Well, I'm by no means a Palytoxin expert, so take this with a grain of salt, please.
I was wondering the same thing a while back when I first heard of this, and went on a hunt for the answer. I believe, again I could be wrong, that the reason it doesn't effect fish and can affect humans is the osmoregulation and homeostasis factor of fish. All that the toxin does is reverse the electrical flow of ions into the system and affects sodium uptake. Since marine fish are constantly flushing water out of their systems, I am asssuming it doesn't effect them.
So, when we ingest it or get it into cuts, it creates pain because it is affecting concentrations in or muscels. When it's ingested or something in large amounts, in effects the life muscle...the heart.
But..not all species of zoos do this and it would take alot to get sick from it. I wouldn't worry about it. Just don't lick your hands after you handle your zoos or put your hands in the tank.
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