The Isles of Scilly Biodiversity Audit 2008 - Cornwall Wildlife Trust

2.2.1.22 Mollusca – Slugs, snails,bivalves and relatives (terrestrial andfreshwater)OverviewThe Isles of Scilly is impoverished in terms ofmolluscan species compared to the mainland withsome species which are common on the mainlandbeing completely absent on Scilly, however for thearchipelago’s size the diversity of molluscs is relativelyhigh. Some molluscs show variations from mainlandforms due to their long separation from the mainlandfauna (Turk et al, 2001).There are 70 species of terrestrial and freshwatermollusc recorded on Scilly, over half the UK species.The granitic rocks are an acidic substrate which is lessthan ideal for providing habitat for snails which needcalcium to create their shells.Important AreasThe supralittoral zone or spray zone at the top of theshore is important for many gastropods which areoften found in caves and strandline vegetation.The sand dunes of Scilly are generally graniticnot calcareous like those in Cornwall but thereare areas of shell sand that still support the samespecies assemblage as Cornwall apart from theround–mouthed snail Pomatias elegans which is absentin Scilly. Maritime grassland and heathland areimportant for snails in Scilly.The shortage of freshwater habitats in Scilly meansthat there are few wetland mollusc species. Severalpond snail species and five species of pea mussel arepresent (Turk et al, 2001).St Mary’s has the largest mollusc fauna and supportsnearly all the species recorded in Scilly whereas thesmaller islands have lower species richness (Cameron,2006).Conservation ImportancePaludinella littorina the lagoon snail is protected bythe Wildilife and Countryside Act, 1981. The Isles ofScilly is considered the stronghold for this specieswhich lives in the supralittoral zone in rocky creviceson rugged granite boulder shores (Light and Kileen,2001). This species lives just on the boundaries of theSSSIs which are drawn at the high water mark.None of the other terrestrial or freshwater molluscson Scilly are BAP species or species of conservationconcern however they are important prey for birdssuch as the song thrush.Trends and IssuesResearchRichards and Robson (1926) recorded 42 species ofmollusc on Scilly in 1925.Holyoak (2003) found ten mollusc species which werenew to Scilly including local and introduced species.Cameron (2006) recorded 51 species from islands.Introduced speciesA number of snail species have been introduced to theislands. These include the garden snail Cornu aspersumwhich is abundant in dunes and coastal habitatsincluding being widespread on the uninhabited islandof Annet (Holyoak 2003; Parslow, 2007). Introductionsmay have been by human inhabitants or smallerspecies could have arrived attached to birds (Cameron,2006).No exotic species of mollusc have been recordedin Scilly but Holyoak (2003) speculates that theintroduction of exotic plants to Tresco Abbey Gardensmay have led to the introduction of associated molluscspecies yet to be found. There is conjecture that theslug Boettgerilla pallens was introduced in this way(Cameron, 2006).White lipped snail Cepaea hortensis. Photo: Colin ButlerIsles of Scilly biodiversity audit Page 35