Separating Blacksaddled Toby and Mimic Filefish
The two fish Blacksaddled Toby, Canthigaster valentini, and Mimic Filefish, Paraluteres prionurus, look very much alike. This is not coincidence. It is astonishing how similar they look, since they are from different families, the Toby being a puffer and the other a filefish. In the photo below, the upper fish is the mimic, the lower one is the puffer.
The filefish mimics the toby because pufferfishes are poisonous. Predator fishes learn this because puffers taste bad and make then sick. By mimicking this poisonous fish, predators also avoid eating the filefish which are otherwise vulnerable.
The easiest way to separate these is by the shape of the dorsal and anal fins, outlined in red in the photo below. On the Blacksaddled Toby, these fins are small and fan-shaped. On the Mimic Filefish, they run half the length of the body.