Okay, crazy theory fun time! This is one I'd been gnawing on before it seemed the joining was imminent: maybe Aladdin gave Jinbe that scar. It could have even happened right after the funeral. Maybe he was upset Jinbe would still work with humans after Otohime's (supposed) assassination by one, how he even wanted to cover it up, and so they had a falling out. He worked under and respected the queen like Jinbe, but he clearly had to take a moment to put aside his anger and help the Celestial Dragon when she asked, and even Arlong asked whether there was some anger pent up in there.
I figured Aladdin should be a bigger part of the story considering how much the flashback spotlighted him, and maybe he and Jinbe, similar in many respects, differed in that he had some resentment he couldn't release and which took over when the queen was killed seemingly by a human, while Jinbe could set it aside for the success of her dream.
Anyway, now that the true killer, Hody, is known, maybe Jinbe wants to find and make amends with him and the other Sun Pirates, tell them that Tiger and Otohime did have it right, and help them find a way to live safe from the Marines.
I could be way off base, but that's crazy theory fun time for ya.
Yay for crazy theories! Anyways, I had the same type of theory but instead of just Aladdiin, I said the whole crew did not like how Jinbe continued to work for humans, especially after the death of their Queen and perhaps caused Jinbe's scar in their fit of rage. And remember Camie did tell Luffy (found here: http://www.manga-access.com/manga/O/One_Piece/chapter/609/7) that there were a lot of detail more to the story that would take too long to explain at the time.