The Arlong Pirates were a pirate crew led by Arlong. Every member of the crew was a fish-man, except for Nami (who later left). They existed before the Sun Pirates but joined with them when they were formed. After the death of Fisher Tiger, they split from Jinbe's crew after he became a Warlord and became their own crew once again. They believed they were above humans as the stronger species and looked down on them as inferior. After they were defeated, they were arrested. Due to their actions and role, they are the main antagonists of the Arlong Park Arc. After their leader's defeat by the Straw Hats, they are arrested and the crew itself is disbanded. Momoo was never taken into custody and Hachi later escaped.[3]
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 Fish-Man Pirates, Hody Jones and Hammond in particular, also have this tattooed on to their bodies, out of respect for Arlong and his ideals.
Not only did they have the advantage of being born Fish-Men, but the crew were fearsome fighters and almost unbeatable underwater. Members of this particular crew also seemed to be stronger than most other fish-men, though this is probably to be expected, having been once led by a Warlord-level pirate, Jinbe, and having experience from the Grand Line. Hatchan, in particular, was able to best three other fish-men, much like himself, from time to time in a matter of seconds. For the Straw Hats, this was the toughest of their early fights, pushing them all to their limits (though much of the pressure on Sanji and Zoro was due to Luffy's recklessness and Zoro's injury).
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. 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 increased even further, as, among other things, they could take advantage of ocean currents by blocking them with boulders to create whirlpools. 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.
Other Information
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 that, at the very least, they had ties to the events of their era, with a few, or possibly all, of those former members having been driven by those personal experiences as former slaves.
The crew's loathing towards humanity was quite well known, and had a huge influence on the New Fish-Man Pirates, who used Arlong's Jolly Roger as "an empty symbol for human-hating fish-men to gather under." To this end, Hody Jones and Hammond even tattooed the symbol on their own bodies, despite having lost respect for Arlong himself for being defeated by Luffy.
History
Past
The Early Years
They existed before the creation of the Sun Pirates, simply bullying Fish-Man Island citizens for supporting Otohime's ideals of interspecies cooperation, but gladly joined with them upon their creation.[4] After the death of Fisher Tiger and when Jinbe became one of the Warlords of the Sea (which led to Arlong being released from Impel Down), 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 Vander Decken IX, the first turning him down after realizing he wouldn't be able to pay for his services, and the second refusing to be the subordinate of another.[5] While Arlong was invading the East Blue, Hody Jones was in the Neptune Army gaining the strength to one day become Arlong's right-hand man.[6]
Conquering the Conomi Islands
The arrival of the Arlong Pirates.
Ten 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 per adult, and 50,000 per child. Bell-mère was only able to pay for her daughters, so Arlong killed her as an example, leaving her adopted daughters Nami and Nojiko alone. Seeing how skilled at map-making Nami was at such as early age, Arlong forced her to become the navigator of his crew. In order to convince her, he struck a deal—if she could somehow gather 100,000,000 and give it to him, he would free her village and 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 an illicit deal with Captain Nezumi of the 16th Branch in order to keep their activities a secret from the rest of the Marines and started mobilizing their forces to conquer all of the East Blue.
East Blue Saga
Arlong Park Arc
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 devising their strategies. Thus, he told Nezumi where Nami kept all of her money and had him confiscate all of it, but Nami and the other villagers instantly realized that Arlong was behind it and finally decided to fight back.
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Translation and Dub Issues
They were the earliest known example of the misnaming of Fish-Men as "Mermen," and many earlier scans exist still baring this mistake. 4Kids themselves used this name for them.
Early One Piece
In One Piece Green: Secret Pieces, an early concept of the Arlong Pirates revealed that Hatchan would have originally been a squid fish-woman named Maple Lead (メイプル・リード,Meipuru Rīdo?). There would also have been a starfish fish-man named Hitodenashi (ヒトデナシ,Hitodenashi?).
↑One Piece Manga and Anime — Vol. 63Chapter 621 (p. 11-17) and Episodes 540–541, Arlong is introduced as he bullies fish-man citizens for signing for Otohime's ideals.
↑One Piece Manga and Anime — Vol. 63Chapter 615 (p. 8-9) and Episode 534, Hyouzou and Decken revealed why they refused Arlong's invitation
↑One Piece Manga and Anime — Vol. 63Chapter 617 (p. 5) and Episode 536, Hody reveals to the Straw Hat Pirates that he is carrying on Arlong's legacy.
100,000 per adult, and 
8,000,000