Fish-Men

Fish-men are one of two humanoid races that inhabit and rule the seas (the other being the merfolk). In terms of leadership, Fisher Tiger, Jinbe, and Arlong were said to have the highest respect amongst their people. They are classified as Type C creatures, "Big Savage". 

Appearance

Fish-men are more fish-like than merfolk, usually looking like a combination between a man and a fish or other aquatic creature, such as an octopus, manta ray, or sawshark; however, they still have legs (in fact, Decken has four). They also have gills between their shoulders and necks, sometimes covered by their clothing, as well as often having webbed hands. Depending on species, they may have multiple limbs (mainly extra arms).

Size may also differ greatly between individuals, as Arlong was much taller than any of his crew members, but was small compared to Fisher Tiger, while Wadatsumi is even bigger than an ancient giant like Oars. For fish-men pirates in pirate crews that are primarily fish-man based (Sun Pirates, Arlong Pirates, and New Fish-Man Pirates), it appears customary to have that crew's jolly roger tattooed somewhere on the person's body, showing allegiance to a crew.

Overall Strength

From birth, fish-men are physically ten times as strong as normal humans. This difference is magnified underwater, where a human's power is cut in half, while fish-men are not only completely unhindered, but are even strengthened. Jinbe himself noted that on dry land, he had little power to offer. Considering the impressive strength of a fish-man as powerful as him, this shows the massive difference in the potential of any fish-man when in the water compared to being on dry land. Overall, the fish-man race can be considered unstoppable in battle while in their natural habitat, the sea. As expected, they are excellent swimmers, and are capable of swimming through the water at a great speed and force. However, their speed is no match for the merfolk, who are the fastest swimmers in the world. In addition, they can still talk under water and cannot drown. However, breathing air into their gills makes them suffer like a normal fish would. While they can use their gills to breathe in water, they have to switch back to their lungs to breathe in air. If a fish-man were to eat a Devil Fruit, they, like anyone who partakes of the fruit, will lose their ability to swim forever. They will still be able to breathe underwater, though they will be unable to move while in it.

Like merfolk, fish-men also possess the ability to communicate with sea creatures. In particular, fish-men seem adept at communicating with large sea monsters, and possess a unique capacity to bring out the primal rage in such creatures. This ability was first demonstrated when Arlong induced Momoo into a primal frenzy to have him attack the Straw Hats during the battle at Arlong Park, despite Momoo's initial reservations about doing so. The ability was shown again in more detail when Hody and, to a more limited extent, the officers of his crew used it to induce the large Sea Monsters the New Fish-man Pirates had in their possession into a similar rage to the one created by the Energy Steroids they had been eating. The animal being influenced has to be at least somewhat willing to comply, as was shown when Surume quickly stopped squeezing Shirahoshi on Hody's command due to Luffy's intervention, and afterwards refused to follow any more of Hody's orders. Just as with normal merfolk, fish-men are still unable to communicate with Sea Kings; only the Mermaid Princess who awakens the power of the ancient weapon Poseidon, currently Shirahoshi, is capable of doing so.