Arlong Pirates

The Arlong Pirates (アーロン一味, Āron Ichimi) are led by Arlong. Every member of the crew is a Fishman, except for Nami (who later left). They split from Jinbei's crew, presumably the Sunny Pirates, after he became a Shichibukai. They believe they are above humans as the stronger species, looking down on them as mere maggots.

They are often incorrectly referred to by fans and the English dub as "Mermen", though this is inaccurate as Mermen are completely different species in the One Piece world.

History
Originally they were part of the Sunny Pirates, but when Jinbei became one of the Shichibukai the crew split up for as of yet unknown reasons, leaving Arlong in charge of his own crew, who he led out of the Grand Line and towards East Blue, which had much weaker prey.

Eight years before the present storyline, the Arlong Pirates invaded an island and forced the residents to pay a large monthly tax for their own survival--100,000 beli per adult, and 50,000 per child. Bellemere was only able to pay for her daughters, so Arlong killed her as an example, leaving her adopted daughters Nami and Nojiko. Seeing how skilled at map-making Nami was at such as early age, Arlong kidnapped her and forced her to become a member of his crew making maps for her. In order to convince her, he struck a deal--if she can somehow gather 100 million berries and give it to him, he will free her village. Thus, Nami began her life as a thief. As a show of their power, the crew constructed their headquarters, Arlong Park, in clear sight. They struck deals with corrupt Marine officials like Captain Nezumi in order to prevent attacks and started to build up their forces to take over all of East Blue.

During the present day, Arlong had already conquered over 20 villages but Nami was getting dangerously close to her goal, and Arlong was getting worried. If Nami left, he would lose a valuable part of the team, as her maps were essential for coming up with strategies. Thus, he told Nezumi where Nami kept all of her money and had him confiscate all it, but Nami and the other villagers instantly realized that Arlong was behind it and finally decided to fight back.

After several long and difficult battles, Arlong Park was toppled by Luffy, freeing the entire island from his reign. He and his surviving crew were arrested, though Hatchan managed to escape and start a takoyaki restaurant.

Pirate Flag
Arlong's flag is that of a saw-nosed shark turned to the left with the tail curving inward. Around the shark is a semi-oval with three curves emitting from it. Behind the shark is a crossbone. This flag is tattooed onto all members of the crew.

Another symbol that may or may not be the Sunny Pirates' flag is a sun-shaped tattoo that appears on several members of the Arlong Pirates. However, this has yet to be confirmed and Momoo doesn't have it (despite being a secret weapon).

Crew strength
As Fishmen, they are twice as strong as ordinary humans, 10 times stronger under water, and are not afraid to use cruel measures to incite fear and show how much stronger they are than humans. They are fearsome fighters and almost unbeatable under water. For the Straw Hats, this was the toughest of their early fights, pushing them all to their limits. Their leader Arlong was not bothered by Luffy having a Devil Fruit power and took advantage of his weakness to the effects of the sea. With Nami's maps, their abilities increase even further, as they can take advantage of ocean currents by blocking them with boulders to create whirlpools, among other actions. With the Marines focusing their attention on the Grand Line and the incapacitation of all other major East Blue pirate bands by the Straw Hats, there would have been nothing to stop them from taking over East Blue if the Straw Hat Pirates hadn't intervened. Members of this particular crew also seem to be stronger than most other Fishmen, though this is probably to be expected, having been once lead by a Shichibukai-level pirate, Jimbei. Hatchan, in particular, was able to best three other Fishmen, much like himself, from time to time in a matter of seconds