Slugs Everywhere

While the insects may be noticeable by their absence this year, slugs seem to be everywhere. A mild, wet spring, and cooler than usual summer have combined to create perfect conditions for gastropods. Slugs have a reputation for destroying plants, but they do have an essential role to play in a garden ecosystem. They recycle waste, and are a source of food for wildlife such as birds and hedgehogs. Of the forty or so known species of slug in the UK, about nine are considered to be pests. Most are detritivores and feed on dead plant matter, animals, and fungi.

Evolution

Snails came first, and are thought to have evolved around 350 million years ago. Slugs subsequently evolved from snails over millions of years. The biggest visible difference between the two is the absence of an external shell on a slug. The evolutionary process took place independently in a number of different types of snails in different parts of the world. This suggests there must have been some evolutionary benefit to losing the external shell. Some slugs retain a very small internal shell, which is used for storing calcium.

Lifecycle

Slugs are hermaphrodites, meaning they have female and male reproductive organs. Slugs lay over a hundred eggs, which they might do several times a year. Baby slugs (neonates) hatch directly from the egg. Neonates reach adulthood after 2 – 12 months, depending on the species. Their total life expectancy is around 18 months, though some are known to live for up to 6 years.

Physiology

Although slugs can fertilise their own eggs, they also mate. Slugs find a mate first by smell, then courtship, then by locking onto each other. Both slugs evert their genitalia, and drop a sperm packet into their partner’s storage organ. In this way, when mating, both slugs may have their eggs fertilised.

Slugs’ mucus is designed to hold water to stop them from drying out. Without a shell to shade them, this is particularly important. They can navigate through the smell of their mucus, following a trail to return home. The water retention qualities make it very difficult to wash slug mucus off skin. The best approach is to wipe as much away with paper as possible, before washing.

Slugs have thousands of teeth (radula), which they shed and regrow. The teeth act like sandpaper when they eat and they consume around forty times their bodyweight in a single day. Defecation is via the hole in the right hand side of their head, which they also breathe through.

Different Types of Slug

Finally, here’s just a small selection of the slugs found in my garden the other night. I’ve done my best with the identification, but please feel free to get in touch if you disagree with any of the labels.

Green cellar slug – eats mould and algae, and often found in compost heaps. Also partial to pet food.

 

Dusky slug – colours range from light yellow grey to dark brown grey. Most common colour is orange brown. This slug has an omnivorous diet and feeds mainly on fungi and decaying plant matter.

Keeled slug – tends to spend much of its life underground. A common variety, it lives in gardens all year round.

Main picture is a dusky slug. I think the small grey slug is a netted slug. This is the most common slug in Europe, and is widespread in Britain. Often found in gardens where soil has been disturbed.

Further Information

The Wildlife Trusts have produced a guide to slugs and snails which is available to download here: Making Friends With Molluscs.

Find out more about why slugs (and snails) are important on this CBBC page.