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The real science · cleaner wrasse

A service economy on a coral reef.

The cleaner-wrasse mutualism is one of the best-studied cooperation systems in nature: a marketplace with clients, cheating, punishment, and reputation. Here's the science behind each mechanic.

Two thousand clients a day

The bluestreak cleaner wrasse (Labroides dimidiatus) holds a small territory, a "cleaning station," where other fish come to have their ectoparasites picked off. A single cleaner inspects on the order of 2,300 clients per day and eats over a thousand parasites doing it1. Clients queue, pose with fins spread, and hold still for a stranger's mouth. Both sides profit, as long as both sides behave.

The temptation

Here's the catch that makes it a cooperation problem: given the choice, cleaners prefer client mucus over parasites2. Every honest inspection is a small act of self-restraint. Cheating (biting mucus) makes the client visibly jolt, which is what makes the system measurable in the field.

Partner control

What keeps cleaners honest is that clients have options. Resident clients with small home ranges are stuck with their local cleaner, and they respond to cheats by punishing (chasing the cleaner around the coral, which costs it foraging time). Visitors with large ranges simply leave and take their custom to a rival station3. Cleaners essentially never cheat predatory clients, where the "punishment" would be getting eaten.

Reputation

Bystander clients observe ongoing interactions and use what they see. In experiments, clients preferentially approached cleaners they had watched cooperating and avoided ones they had seen cheat4. A cleaner's honesty toward the client in front of it is simultaneously an advertisement to everyone in line, which is why, in the game, cheating with a bystander present costs future customers.

The massage

A cleaner can also hover above a client and drum its pelvic fins along the client's back, a "massage" that provides no food. Experiments show cleaners use it strategically to entice new partners, calm predators, and reconcile with clients they've just cheated 5. The touch itself is a genuine service because it measurably lowers client stress hormones6.

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References

  1. Grutter, A.S. (1996). Parasite removal rates by the cleaner wrasse Labroides dimidiatus. Marine Ecology Progress Series 130, 61–70. doi:10.3354/meps130061
  2. Grutter, A.S. & Bshary, R. (2003). Cleaner wrasse prefer client mucus: support for partner control mechanisms in cleaning interactions. Proceedings of the Royal Society B 270 (Suppl.), S242–S244. doi:10.1098/rsbl.2003.0077
  3. Bshary, R. & Grutter, A.S. (2002). Asymmetric cheating opportunities and partner control in a cleaner fish mutualism. Animal Behaviour 63, 547–555. doi:10.1006/anbe.2001.1937
  4. Bshary, R. & Grutter, A.S. (2006). Image scoring and cooperation in a cleaner fish mutualism. Nature 441, 975–978. doi:10.1038/nature04755
  5. Bshary, R. & Würth, M. (2001). Cleaner fish Labroides dimidiatus manipulate client reef fish by providing tactile stimulation. Proceedings of the Royal Society B 268, 1495–1501. doi:10.1098/rspb.2001.1495
  6. Soares, M.C., Oliveira, R.F., Ros, A.F.H., Grutter, A.S. & Bshary, R. (2011). Tactile stimulation lowers stress in fish. Nature Communications 2, 534. doi:10.1038/ncomms1547