Vol 4 Test 5 Part 4

Good morning everyone. In this module we've been looking at various field survey methods. Now I'm going to give you a brief introduction to the specific organism that we'll be focusing on during our first field trip, and that's the gastropod.

Gastropods are invertebrates—organisms without a backbone—and they're more commonly known as snails or, if they haven't got a shell, slugs. In terms of their evolution, gastropods go back a long way. As with other primitive animals, the minerals in their body fluids are very similar to the mineral composition of the sea, indicating that they began life there. Their fossilised remains have been found in rocks dating back at least 500 million years.

Looking at the physical features of gastropods, they all have one muscular foot for swimming or walking. This foot is lubricated with mucus, and it's this that leaves the sticky trail you see behind snails and slugs. They also have an organ called a radula, which they use for feeding. Snails, though not slugs, have a shell.

In terms of their dimensions, snail shells in Britain range from the dwarf snail, which is 1.5 millimetres long, to the Roman snail, whose shell can be as big as 50 millimetres. As well as size variations, there are many differences in shell form. For example, most shells coil to the right, but a few coil to the left. Some shells, such as that of the plated snail, have regularly spaced ribs, while the prickly snail has prominent spines. There are even species with hair on their shells. Colours and patterns also vary widely, so appearances are extremely diverse.

Let’s turn now to feeding habits. Most of you who are gardeners will know, to your cost, that some gastropods eat healthy plants, but these are actually a minority. Most species feed on rotting plants, fungi, or algae. Some species eat dead animals, and others are carnivorous, preying on live animals. For example, the shield slug feeds on worms.

Apart from their own feeding habits, gastropods themselves have many predators. Birds and frogs are the main ones, but there is also an entire family of flies that has specialised in preying on gastropods. And, last but not least, humans are predators too. The Romans probably first introduced edible snails into Britain in the first century, and they are still eaten today. In fact, the collection of wild snails for cooking has caused local extinctions of one species, which is also under threat internationally.

To protect themselves from predators, gastropods have developed special defences. I’ll give you just one example. The glutinous snail avoids predators by folding its mantle over its shell and turning itself into a very slippery blob of jelly.

Turning now to gastropod habitats, most species prefer damp and shady environments. In general, both very hot and very cold weather is harmful to gastropods, but dry conditions are the least favourable. Snails cope with this by withdrawing into their shells and often forming a thick film of mucus over the shell opening to reduce water loss.

Habitats with a long, stable history tend to support the greatest range of gastropod species. For example, long-established forests usually support more species than newer tree plantations, and old natural meadowland is far better than land that has recently been farmed.

There are also a small number of highly specialised species that live in unusual habitats. For instance, the blind snail spends its entire life underground and has been reported at depths of up to two metres. It apparently prefers cavities in old, buried bones. However, species like this are exceptional.

Wherever they live, gastropods are often sensitive to pollution and do not thrive in areas contaminated by agrochemicals. This makes them a reliable indicator of environmental health.

Now, at this point, I’d like to consider habitat management in related…