Parasitic fish actually switch genders when they eat too much

No animal is more defined by what it eats than the cleaner fish. These helpful parasites are all born female, but the biggest eater becomes the dominant male...and then, as you might expect, institutes a strict diet for everyone else.

Cleaner fish do pretty much exactly what you'd expect them to: they swim up to a bigger fish and eat all the parasites on its body. This cleaning service is itself parasitic, but the cleaner fish try to avoid getting carried away and actually biting their client fish - after all, that's a pretty quick way to lose access to all the parasites on the fish's body.

Researchers have previously observed the harsh punishments that male cleaner fish will mete out against females who bite into the host fish. In fact, this is one of the only non-human examples of something akin to a justice system, in which the punished fish adjusts its behavior and offers exemplary service to parasite-rich clients in future. Dr. Nichola Raihani of the Zoological Society of London explains:

"Cleaner fish and humans may not share many physical traits, but cleaner fish punish cheating individuals, just as we punish people who step outside of the law. In both situations, harsher punishment may serve as a stronger deterrent against future crimes."

The assumption had been that the punishment was strictly in response to the loss of food, but new research by Dr. Raihani and her colleagues reveals another, even more serious consideration for the male. If a female cleaner fish eats too much, it too will become a male and compete for the dominant position in the group.

Cleaner fish live in groups of one dominant male and up to 16 females, but only the male and the largest female will take care of cleaning duties, which creates the potentially risky situation for the male. As Dr. Raihani puts it:

"Our research shows that male cleaner fish are sensitive to their female partner's size. One reason for keeping a cheating female in check may be to stop her eating too much and then challenging his position as the dominant male on the reef."

And this isn't the end of the cleaner fish's remarkable story. The researchers now hope to actually examine the cleaner fish "economy" and understand the "market forces" that cause cleaner fish to give better service to certain client fish while providing only basic cleaning to others.

Via Proceedings of the Royal Society B. Image via.