Swollen Spire Snail

From Rio to Sussex



Species Action Plan for Sussex

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Factors affecting species
National SAP
Current Action
Targets & costs
Action Table
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Swollen Spire Snail

Mercuria confusa

1. Introduction/Current Status


The swollen spire snail Mercuria confusa (until recently known as Pseudamnicola confusa), is a rare and declining species in Britain and where information is available, appears to be equally threatened in Northern Europe (Bratton, J. H. (editor), 1991). In Britain the species is categorised in the Red Data Book (Bratton, J. H. (editor) 1991) as endangered (RDB 1). The species is also placed by the UK Biodiversity Steering Group (UK Biodiversity Group, 1995) as a Long List species (now redefined as a 'species of conservation concern'). There is some evidence for population decline of Mercuria confusa elsewhere in Northern Europe. Thus in the Netherlands there is evidence (Gittenberger, E. et al., 1999) of a 75% decline between 1970 -1997.


This is a mainly western Mediterranean species, which has scattered outposts on the Atlantic coast of Europe extending northwards to the Netherlands and the U.K. There is some uncertainty as to whether, due to differing habitat requirements, the species found in southern France and those from England and Holland are actually conspecific (Dieter Kodolsky; pers. comm.). In the U.K. M. confusa has a very restricted distribution (Kerney, M. P., 1999) being known from only four sites; small populations may live on the River Alde in Suffolk and in the Thames Estuary at Barking Creek. The two strongholds are on the Suffolk/Norfolk borders near Oulton Broad and including the lower reaches of the Rivers Waveney and Yare and in West Sussex on the lower River Arun. There are also a number of populations in Ireland (Kerney, M. P. 1999, Janus, H. 1982) on the estuaries of the Shannon, Suir, Barrow and Nore. In Sussex the species is found adjacent to the lower River Arun (Abraham, F. & Willing, M. 1996; Abraham, F. & Willing, M. 1997; Willing, M.J. 1999; Abraham, F. et al. 1998) with populations shown to be present in river-side drainage ditches and marginal emergent vegetation from near Coldwaltham in the north, to pools lying close to the river at Lyminster downstream of Arundel, in the south. Specific locations known to support the snail are present at Lyminster, immediately upstream of Arundel, Burpham, South Stoke, Houghton Bridge and near Coldwaltham. The Sussex Arun distribution spans three English Nature designated Natural Areas these being (1) the South Coast Plain and Hampshire Lowlands, (2) the South Downs and (3) the Wealden Greensand. It is also possible that the species extends into (4) the Low Weald and Pevensey.

It has been stated (Bratton, J. H. (editor), 1991) that the largest surviving populations of M. confusa are probably those in Suffolk. Comparisons of data from studies (Abraham, F. & Willing, M. 1996; Abraham, F. & Willing, M. 1997; Willing, M.J. 1999; Abraham, F. et al. 1998; Baker, R., Clarke, K. & Howlett, D. 1999) in both of these areas tends, however, to indicate that the Arun populations are probably both the most extensive and in certain areas, the richest in the UK. The greatest recorded abundances for the snail in Suffolk (Baker, R., Clarke, K. & Howlett, D., 1999) reached 500 per m2 whereas in habitat adjacent to the Arun at Burpham densities of 15, 000 per m2 have been recorded (Abraham, F. & Willing, M., 1997).

Habitat Requirements

M. confusa has very specialised habitat requirements. It is typically found on bare mud exposed at low tide beneath emergent vegetation such as Phragmites australis or Glyceria maxima. It is sometimes described (Ellis, A.E., 1926) as a brackish water species, but it is not found on salt marshes with such typical estuarine snails as Hydrobia ulvae. Instead it requires water with a very low salinity (1 - 5ppt NaCl) and is typically found in association with freshwater molluscs such as Lymnaea palustris and L. truncatula and wetland species including Zonitoides nitidus and Carychium minimum. Some authorities (Baker et al., 1999; Kerney, M.P. in Bratton, 1991) believe that it is more accurate to consider M. confusa as a freshwater snail that requires periodic or occasional contact with very slightly saline water. On both the Arun and Broadland sites M. confusa appears to be most frequent in association with Glyceria maxima, often when lightly grazed by cattle. At Burpham the very high population densities of the snail are found in less shaded, cattle grazed area of Glyceria maxima where livestock poaching has created a mosaic of partially trampled vegetation and small pools.


2. Current Factors Causing Loss or Decline [top]

Kerney (Bratton, J. H. (editor) 1991) considers that all UK populations of M. confusa are at risk. Two factors in particular seem to pose a threat to the species; salinity changes and disruption to bankside management and vegetation. The main threats include:

· Any factors such as barrages, sluices and water flow management schemes that may alter or disrupt the narrow salinity range or frequency of exposure to brackish waters.

· Global warming causing higher sea levels and the greater chances of saline intrusion could disrupt salt levels causing the loss of some marginal Arun populations. It could be argued, however, that such effects could also lead to the creation of new areas of suitable habitat upstream, possibly counter-balancing the loss of some populations.

· Wash from boat traffic scours the muddy banks of the river removing sediment and dislodging Glyceria beds. There is circumstantial evidence that very low numbers of M. confusa on certain stretches of the Arun near to Arundel may be due to the incidence of frequent pleasure craft traffic on the river.

· Maintenance and reinforcement work to banks and berms. This may be particularly damaging where fringing vegetation is removed exposing otherwise sheltered areas of bank only covered at high tide. The removal of sediments from the river in order to raise bank heights is also likely to be damaging. Fencing bankside areas so that cattle grazing ceases to contain the spread of Phragmites australis, at the expense of Glyceria maxima, may also reduce populations of M. confusa.

· Chance incidents of water pollution also pose a risk.


Standing Fresh Water HAP

3. National Species Action Plan[top]

There is no National Action Plan for this species.

4. Current Action


There is currently no known national conservation project being undertaken for this species. The Ted Ellis Trust at Wheatfen, Surlingham, is continuing monitoring populations of M. confusa in the Wheatfen and Surlingham areas of Suffolk.


Mercuria (Pseudamnicola) confusa is mentioned on the SSSI citation and criteria for Amberley Wildbrooks, but not on those for Pulborough Brooks SSSI and Waltham Brooks SSSI. The snail is also mentioned in the Ramsar citation for the area covering these three SSSIs. Further south M.(Pseudamnicola) confusa appears on the Arundel Park SSSI citation, but not on that for Arun Banks SSSI.

5. Objectives[top]

The overall aim of this Plan is to protect and increase the distribution and population of Mercuria confusa in Sussex. This broad aim translates the specific objectives set out below.

i. Maintain the existing populations and range of Mercuria confusa in Sussex.

ii. Promote education, communication and awareness of the status and ecological requirements of Mercuria confusa in the lower Arun valley.

iii. Seek to ensure all necessary research and monitoring of the species is carried out and the results of such study disseminated to appropriate organisations and land managers.


Standing Fresh Water HAP

6. Targets and Costs[top]

1. Undertake further survey work in order to develop a complete picture of the range and population densities of Mercuria confusa on the Arun by 2005.

2. Undertake ecological work at selected sites in order to ascertain in greater detail the habitat requirements and ecology of the snail by 2005.

3. Undertake surveys of appropriate stretches of the rivers Adur and Ouse to determine if further populations of the Mercuria confusa are present by 2005.

4. Establish a monitoring protocol and a suitable monitoring programme by 2002, and carry out monitoring on a biannual basis (ongoing, review every five years).

5. Ensure that M. confusa is included on the criteria and/or citations of all Arun valley SSSIs where the species is present.

6. Inform landowners and river banks managers of the presence of Mercuria confusa and seek to negotiate sympathetic physical management and/or suitable access for grazing animals. A paragraph about the snail should be included in each appropriate Environment Agency L.E.A.P.

7. Research the impact of boat traffic speed and density on populations of M. confusa and, in the light of findings, seek to ensure the development of a river-traffic regime that avoids damage to populations of the snail.

8. Inform the Environment Agency of the risks to M. confusa of any works that might disrupt the narrow salinity range or frequency of exposure to brackish waters required by the snail.

9. Produce a leaflet for land owners, conservation workers and the general public publicising the rarity and vulnerability of the snail and details the key features required for habitat management.

10. Undertake actions to raise funding to implement this SAP.

Survey work to be carried out by a consultant or invertebrate ecologist with experience of the species and requisite professional skills.

Identification leaflets

Survey of brackish pools south of Lyminster £400

Survey of marginal Phragmites and Glyceria north of Watersfield £400

Linear survey of the Arun corridor £1200

Environmental factors survey £1100

Surveys of potential sites on the Adur and Ouse for M. confusa £600

Total estimated budget to implement plan £4700

7. Potential

Opportunities for the species in Sussex:

Within the salinity range in the River Arun, Mercuria confusa has the potential to colonise all suitable upper tidal mud, especially where this is shielded by emergent vegetation such as Phragmites and Glyceria.

Limiting factors for the species in Sussex:

A number of factors could act to limit or reduce populations of Mercuria confusa in habitats adjacent to the River Arun. These include:

· An increase in boat traffic or maximum permissible traffic speeds;

· engineering works on the river bank or berm including the removal of mud or the deposition on these sites of sediments dredged from the river;

· barrage works altering the salinity range or flow regime of the river;

· fencing of marginal fens that are currently open to grazing cattle.

8. Action Plan [top]

See Action Plan Table (on menu above)

9. Monitoring/Review

This action plan will be monitored biannually as appropriate and reviewed every five years by the Sussex Biodiversity Partnership.

A scheme monitoring the distribution and population densities of the species in Sussex will be carried out once a monitoring protocol has been established.

10. References [top]

Abraham, F. & Willing, M. 1996, A short survey of the distribution and abundance of Pseudamnicola confusa at selected locations on the 'Burpham Loop' of the River Arun (August 1996), Unpublished report for the Environment Agency, Worthing.

Abraham, F. & Willing, M. 1997, A survey monitoring the abundance and distribution of populations of the aquatic snail Pseudamnicola confusa at selected locations on the River Arun at Burpham and South Stoke, August 1997, Unpublished report for the Environment Agency, Worthing.

Abraham, F.; Allen, S.; Hodge, P.J. & Willing, M.J. 1998, A survey of the flora and selected invertebrate groups of the ditches of the lower Arun Valley, Arun Valley Countryside Project, Arun District Council Bognor Regis.

Baker, R., Clarke, K. & Howlett, D. 1999, A survey of the Broadland distribution of Pseudamnicola confusa (Frauenfeld), (Ted Ellis Trust), English Nature Research Reports No. 319.

Bratton, J. H. (editor) 1991, British Red Data Books:3. Invertebrates other than insects. J.N.C.C., Peterborough, 1991.

Ellis, A.E. 1926, British Snails, Oxford University Press.

Gittenberger, E., Janssen, A.W., Kuijper, W.J., Kuiper, J.G.J, Meijer, T., Van de Velde, G. & De Vries, J.N. 1999, De Nederlandse zoetwatermollusken. Recente en fossieleweekdieren uit zoet en brak water. Nederlandse Fauna 2. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & EIS-Nederland, Leiden.

Janus, H. 1982, The illustrated guide to molluscs. H. Starke Ltd.

Kerney, M. P. 1999, Atlas of the Land and Freshwater Molluscs of Britain and Ireland. Harley Books.

UK Biodiversity Group 1995, Biodiversity: The UK Steering Group Report, Volume 2 Action Plans, Department of the Environment, HMSO, 1995.

Willing, M.J. 1999, Molluscan survey of drainage ditches at Watersfield, near Coldwaltham, West Sussex (May 1999), English Nature contract report (No. 18/16/E/99-00).

11. Consultation[top]

Author: Dr. Martin J. Willing, molluscs@willing.fsbusiness.co.uk

12. Appendix [top]




Standing Fresh Water HAP