How are mutualisms maintained when there is so much incentive for partners to cheat?? Do species interactions shift from cooperative to antagonistic or vise versa? If so, how? I’m very fascinated by these questions, as many ecologists are. In my previous post, I wrote about crayfish-worm symbiosis and how their interactions could shift from mutualism to parasitism, depending on the worm abundance.
Today, I’ll talk about the work by Gingins and colleagues on the cleaner-client fish mutualism. A cleaner fish, in this case Labroides dimidiatus, cleans a ‘client’ fish by eating the parasites that live on the client’s surface (ectoparasites). These “good” cleaners can sometimes be bad for the clients by eating client’s mucus, a preferred food to ectoparasites. Clients cannot eat a cleaner so the exploitation is a one-way direction. Because of this asymmetrical dynamic, clients face the challenge of maintaining a good cleaning service to remove ectoparasites, but not mucus. So how do they do it? One hypothesis is that cleaners quickly shake off the cleaners and terminate the interaction as soon as they’re exploited. They predicted a cooperative outcome if: a) “the temptation to cheat is low enough” and b) “the threat of termination is strong enough”. This makes sense to me. Let’s anthropomorphize for a second: If you’re in a relationship, you wouldn’t cheat if a) there is no temptation presented that could distract you from your partner, and b) there would be serious consequences of cheating (e.g. a breakup).
What they did:
To test whether clients’ threat to terminate the interaction explains suppression of exploitation by the cleaners, they conducted a lab experiment manipulating two factors: food quality (preferred vs. less-preferred) and time to respond to exploitation by a host (quick vs. slow motion). Because it’s complicated to conduct an experiment using a client without introducing confounding factors (e.g. client’s size, behavior), they prepared Plexiglas plates as a client. They varied the proportion of preferred vs. less-preferred food (high or low). They also simulated the response of a client fish by either quickly (<1 sec) or slowly (2-3 sec) removing the plate from a tank when a cleaner feed on a preferred food. Additionally, they performed the same experiment using a closely related non-cleaner fish (Halichoeres melanurus) to compare their results with cleaner fish.
What they found:
Cleaners altered their behavior depending on the food quality and the time of the host’s response to exploitation. When the proportion of less-preferred food was high, they ate more low quality food than expected by random chance. This was true regardless of a ‘client’ response (i.e. how fast a plate is removed). However when there are higher proportions of preferred food on a plate and a host responds slowly, they eat significantly more good quality food than expected from random foraging (i.e. exploitation). However, if a ‘client’ responds fast, then clients eat more of the less preferred food than expected (i.e. cooperation). So the results suggest that high response is a good mechanism to stop exploitation.
Non-cleaner fish, on the other hand, always ate preferred food regardless of different treatments: This makes sense because they feed on small invertebrates found on substrates in nature so they don’t have to adjust their feeding behavior according to their prey. Cleaners alter their behavior depending on the food quality and the time host responds to exploitation.
There are many hypotheses to explain the mechanisms of maintaining mutualism (e.g. sanction, punishment, and host switching) and there are an increasing number of experiments to test these hypotheses. This study is one of them.
It’s a fairly simple experimental design but they addressed the hypothesis to explain how mutualism can be maintained in clean-client interactions. Plus, as a plant person, it’s nice to change it up and read about charismatic animals like pretty fish!
Gingins S, Werminghausen J, Johnstone RA, Grutter AS, Bshary R. 2013. Power and temptation cause shifts between exploitation and cooperation in a cleaner wrasse mutualism. Proc Biol Sci. doi: 10.1098/rspb.2013.0553