Ichthyotoxin
A poison aimed at fish rather than at mammals, usually attacking the gills - including pardaxin, the Red Sea sole secretion that makes a shark let go.
An ichthyotoxin is simply a poison aimed at fish. The word describes the target rather than the chemistry, and it covers a good deal of what is on this site: ostracitoxin from boxfishes, grammistin from soapfishes, the saponins of sea cucumbers. The common thread is the gill - an enormous, exposed, single-cell-thick surface that a swimmer simply does not have, which is why so many reef poisons are lethal to a fish in a bucket and unremarkable on a human hand.
The most interesting one in this sea belongs to a flatfish. The Finless Sole, Pardachirus marmoratus, seeps a bitter milky fluid from pores along the bases of its fins, and in the 1970s divers here found that a shark taking one into its mouth releases it at once, jaws locked open, and swims off. The active peptide was named pardaxin, and it was pursued for years as a possible shark repellent - the best-known piece of Red Sea marine chemistry there is.
First aid
By definition these are aimed at gills, which is why they are so much more dangerous to a fish in a bucket than to a hand in the sea. Wash your hands after handling any of the animals that carry one, keep them away from your eyes, and never transport them in a container with other fish.
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