Cleaner wrasse plan ahead for the future

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Service quality in cleaner wrasses (Labroides) differ according to size of the home range and the chance of future encounters with the same client, according to several studies published recently or to be published in the upcoming months.

Cleaner wrasses feed on and remove ectoparasites from client fish, but prefer to cheat by feeding on client mucus instead of parasites, leading to conflict. 

A study published in a recent issue of the journal Ethology by Jennifer Oates, Andrea Manica, Redouan Bshary and another to be published by Suzanne Mills and Isabelle Côte in an upcoming issue of the Proceedings of the Royal Society B: Biological Sciences have discovered that the propensity of the cleaner wrasses to cheat on clients is largely related to the size of the cleaner wrasse's home range. 

Both studies demonstrate that the Bicolor cleaner wrasse (Labroides bicolor) is more likely to take a bite out of the clients and roves over a much larger area when performing its cleaning duties than its better-studied cousin, the Bluestreak cleaner wrasse (Labroides dimidiatus), pictured above.

A third study, published in a recent issue of the journal Current Biology, has shown that cleaner wrasse are capable of planning ahead for the future in the manner of humans. The term “shadow of the future” is used in game theory (a branch of applied mathematics most used notably in economics) to describe the way humans behave to another depending on the likelihood that they will meet again.  If a future meeting is more likely, they are more cooperative and if less likely, they tend to cheat.

Jennifer Oates, Andrea Manica, Redouan Bshary have also discovered that the Bicolor cleaner wrasse also responds to the "shadow of the future" in the manner of humans.

The much larger cleaning station that the Bicolor cleaning wrasse occupies is predicted to destabilise cooperative behaviour and lead to more conflict because the frequency of repeat interactions between clients and cleaners is reduced. 

The authors test this hypothesis with a field survey in Mooreea Island in French Polynesia, examining ten adult wrasse for eight 30-minute sessions and recording the species of client, size of client, duration of interaction in seconds, number of jolts by client (jolting occurs when the cleaner wrasse cheats and takes a bite of the client’s mucus) and whether or not the client terminated the interaction in response to cheating by chasing or swimming off.

The results indicated that the cleaner wrasse are more cooperative and more likely to feed on ectoparasites in parts of their home range that they spend the most time (because they are more likely to encounter the same client fish there). Conversely, in areas where they visit less often (and are thus less likely to encounter the same client fish), the wrasses cheat and feed on mucus more often.

These results suggest that cleaner wrasse take future, rather than immediate, consequences into account when deciding on how to behave towards a client fish.

Why do client fishes still visit Bicolor cleaner wrasses despite their penchant for cheating? According to Mills and Côte, this is probably because "the cost of being cheated by a more dishonest cleaner… may still be lower than the costs of travelling (eg. time, aggression by other territorial fish, intrusions onto temporarily abandoned territories) to visit a more honest cleaner…Thus, to be cleaned by a cleaner that sometimes cheats may be a better option than not to be cleaned at all."

For more information, see the papers:
Mills, SC and IM Côte (2010) Crime and punishment in a roaming cleanerfish. Proceedings of the Royal Society B: Biological Sciences doi: 10.1098/rspb.2010.0941

Oates, J, A Manica and R Bshary (2010) Roving and service quality in the cleaner wrasse Labroides bicolor. Ethology 116, pp. 309–315.

Oates, J, A Manica and R Bshary (2010) The shadow of the future affects cooperation in a cleaner fish. Current Biology 20, pp. 472–473.