The blue dragon nudibranch, Glaucus atlanticus, is beautiful but extremely toxic. It feeds on the Portuguese man o’ war and then absorbs its venom, storing it in the tips of its finger-like appendages. So don’t touch it, or you’re risking a painful sting!
If you’re wondering about sea hare ink, it’s definitely not toxic to humans, but it can give lobsters a stuffy nose. It’s a powerful repellant to fish and crustaceans, which is why they use it to defend themselves. Sea hare ink is also toxic to dogs, though, and this becomes a problem when many dead sea hares wash up on the beach and dogs try to ingest them.
This is amusing: some beachgoers have been alarmed by sea hares onshore because they mistake them for human organs.
What’s more amusing is that in Australia, sea hares are called “beach blobbies” (see the above sign).
Now you know.