Cleaner Shrimp's Black Belly

[Delphinus]

I think food can sometimes be seen, and that's definitely dark.

If it is eggs, they are between the swimmorets, and under close scrutiny should be obvious as a mass of little itty-bitty spheres.

I don't know, I guess I just don't have the power of imagination going here, or maybe the opposite, too much, but it just seems to me it could be more than one thing based on the description so far.

A picture would totally help clear things up! Hint, hint.