There was a similar story a few months ago in the Scottish papers.A dog had become very ill after being out for a walk in a park in Glasgow and the vet had put it down to slugs/snails in the park.Can't remember much more about the story.
That article was from an article from an Auckland, New Zealand newspaper. The sea slugs were found to contain a naturally occuring toxin which is leathal to dogs (& probably people if you ate one). Another dog died last week in Thames, New Zealand when the owners stopped at a nearby beach to let the dog out for a walk. Not long after getting back in the car the dog started salivating and fitting. 20 mins later at the Thames vet the dog died.
Thames is about 20mins from Auckland by air.
In New Zealand at the moment if you take your dog to the beach it may eat a poisonous animal. If you take it to the forest it may eat a 1080 bait!!! Keep an eye on your dogs they are too precious to loose to poison. :-[