You are thinking about Cymothoa exigua

Some parasitic isopod, but it does not act like tittle describes. It just eats fish from inside.

Look at Braga cigarra 

, which isn't closely related to either of these animals.

This specimen have relatively long curved spines on the legs. Do C. exigua have such spines? I don't think so.

Fun fact #1: Fishes containing parasitic isopods are believed to be free of toxins. (there is no confirmation).

Fun fact #2 One of parasitic isopods, Rocinela signata, sometimes attacks humans in large swarms. (which is allegedly terrifying)