Some sea slugs contain toxins.
There is one very particular specie of sea slugs - Pleurobranchaea maculata - which has been ddemed responsible for the deaths of on Auckland beaches in 2009.
These slugs contain Tetrodotoxin (TTX), the same toxin found on pufferfish. The lethal (killer) dose of TTX in humans is 1-2mg, that's like eating around 2-3 grams of sea slugs! (ee[link removed])
Hey, thanks for the Points. =)