The Arlong Pirates (アーロン海賊団, Āron Kaizokudan) or the Arlong crew (アーロン一味, Āron Ichimi) were led by Arlong. Every member of the crew was a mermen, except for Nami (who later left). They existed before the Sun Pirates, but joined with them when they were formed and after the death of Tiger Fisher, they split from Jimbei's crew, after he became a Warlord and became their own crew again. They believe they are above humans as the stronger species, looking down on them as an inferior species. After they were defeated, they were arrested. Momoo was never taken into custody and Hachi later escaped. The crew is no longer active. Their ship was Shark Superb.
They existed before the creation of the Sun Pirates, but gladly joined with them upon their creation. After the death of Fisher Tiger and when Jimbei became one of the Seven Warlords of the Sea the crew split from the Sun Pirates and returned to being the Arlong Pirates, who then left the Grand Line for East Blue, which had much weaker prey. Arlong had also attempted to recruit both Hyouzou and Decken Vander IX, the first eventually turned down Arlong after he would fail to pay his hiring and the second refused to be the underling of another.
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 berries per adult, and 50,000 berries per child. Bellmère 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 100,000,000 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 Navy 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, the Arlong Pirates were defeated by Team Natsu and the Straw Hat Pirates, with Luffy defeating Arlong as well as destroying Arlong Park, freeing the entire island from Arlong's reign. He and his surviving crew were arrested, though Hatchan managed to escape and start a takoyaki restaurant.
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 cross bone. This flag is tattooed onto all members of the crew. Several members of the New Mermen Pirates, Hordy Jones and Hammond in specific, also has this tattooed on to their bodies, out of respect for Arlong and his ideals.
Not only do they have the advantages of being born Mermen, but the crew are fearsome fighters and almost unbeatable under water. Members of this particular crew also seem to be stronger than most other Mermen, though this is probably to be expected, having been once lead by a Warlord-level pirate, Jimbei, and having experience from the Grand Line. Hatchan, in particular, was able to best three other Mermen, much like himself, from time to time in a matter of seconds. For the Straw Hats and Team Natsu, this was the toughest of their early fights, pushing them all to their limits (though much of the pressure on Sanji and Zolo was due to Luffy's recklessness and Zolo's injury).
Their leader Arlong was not bothered by Luffy having a Cursed Fruit power and took advantage of his weakness to the effects of the sea. Arlong had a keen eye for talent that could be used for his ambitions and was able to make use of the skills bestowed upon him and his crewmates, even though he had lacked the resources to pay for more experienced soldiers such as Hyouzou. 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 crews by the Straw Hats, there would have been nothing to stop them from taking over East Blue if the Straw Hat Pirates had not intervened.
Though the opinions and ideals of the crew were extreme, they displayed a high level of brotherhood amongst themselves and even towards Nami despite her species divide. Arlong looked after his crew members and even though he betrayed Nami, he did so because he did not want to lose her talent. On top of this, in the final moments of the battle with Luffy, he stated that Nami was his crew member and would be forever, and while he never displayed any regard for her as a friend he was prepared to fight to keep her in the crew. If humans insulted their group, Arlong would react to the insult, and due to the high level of concern amongst his officers they would attempt to keep his temper under control. When weaker members of his crew were hurt, Arlong would seek out revenge on those who had committed the act against those crew members. The level of companionship between the pirate group was therefore quite high in comparison to other crews who do not even show concern when a crewmate is hurt, defeated or even killed. There was also a high level of respect amongst their crew towards Arlong and his officers while Arlong overall gave a high level of respect back to all members of his crew, including Nami.
On top of this, many of their crew members carried the "mark of the sun" and had previously been in the Sun Pirates. Although not all of those who carried the mark were slaves, this is still worth noting as the Arlong Pirates had a number of crew members who had subject to the shared experiences of those on board the Sun Pirates ship. The result would have meant at the very least they had ties to the events of their era, with a few or all of those former members possibly have been driven by those experiences personally as slaves in the past.
The crew's loathing towards humanity is quite well known, and had a huge influence to the New Merman Pirates, who would use Arlong's Jolly Roger as "an empty symbol for human-hating Mermen to gather under". To this end, Hordy Jones and Hammond even tattooed the symbol on their own bodies, despite having lost respect for Arlong himself for being defeated by Luffy.