I mean, obviously when you have something all the way through your hand, it's kind of bad anyway, but in this case, sea urchins have a venom on them that, if you've ever tangled with them, you know that a sea urchin spine in you gives you horrible, painful inflammation.
Avoir la main transpercée, c'est déjà mauvais en soi. Mais en l’occurrence, les oursins sécrètent un venin, et si vous en avez déjà croisé, vous savez que les épines d'oursins causent des inflammations douloureuses.