Kenny George, of Burnside Primary School, asks :-
Why do snails foam when I put salt on them?
Mike Winterbourn, a zoologist at the University of Canterbury, responded.
The garden snail, whose scientific name is Helix aspersa came to New Zealand from Europe many years ago. It is generally considered to be a pest, since it chews the leaves and stems of vegetable plants and various kinds of seedlings. A traditional way that people have got rid of snails and slugs is by sprinkling salt around plants they want to protect. Another way is to sink tins full of beer in the ground as baited pitfall traps. The snails are attracted by the beer, fall into it and drown!
So why did your snails foam up when you sprinkled salt on them? Well, you will have seen that snails produce lots of sticky slime or mucus, not just on the foot they use to glide along on, but also inside the cavity they use as a lung. You may also have noticed a small hole just behind the snail's head and I suspect a lot of the foam was coming from that hole. When the cells that line the lung behind that hole are irritated by salt they produce lots more slime than usual and it gets mixed with air as it is pumped out. The foam you saw is slime mixed with air.
The result of this slime-producing activity is that the snail's tissues dry out (dehydrate) and if it continues long enough the snail will die from loss of water. Dehydration is also the way some of the most successful chemical baits work to kill snails and slugs.