There is nothing quite annoying like having a bad itch. Now imagine a really bad itch. Really, really bad. And now imagine, on top of that, that you can’t scratch yourself because you have no hands. A bad situation. I don’t know for sure if fish subjectively feel an itch. But if they do, a large parasite would probably cause such an itch. I certainly know that fish have no hands to scratch. Look at the title picture of this post. Note the parasite below its gill cover! Unpleasant, very unpleasant! And check out this parasite on this goby’s back:
Very, very unpleasant! The goby really looks quite emaciated.
What can these fish do if they can’t scratch? They can visit cleaner stations! On tropical coral reefs, several species clean fishes of parasites. This typically happens around somewhat elevated rocks or coral blocks. Cleaner wrasses service other fish by removing parasites from them. Note the small blue fish on top of the lizard fish and the grouper:
And cleaner shrimp like this one are also parasite removers:
Both partners in this mutualistic symbiosis win: The big fish gets to live parasite free for a certain while, while the cleaner gets a rich meal by eating the parasites.
If you dive with us in Dauin, you will have ample chances to observe cleaning behavior. Ask your dive guide to point it out to you. It’s important to approach cleaning stations calmly in order to not scare off the cleaners and their clients. Then, just hover in place, give the cleaners and its clients some space, and watch. Often the client fish will line up and wait for their turn. The cleaner wrasses swim in a jerking type of way to signal that they are available for clean-jobs. Once they approach a fish, they will scan its whole body for parasites, like in this episode where two butterfly fish visited a cleaner station:
I have on a few occasions seen that pipefish clean bigger fish. In temperate water Australia, clingfish take over the role of the cleaner wrasses. This one is resting on a sponge between cleaner-jobs:
Marine biologists believe that the larvae of many fishes remain in mid-ocean (they live a pelagic life) to avoid parasites. Parasites are more abundant on the reef, and these fishes only settle down on the reef once they reach a certain minimum body size. Very small fishes simply could not cope with the serious threat of parasites on the reef. If that hypothesis is true then parasites have a very fundamental influence on the life cycles of fishes. They are so much more than a mere annoyance, but an important part of the ecology of the reef.