both species, the small, hard, nut-like fruits may well be dispersed by waterfowl, thus enabling jump dispersal of the species (Preston & Croft 1997). On account of wider geographical distribution and biodiversity aims, C. submersum is included on the NIEA list of Priority Species of special concern requiring local conservation action.

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 'submersum' means 'submerged'. The English common name 'Soft 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 plant is rare and does not have any use in herbal medicine that the present author can detect.

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