Some urchins are poisonous to humans as well as fish. Be careful when dealing with the various species as getting stuck can cause a lot of pain and in some cases hospitalization. The urchin in this story is part of a larger group called “collector urchins” because they camouflage themselves by sticking things on their back. The Tuxedo Urchin is an example of this group. This new discovery is quite interesting. They have been found to shoot out toxic jaws as a means of defense. It’s not clear from the story how much handling is required to trigger this but I do suggest care when handling urchins in your tank. Just in case.
Prey animals are capable of defending themselves in an amazing of ways, but when it comes to mounting a sophisticated biological counter-attack, sea urchins have taken it to another level. When attacked by predatory fish, these humble echinoderms release a hostile cloud of tiny jaws that act independently of the urchin itself, attacking the fish and releasing the venom contained within them.
Source: These Sea Urchins Have a Terrifying Self-Defense Strategy