Study
Image scoring and cooperation in a cleaner fish mutualism
Bshary, Redouan; Grutter, Alexandra S.
Nature, 2006
Cleaner wrasse were offered food representing the honest option — client parasites — and the preferred option, client mucus, whose removal the client resists. Their choices were recorded with and without another client fish able to observe the interaction, and clients were tested for whether they preferred cleaners they had watched behaving well.
- Studied in
- Labroides dimidiatus
- Sample size
- cleaner fish tested with and without an observing client
Cleaners behaved more cooperatively — taking parasites rather than mucus — when an observing client was present, and clients preferred to approach cleaners they had observed behaving cooperatively.
What it means
The authors' reading, and ours. Where they differ, that difference is the point.
How the authors put it
Cleaner behaviour is influenced by bystanders, and client choice based on observed behaviour helps maintain cooperation in the interaction.
How NatureHQ reads it
This is where mutualism stops looking like harmony. The cleaner’s preferred food is the client’s mucus, which the client does not want removed — so the partners want different things, and the honest behaviour needs an explanation. Part of the answer is that other clients are watching, and cheating in front of them costs future custom.
- Aquarium conditions, where an audience is unavoidably present in a way it may not be on a reef.
- Establishes the effect of an observer, not how much of natural cleaning behaviour it accounts for.
What this study is used for on NatureHQ
One study can inform several subjects. Here is everywhere this one is cited.
Mutualism
Mutualism is not harmony. Each partner is in it for itself, the two want different terms, and cheats — partners and outsiders — turn up in nearly every mutualism anybody has examined closely.
Mutualism
A cleaner wrasse would rather eat its client’s mucus than its parasites. It takes the parasites more often when another client is watching — and clients prefer cleaners they have seen behaving well.
Cleaner fish
Cleaners prefer client mucus to client parasites, so honesty needs enforcing. Clients that can go elsewhere leave; clients that cannot chase the cleaner instead — two mechanisms, chosen by what the client can do.
Published by: Nature PortfolioPublished in a Nature Portfolio journal.
Checked against Crossref on 2026-09-28, and they agree on the title, the authors and the year.
NatureHQ summarises research in its own words and does not reproduce published text. Reviewed 2026-09-03.