There are many examples of cooperation between species, however, one of the better examples is the relationship observed between marine ‘cleaner’ fish and their ‘clients’. There are many species of cleaner fish such as the wrasse from the genus Labroides, or the gobies from the genus Elacatinus. Such cleaner fish typically inhabit the coral reefs of the Indian, Pacific or Caribbean oceans and will clean and remove parasites from other marine fish.

The cleaner fish set up ‘cleaning stations’ which other fish like the Molly Fish will visit to be removed of parasites, the cleaner fish will even clean predatory fish. Nearly all cleaner fish nutrition is obtained through removal of parasites. The cleaning process is beneficial for both the cleaner and the client (the visiting fish species), the cleaner gets a meal and the client is removed of parasites which can decrease fitness.

The cleaner fish is described as a parasite and the client fish described as the host. The relationship between this cleaner and its client is not entirely symbiotic, if the client lacks the ectoparasitic Gnathiidae species which the cleaner typically feeds on, then the cleaner fish may consume client mucus. Whilst consumption of Gnathiidae is beneficial to the client, consumption of mucus is not. As a result an interesting relationship between client and cleaner species forms.
Another clear example is between snakes, the environment also affects a lot if you find the best snake terrarium, your chances of two different species coexisting will increase considerably.

Client – Cleaner Relations

Cleaner fish will happily consume Gnathiidae parasites on client fish as this constitutes as nutrition for them, however, it is much easier for the cleaner to consume the mucus of the client species. Consumption of mucus is costly to the client however, as a result client species will attempt to defer mucus consumption by the cleaner by the ‘threat of reciprocity’. A cleaner fish which consumes mucus is essentially cheating in the cooperative relationship, as a result, predatory clients will not hesitate to consume cheating cleaners (they do not normally consume them as the benefit provided by the cleaners is much greater than the nutrition they would provide).  As a result, a cleaner will almost never cheat on a predatory client.

Non-predatory client species are unable to provide a threat of reciprocity. Instead, they employ one of two methods if a cleaner fish is caught cheating, either:

1. Partner switching – If the cleaner fish cheats on the client by consuming mucus instead of ectoparasites the client fish will simply leave the cleaning station and search for another
2. Punishment – Cheating cleaning fish may also be chased away by the client

In both of these cases, an immediate cost is incurred to the client as they lose a cleaner. The choice of strategy adopted depends on the current availability of alternative cleaners in the vicinity.

The Effect of Choice on Cheating

Cleaner fish set up their stations on coral reefs; it may be the case that there is only one station in the vicinity. If there is only one station then there is only one choice of cleaner i.e. no choice in cleaner. Where there are multiple cleaning stations there are more cleaners to choose from and thus client species are able to exhibit choice.

When there is only one cleaning station, it is not possible to employ the partner switching technique on cheating cleaner species. As a result clients with no choice in cleaner are more likely to display aggression. Upon the next encounter with the cleaner, it is likely they will remember the client’s display of aggression and thus be less likely to cheat i.e. a greater level of cooperation. It was shown that clients without choice were around 3 times as likely to display aggression when compared to those with choice. This is simply because clients with choice are able to partner switch and do not need to resort to aggression.

Clients express jolting behaviour towards a cheating cleaner to indicate irritation. It was shown that following displays of aggression in this manner, cleaners were much less likely to display cheating behaviour.

These observations were taken in the wild, but an experiment has been conducted in laboratory conditions which showed the same thing:

• Two treatments were set up in an aquarium, glass dishes were decorated to display different client species
• On each dish there were both prawns (used to indicate mucus) and fish scales (indicate typically ectoparasite situation). If the cleaner consumed prawns, this was the equivalent of displaying cheating behaviour in the wild
• Before any form of learning behaviour, the cleaner fish preferred to consume the prawns
• A teaching period involved either emulated fleeing or punishment when prawns were consumed by the cleaner fish
• Following the teaching period, the cleaner fish expressed preference for the fish scales over the prawns showing they had adopted cooperative behaviour following punishment or fleeing by the ‘client’

Image Scoring by Eavesdroppers

It is suggested that client fish image score their potential cleaners by watching their interactions with other clients. Image scoring in this case, is the determination of whether the cleaner fish will cooperative or be likely to cheat. By eavesdropping (observing the third-party interactions) the client is able to provide direct benefits to itself as it can determine whether the cleaner is likely to cooperate. It is thought that the presence of other clients or ‘bystanders’ during the cleaning process will alter the behaviour of the cleaner fish. This is known as the audience effect and in this example; the cleaner fish will be less likely to cheat when influenced by an audience.

It has been show in experiments (by mimicking cooperative and un-cooperative cleaner fish) that client fish species are more likely to stay near cooperative cleaners, spending around 60-80% of their time near these cleaners. It was also shown that any bystanders who observe un-cooperative behaviour (cheating) in cleaner fish will swim off and find another cleaner fish (as long as choice is available).

Cooperation between Cleaner Fish

Male and female cleaner fish often form stable mating pairs, these pairs will clean client species together. If either member of the pair cheats then it is highly likely that the client will leave.  Therefore to retain the client fish, both the male and female must behave in a cooperative manner – that is consume the ectoparasites and not the mucus of the client.

This is reminiscent of the prisoner’s dilemma in that:

• Should both the cleaners cooperate they both receive a reward in the form of a meal
• If one defects (i.e. consumes mucus) then the temptation payoff (the defector’s payoff) is they got a mucus meal, the suckers payoff (the co-operators payoff) is that they get no meal
• Should both defect, the punishment payoff is that they have to wait for their next meal – however it is here where they situation differs slightly from a typical prisoner’s dilemma scenario as both of them essentially received a temptation payoff as they both got a mucus meal

It has been shown though, that despite the temptation of defecting, cleaner fish which work in pairs are more likely to cooperate with the client than cleaner fish who work alone.

Cooperation between cleaner fish occurs because the males punish females if they cheat i.e. they police the female’s behaviour. When males are allowed to punish females, it has been shown that the females are less likely to cheat. Males are willing to police female behaviour, because by doing so they essentially are allowing themselves more to eat as if the female behaves, they are less likely to lose their client.