Re: How do snails and slugs communicate?

What a super question! After much reading (and sorry for the time taken to 
do so), it appears that land snails and slugs (both called 'gastropod 
molluscs') have two potential ways to communicate to members of their own 
kind, or species. If you have any live snails handy (best time to look for 
them is after rain in early summer), take a look at their head and face 
area. You should be able to see two pairs of tentacles. The top ones are 
longer and have simple eyes on the end. It has been found that they are 
really only good at seeing large objects - like a bird coming to eat them. 
Thus they don't likely use this to communicate with each other. 
However, the lower tentacles are used for touch and chemical reception 
(like the ability to taste in humans) - which would be valuable for two 
snails when they meet. If they decide to mate with the snail they meet, 
they will exchange 'love darts' (much like being shot by a cupid's arrow) 
to identify the species they belong to (but not whether they are male or 
female, because snails are hermaphrodites and are both male and female at 
the same time). If the love dart is received by the right partner, 
courtship will proceed.
If you watch a snail exploring it's environment, it will wave all it's 
tentacles through the air to pick up as much information from the air as 
possible. But there is another way to communicate that is often overlooked. 
 The slimey mucous trail that snails and slugs leave behind when they slide 
from one place to another is a great place to leave behind a semi-permanent 
message for a passing snail to cross over and 'read'. The use of a special 
chemical signal called a pheromone (which is much like a perfume but made 
of hormone-like substances) has recently been studied by scientists. 
Pheromones are left behind, mixed in with the slime trail. In 1992 a 
scientist named E. Pakarinen wrote a research paper on communication of 
stress (from being pecked at by a bird) through slimey pheromone trails. If 
I remember correctly, it was found that a snail sliding upon the trail of 
another snail that had been stressed made it move away as quickly as 
possible - a good reaction if there is a chance the bird is still around. 
You can check this for yourself in the Journal of Animal Behavior, volume 
43: pg. 1051-52.
Happy gastropoding!

Neala MacDonald
Zoologist and Science Outreach Coordinator
University of Western Ontario
London, ON, Canada