I've been thinking about this for quite some time:
A lot of you might have already noticed, but the red tattoo Arlong has on his chest is quite similar to the one Hacchi has on his forehead. Kuroobi has an apparently similar one on his chest too: too bad it's always covered by his shirt….>_<
Judging only from these hints, I had the idea of that being the symbol of Jinbei's crew (Just like Bellamy sported DoFlamingo's symbol), but since only Arlong and Hacchi had it, id didnt seem enough proof to me.
BUT, yesterday I was looking into Nami's flashback: when Arlong and his cohorts are checking Bellemer's house, you can notice the drawing on Kuroobi's hat....it's the carbon copy of Hacchi's tattoo.
Now, this doesn't say a whole lot, but it doesn't seem such an absurd theory either, right? That symbol might be reserved for high ranking officers; Chuu might not have it because he's a recent addition to the crew (or perhaps he has it somewhere on his body, we just havent seen it...Chuu is dressed too much)
The tattoo resembles a sun, but with some imagination it might represent a crab too...at least I see it that way...
P.S.: I've just realized something: the fact that Arlong's tattoo has more "spikes" than Kuroobi's or Hacchi's might be an indicator of his higher status within the crew....I'm just throwing around ideas, anyway.....