LAND AND FRESHWATER MOLLUSCA (GASTROPODA & BIVALVIA)

 G.A. Holyoak

Cornwall is not a ‘rich’ region for land and freshwater molluscs. Nationally, 213 species (excluding glasshouse aliens) have been recorded in Britain (Anderson, 2005) compared with 122 species in Cornwall. The most important reasons for this impoverished fauna are the lack (or near absence) of calcareous substrata, and the predominance of soft-water, rocky and fast-flow streams and rivers. Earlier fieldworkers, especially Stella Turk, and Jean Paton in the 1960s contributed many of the records of molluscs in Cornwall and both added species to the Cornish list. Stella Turk should also be credited for her work compiling a database of distributional records, which resulted from her own fieldwork, her research on the historical records and museum specimens in Cornwall, and not least, her encouragement of others to record and submit records. Thus, a large number of distributional records accumulated, but despite the number of records it is now clear that many rarer species were not recorded. This is largely because there have been very few competent conchologists recording in Cornwall, so there have been few or no recorders with the knowledge necessary to seek out ‘specialised’ habitats supporting the rarer species, or with the knowledge to employ suitable sampling techniques to find and identify very small snails. The present author has been recording in Cornwall since 1998, mainly mapping species distribution on a tetrad (2 x 2 km grid square) basis. Since 1998 almost 6500 distributional records have accumulated and eight species have been added to the Cornish list.

This account updates that in the First edition of the Red Data Book for Cornwall and the Isles of Scilly (Spalding, 1997). It takes 1965 as the starting date for recent records in order to remain comparable with the national Atlas (Kerney, 1999) and the Cornish Atlas (Turk et al., 2001). It includes records made up to the end of August 2008. Hazel Meredith and Tim Geatches (Environment Agency, Bodmin) are acknowledged for their contribution of records.

Nomenclature follows Anderson (2005). The sequence of taxa under each heading is alphabetical. Because of changes in taxonomy and nomenclature of the British Mollusca in recent years, synonymous names are listed (prefaced by ‘syn.’) where different names have been used in other recent publications (Kerney, 1999; Turk, 1997; Turk et al., 2001).

Full assessment of the threat-status of British Mollusca using the IUCN threat criteria has not yet been completed. No ‘Nationally Rare’ species (i.e. those present in 1-15 10 km grid squares in Britain) occur in Cornwall. In the present review detailed accounts are included for two species of gastropods which were listed in the British Red Data Book (Bratton, 1991) and one freshwater bivalve specially protected under the Wildlife and Countryside Act (1981). These three species of Mollusca are also covered in the UK Biodiversity Action Plans (following the list agreed by DEFRA in 2007).

‘Nationally Scarce’ species (those present in 16-100 10 km grid squares and ‘Locally Scarce’ species (those recorded in four or fewer tetrads in Cornwall, but not qualifying as Nationally Rare or Nationally Scarce) are listed with brief notes. Species covered in the first edition of the Red Data Book for Cornwall and the Isles of Scilly by Turk (1997) but excluded here are also listed below with brief comments explaining their exclusion.

1. Accounts of Red-list species

Margaritifera margaritifera Freshwater Pearl Mussel

Omphiscola glabra Mud Pond Snail

Vertigo moulinsiana Des Moulins’ Whorl Snail

2. Nationally Scarce species

Phenacolimax major Greater Pellucid Glass Snail

Ponentina subvirescens Green Snail

Testacella maugei Mauge’s Slug

Testacella scutulum Shield Slug

3. Locally Scarce species

Acroloxus lacustris Lake Limpet

Anodonta cygnea Swan Mussel

Bithynia tentaculata Common Bithynia

Cochlodina laminata Plaited Door Snail

Dreissena polymorpha Zebra Mussel

Paralaoma servilis

Pisidium henslowanum

Pisidium hibernicum

Pisidium moitessierianum

Planorbarius corneus Great Ram’s Horn

Pomatias elegans Round-mouth Snail

Pyramidula pusilla Rock Snail

Spermodea lamellata Plated Snail

Sphaerium corneum Horny Orb Mussel

Vallonia pulchella Smooth (or Beautiful) Grass Snail

4. List of species of molluscs included in Red Data Book for Cornwall and the Isles of Scilly, 1st ed. (1997) but now excluded.

Hygromia cinctella (Draparnaud) No longer regarded as nationally or locally scarce.

Theba pisana (O.F. Müller) No longer regarded as nationally or locally scarce.