#cleanerfish

This cleaning service is a form of symbiosis, a mutual relationship between two animals in which both animals benefit. In this case, the fish receiving cleaning, known as 'clients', benefit from the removal of harmful ectoparasites they couldn't remove themselves while the cleanerfish derive a variety of benefits. Beyond the free (and energetically cheap!) food supply their clients provide, cleanerfish are rarely predated upon by other animals (as the service they provide is more beneficial) and the presence of large fish such as sharks further deters predation on the cleanerfish.

(See how cleanerfish services are so valued, that they're perfectly comfortable swimming into a predator's mouth for a spot of cleaning!)

However, the most fascinating part of cleanerfish isn't just the service they provide but that many species set up shop within coral reefs, marking 'cleaning stations', each receiving over 2000 clients a day, that animals will travel long distances to for cleaning (even waiting around patiently for other clients to be cleaned like a queue). These cleaning stations are run like a business and cleanerfish will employ many strategies to maximise the benefit they get from the service they provide, many of which mirror business tactics used by humans!

In Part 2, I will go through some of these strategies and show you what intelligent fish this group is!

Some handy references:

Grutter, A. (1999) 'Cleaner fish really do clean', Nature, 398, pp. 672-673.

Grutter, A. (2010) 'Cleaner fish', Current Biology, 20(13), pp. 547-549.

The mutualistic relationship between a cleaner and client, much like a business and its customers, is built on mutual cooperation in which both animals behave in a way that benefits the other and must maintain trust that neither member of the agreement will defect in order to get a little extra from the interaction. Here, cleanerfish cooperate by only cleaning the client and clients cooperate by not eating the cleanerfish. Despite how peaceful this sounds, mutual cooperation between animals often spawn conflicts of interest.

There's one major conflict of interest between cleaner and client: dead skin and ectoparasites aren't the most nutritious food source. For this reason, cleanerfish sometimes 'cheat' their clients while cleaning, instead biting off a piece of protective mucus or healthy tissue during cleaning. The fish get more energy from cheating this way but hurt their client who might chase them around in retaliation or, if they're big enough, just make the cleanerfish their next meal. (This is an example of how mutual cooperation is maintained: defection - or 'breaking the agreement' - is punished to discourage future defection.)

However, cheating isn't just punished by the client. Other clients, seeing a client get bitten by the cleanerfish, may also clear off to avoid getting hurt themselves, meaning the cleanerfish misses out on a large gathering worth of tasty pickings. Not to mention, there's often never just one cleaning station in a coral reef and, like stalls at a market, cleanerfish are constantly competing with each other for clients. Any cleanerfish renowned for biting their clients won't get much business compared to their peers and the threat of a client taking their business elsewhere is a good motivator to adhere to the agreement.

Therefore, cleanerfish are often cited in behavioural ecology as an excellent example of game theory, a branch of mathematics based on investigating strategic decision-making in interactions (or 'games') where the outcome of a specific decision for an individual is determined by the other participant's decisions.

In this case, the cleanerfish could cheat every client but they would run out of clients and constantly risk punishment, including being eaten by a larger predator. Of course, they could also never cheat clients but then they don't maximise energy gain for themselves. However, by factoring in how the clients will 'play the game', they can select when and who to cheat strategically in order to both maximise profit and minimise risk. This may sound far too complex for a group of fish to calculate but cleanerfish have shown many business strategies that adhere to the principles of game theory:

Cleanerfish can recognise up to 100 clients at once and can categorise them to determine which groups of clients are the least risky to cheat. For example, cleanerfish are less likely to cheat 'local clients' with home ranges near the cleaning station as they are more likely to visit regularly.

Cleanerfish are also more likely to cheat clients that don't have the option to switch cleaning station where the risk of losing a client is practically non-existent.

Social recognition (essentially your reputation among other animals) is highly valuable to cleanerfish. Visiting clients will eavesdrop how local cleaners behave and only interact with cleaners that cooperate so cleanerfish manage their reputation carefully, cooperating more often in the presence of high-value eavesdroppers (those with heavy parasite coverage) they're hoping to draw in and selecting when they can cheat a client without too many potential clients noticing.

If a client reacts negatively to being cheated, cleanerfish will give those clients a 'premium' service in exchange, adding a sort of massage that convinces them to return. Cleanerfish may also give a higher quality service to clients they've recently cheated to compensate for their previous experience.

Cleanerfish have also been observed using these massages to build relationships with new clients and prolong the time a client sticks around for cleaning.

Some handy references:

Bshary, R., Würth, M. (2001) 'Cleaner fish Labroides dimidiatus manipulate client reef fish by providing tactile stimulation.', Proc. Biol. Sci., 268(1475), pp. 1495-1501.