Wrasse Punish Cheaters


The bluestreak cleaner wrasse (Labroides dimidiatus) operates an underwater health spa for larger fish. It advertises its services with bright colours and distinctive dances. When customers arrive, the cleaner eats parasites and dead tissue lurking in any hard-to-reach places. Males and females will sometimes operate a joint business, working together to clean their clients. The clients, in return, dutifully pay the cleaners by not eating them.

That’s the basic idea, but cleaners sometimes violate their contracts. Rather than picking off parasites, they’ll take a bite of the mucus that lines their clients’ skin. That’s an offensive act—it’s like a masseuse having an inappropriate grope between strokes. The affronted client will often leave. That’s particularly bad news if the cleaners are working as a pair because the other fish, who didn’t do anything wrong, still loses out on future parasite meals.

Males don’t take this sort of behaviour lightly. Nichola Raihani from the Zoological Society of London has found that males will punish their female partners by chasing them aggressively, if their mucus-snatching antics cause a client to storm out.


At first glance, the male cleaner wrasse behaves oddly for an animal, in punishing an offender on behalf of a third party, even though he hasn’t been wronged himself. That’s common practice in human societies but much rarer in the animal world. But Raihani’s experiments clearly show that the males are actually doing themselves a favour by punishing females on behalf of a third party. Their act of apparent altruism means they get more food in the long run.

Posted on January 20, 2010 at 1:26 PM9 Comments


Roger January 20, 2010 2:18 PM

Fascinating — but not really surprising. The more realistic of the numerous variants of the “Prisoner’s dilemma” games have generally shown that in any game that is sustained for a large but unknown number of rounds, the best policy is some sort of “punishment with forgiveness.” This example might look like the male wrasse is acting altruistically on behalf of the “client fish”, but it is really just looking out for its own interests as the client fish here is playing the role of the warden in Prisoner’s dilemma; it is infrastructure, not an actor.

We might wonder why only males do this, but the answer is apparent when we read about the unusual sexual biology of this fish. Bluestreak cleaner wrasse are “sequential hermaphrodites”, and whenever a male dies the strongest female undergoes a sex change!

DayOwl January 20, 2010 3:28 PM

Are we to understand that only females violate the contract? If then, why? Is there a remedy? Sometimes good security means looking past the surface.

Juergen January 20, 2010 3:45 PM

As far as I understand the article the males cheat, too, and get away with it because they are bigger.

blue92 January 20, 2010 4:20 PM

Roger: The fun thing here is that human altruism can be similarly deconstructed.

Is the depth of our altruism in comparison to our finny friend in question merely a function of our own social (and probably neurological) sophistication? What looks at first glance like compassion is often empathetic warm fuzzies, quid pro quo, or, as in this case, social structure with means to an end.

Clive Robinson January 20, 2010 5:59 PM

@ Bruce,

“Their act of apparent altruism means they get more food in the long run.”

Actually this may not be the main reason at all.

But one of the number of clients, mating and progeny survival.

If a female cheats regularly then the most usefull clients will take their business else where not just once but permanently. Thus this makes cheating self limiting in many respects. That is unless there are sufficient clients that it does not matter if they never came back detering the “good meal clients” leaves only the “poor meal clients”.

This means the female who cheats is less likly to thrive and thus be a less desirable mate as the progeny will be reduced. Thus it is in the males interest to ensure the female thrives by not allowing her to drive away “good meal clients” future visits for a very short term gain.

Simon January 24, 2010 9:35 AM

There’s also a ‘false cleaner wrasse’ that mimics the appearance and behaviour of the real thing. It uses its disguise to get in close and bite a piece out of ‘clients’ that have been suckered into posing for ‘cleaning’ (it has much larger teeth).

The false cleaner isn’t even a wrasse, it’s a blenny (Aspidontus taeniatus).

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