Grutter told IFLScience she thought this was because it was a harder test, which the fish couldn't manage when their bodies were busy fighting off parasites. Even internal parasites were higher without cleaners, as the immune system of the fish had to fight on two fronts at once.
These heavily infected fish came from reefs deliberately cleared of cleaners for research purposes, but Grutter told IFLScience cleaner wrasse are popular in the aquarium trade. Since they seldom thrive in tanks, this has affected cleaner numbers in the wild, suggesting there could be a lot of clients out there struggling to learn.
Humans may not be immune. Children in developing tropical regions are often infected, and some aid workers regard parasite removal as one of the best ways to improve educational performance.