This is a chapter about British and Irish animals without backbones, other than insects (see Chapters 14 and 15) and marine animals (Chapters 19 and 20). These ‘other invertebrates’ include terrestrial and freshwater snails, slugs, spiders, scorpions, crustaceans, worms, flatworms and freshwater jellyfish.

Molluscs

Let’s begin with the molluscs, the slugs and snails. There are over 200 of these species on the British and Irish list, of which about 60 are in decline. These include 38 land snails, 6 land slugs, 11 freshwater snails and 11 freshwater bivalves. A few, as shown in Table 16.1, are recent arrivals. Of the overall molluscan fauna of Britain, 37% are in decline – and a similar percentage is also true for Ireland. It is easy to attribute the loss of the freshwater molluscs to pollution and drainage of ponds and waterways, but many terrestrial species are also in decline, as shown in Table 16.2.

The freshwater pearl mussel is one of the most endangered animals in Britain and Ireland, and seems to be declining because of the loss of clean gravel beds needed by the immature animals. The decline of this interesting bivalve is estimated to have exceeded 90% in Europe during the twentieth century. Until recently the species was sought as a source of the grey freshwater pearls, much prized by the Romans from the Scottish River Tay. Asiatic clams and zebra mussels, both introduced molluscs, are believed to be a cause of decline in some other freshwater mussels, such as duck, swan and painter’s mussels. In a number of British and Irish sites captive breeding of pearl mussels is being attempted to help mitigate the declines.