Fish
Cleaner Wrasse
Labroides dimidiatus · also called cleaner wrasse, bluestreak cleaner wrasse
The cleaner wrasse is a small, electric-blue fish that runs a business. It holds a fixed spot on the reef where other fish — including ones big enough to swallow it whole — line up to have their parasites and dead skin picked off. It advertises with a dance, manages its reputation, cheats when it thinks no one is looking, and, in a much-argued 2019 experiment, passed a test for self-recognition once thought impossible for a fish.
Overview
The cleaner wrasse lives on coral reefs across the tropical Indo-Pacific. It is about 10 cm long, with a bright blue body and a black stripe that widens toward the tail. It eats parasites, dead skin, and mucus picked off client fish.
The cleaning station
The wrasse works a fixed patch of reef where clients — from tiny damselfish to groupers, moray eels, and manta rays — come and pose, and a single cleaner may service over a thousand in a day. It attracts and calms clients with a bobbing dance and gentle fin contact.
Cheating and reputation
A cleaner would rather eat a client’s nutritious mucus than a parasite, so it sometimes cheats. Clients chase cheats or switch stations, and cleaners work more honestly when other clients are watching. A blenny, the sabre-toothed blenny, mimics the wrasse to sneak in and bite chunks out of fins.
The mirror test
In 2019, a cleaner wrasse marked with a gel it could only see in a mirror tried to scrape the mark off — a response taken as evidence of self-recognition, which had rarely been shown outside large-brained mammals and birds. Whether it means the same thing in a fish is still argued.
Status
The cleaner wrasse is Least Concern, common and widespread. Removing it from a reef patch leaves fewer fish and less variety.
Cleaner Wrasse: common questions
What actually happens at a cleaning station?
A client fish arrives at the wrasse's patch of reef and holds still, often adopting an odd head-up or head-down posture and flaring its gills and mouth to signal that it wants to be cleaned and will not attack. The cleaner wrasse then works over the client's body, moving into the gill chambers and even inside the open mouth, picking off parasites such as isopods and copepods, along with dead skin, fungus, and mucus. Large predatory clients wait their turn in a loose queue and do not eat the cleaner. When it is done, the client shudders or changes colour and swims off, and the next one moves in.
Does the cleaner wrasse really recognise itself in a mirror?
That is the claim, and it is debated. In the 2019 study, cleaner wrasses shown a mirror first reacted as if to another fish, then settled down, and when researchers put a coloured mark on a spot the fish could only see in the mirror, the wrasses spent time positioning themselves in front of the mirror and then scraped that body part against rocks and sand, apparently trying to remove the mark. They did not do this for a colourless mark or without a mirror. Some scientists accept this as passing the mark test, a classic measure of self-awareness; others argue the fish may simply be responding to an irritating visual stimulus without any sense of self, and that the test does not mean the same thing in a fish as in an ape.
Is the relationship good for the client fish?
Mostly, but not perfectly. Clients that visit cleaners carry fewer parasites, heal better, and are less stressed, and reefs from which cleaner wrasses have been removed end up with fewer fish and less variety, so the service clearly matters. However, the cleaner would rather eat the client's nutritious mucus than its parasites, so it is always tempted to cheat, and a bite of mucus is a small injury to the client. The system stays cooperative because clients monitor the service they get, remember which cleaners are reliable, and impose costs on cheats, which keeps most cleaners mostly honest.
How does the sabre-toothed blenny fool other fish?
The false cleanerfish, a type of blenny, has evolved almost the same electric-blue body and black stripe as the cleaner wrasse and even copies its distinctive bobbing swimming motion. Client fish approaching what they think is a cleaning station let the blenny come close, and it then lunges and tears off a piece of fin, skin, or scales and flees. Because clients learn to be wary at stations where they have been bitten, the mimic does best where real cleaner wrasses are common enough to keep clients trusting, and it tends to attack from behind or target fish that are new to the area.