Sick

When we get sick, we go to the doctor or the medicine cabinet.

When fish get sick, they look for waving antenna. A variety of shrimp and small fish serve to pick parasites off of larger fish. Shrimps specifically will wave their long antenna around in order to advertise that they are there.

Much larger predator fish will swim up and open wide. In this shot, the Red Hind (large red polka-dotted fish) is being "cleaned" of parasites by a pederson shrimp (small blue + translucent) and a neon goby (dark blue tiny slender fish) up top.

It is generally understood that the shrimp will not get eaten while doing its job, but there are simply no guarantees in this world. As an aside, if you gently put your finger near the shrimp, they will climb on and give you a cuticle =)