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.
Quiero decir, obviamente, cuando tienes  algo atravesando tu mano, es malo de todos modos, pero en este caso, los erizos de mar  tienen un veneno que, si alguna vez se los han clavado, saben que producen  una inflamación horrible y dolorosa.