Fish-Men

Fishmen are one of two species that dominate the seas (the other being the Merfolk). In terms of leadership, none are mentioned, however Fisher Tiger, Jinbei and Arlong were said to have the highest respect amongst their people.

Biology
Appearance-wise, they are more "fish-like" than merfolk, looking like a cross between a man and a fish of the ocean such as an octopus, manta ray, or sawfish; however, they still have legs. They have their gills near their necks which only work when they are in water. On land they can breathe with their lungs like humans do. They are ten times as strong as normal humans from birth, and this difference is magnified underwater, where a human's power is cut in half (while Fishmen are completely unhindered, and perhaps even strengthened).

Even though Fishmen are part fish, they are still mammals and give birth to live young.

As was revealed during the Davy Back Fight arc with Big Pan Fishmen can also breed with Giants, producing a hybrid who has biological abilities of a Fishmen and a huge body of a Giant (but still smaller than a full one, as related by Robin), they are known as Wotan. Although never shown it's clear that Fishmen and humans can also breed the same way Merfolk and humans can breed.

Inter-species Relationships
Their view of life under the sea is dominated by their views that they should rule over all other fish in the sea. Despite this view, usually they get along well with the Merfolk who share the opposite view on life, however it is not unknown for Fishmen to take advantage of the Merfolk's nature. Because of their strength, they appear in One Piece on a regular basis.

A stereotypical Fishman considers humans an inferior, weaker species. However, this does not apply to all Fishmen: for example, the pufferfish shipwright Tom cared and looked after two human children (Iceburg and Franky) and even protected them from the actions of Spandam and CP5, and even the Shichibukai, Jinbei, considers the Whitebeard Pirate, Portgas D. Ace an equal and holds the Whitebeard Pirates as a group in high regard because of Whitebeard protecting Fishman Island.

On the opposite end of the spectrum, human relationship to the Fishmen was defined with their definition of both them and merpeople as "fish" instead of "men". This lead to centuries of discrimination as the Fishmen fought to prove their worth to the world. Only two hundred years ago did the World Government began to bridge the gap, making an alliance with Fishman Island to remove the problems between the two sides. While attempting to fix the problem, centuries of bad relations remained: many humans continued to see Fishmen as freaks or, in the case around Sabaody Archipelago, slaves for the Tenryuubito. On the opposite end, certain sects of Fishmen attempted to create their own societies where they dominated over the humans, such as Arlong Park in East Blue.

Translation and Dub Issues
The kanji for Fishmen (魚人) is the reverse of the name for Mermen (人魚). They are often incorrectly referred to by fans, and the English dub made by 4Kids, as Mermen for that reason.

Trivia

 * Originally Oda planned on introducing Fishmen as far back as Chapter 3. However, due to his editor he then put it back, ending up introducing the concept during the Arlong arc instead.