Garden slugs are all members of the air breathing "pulmonates
" group of gastropods alongside their shelled compatriots the snails
. In fact, all our slugs are descended from snails which have evolved to lose (or highly reduce) their shell. This has happened many times in evolution, and the various families of slugs are not all closely related to each other, except back through shelled snail ancestors.
Why be a slug?
Snails are well adapted to life on land. Their hard shell gives a useful degree of protection against predators, but it is especially important in protecting the snail from drying out in warm dry air. The mucus layer on the rest of the snail's body when out of the shell also helps, but this the only dessication protection available to slugs. There must be a good reason for losing the shell - since several lineages of slugs have done so, in the sea as well as on land. What are the advantages of losing the external shell? Here are Robert Cameron's suggestions, from his highly recommended book listed below:
• Shells are heavy, and slugs generally can move faster than snails (still not very fast!)
• Shells require a lot of calcium, which has to come from food. In chalk places this is easy, but where the soil is
acidic, calcium is hard to get and snails are much less common than slugs. Even when calciu is abundant, there
must still be some metabolic cost in making the shell.
• Losing the shell means the slug is entirely soft-bodied. and can squeeze through small apertures, and especially
between particles of soil, in a way impossible to snails. Instead of carrying a defensive home on their backs,
they can hide away in all sorts of confined spaces. Many land slugs are largely soil-living, and it seems the
main advantage of "sluggishness" is the greater ability to make use of small spaces.
Most slugs have very strong mucus-secreting powers, and in addition taste foul, so they are better defended than they appear.
The shell of snails was almost certainly the main reason why pulmonates were able to evolve to live on land, but slugs have become successful precisiely because - once adapted to land - they were able to ditch the shell. An exact parallel can be seen in the cephalopod molluscs
They were able to become swimming predators by using their coiled shell for bouyancy - as in modern Nautilus
and fossil ammonites. However, the modern cephalopods, especially the squid and octopus, have become even more successful through abandoning the bouyant shell, and maintaining their position in the water by other means.
Having said this many slugs do retain a vestige of an internal shell - a useful calcium store, and some such as Testacella have a very small external shell. Equally, especially in the tropics, there are many species of semi-slugs which are snails which have reduced their shell to a greater or lesser extent.