Monday, 29 March 2010

An overlooked invertebrate

The other morning, while moving some pots from one part of the garden to another, I came across a cluster of snails. They had tucked themselves away in the relatively stable microclimate between the pots and an old flint wall. Part hidden by ivy, this appeared to be a suitable place for them to have sat out the bitter winter we have just experienced. I felt somewhat guilty for having disturbed them but I am sure they would get their own back if I failed to properly protect my spring salad crop with mesh netting.

Snails are so accessible that they quickly become an object of childhood fascination and I remember marking some as a child with spots of model paint, delicately applied with a fine brush. Following the instructions laid out in some book or other on becoming a naturalist, I’d marked the snails to see where they moved to around the garden; did they always return to the same spot or did they move about and mix with others? I wonder whether, if I repeated that experiment now, I would find that some of them turned up on the thrush’s anvil during those few summer weeks when snails become an important food for this industrious bird.

While the garden snail Helix aspersa is a familiar creature, most of our other snails go completely unnoticed. Many are tiny, the smallest just 1.5mm wide and 1mm tall, and are easily overlooked. Others may go unrecognised because they do not conform to the familiar shape that we expect to see in a snail, with its round shell carried on a pale, muscular body. The origins of our snails are aquatic in nature and there is still a strong requirement for damp conditions and habitats. This is one reason why they are predominantly nocturnal in habits, although being nocturnal probably reduces their exposure to potential predators. Many snails are found associated with rivers and ponds and some of the larger species may be familiar to those who have been pond dipping. Soil type also has a strong influence on snails, with a greater number of species associated with lime-rich soils and very few species found on acid soils. This association is linked to the snail’s shell, with the lime used in shell formation. Those snails found on acid heathland tend to have shells that are very thin and fragile.

Invariably, where you find snails you also find slugs and many people regard slugs as just snails without a shell. In fact the division is less clear-cut than this because some snails have very small shells, relative to body size, and while very few slugs have an external shell, others carry one internally.

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