When do client reef fishes know that they have been cleaned of ectoparasites by cleaner gobies (Elacatinus spp.)? When they get a jolt from a bite, according to a paper by Marta Soares and co-authors published in the most recent issue of the journal Ethology.
The authors compared the cleaning behaviours of the two most common cleaning fishes, the Cleaner wrasse (Labroides dimidiatus) and the cleaner gobies (Elacatinus spp., also known as neon gobies), and noted that the Cleaner wrasses shows a propensity to cheat and take bites of scales and mucus of its clients.
In response, the clients of Cleaner wrasses ensure compliance by threatening to reciprocate (e.g. to eat the cleaners) or the wrasses (which frequently work in pairs) police themselves by switching partners or aggressive chasing when one of them cheats.
Although scales and mucus have been found in the guts of cleaner gobies, the behaviours associated with dishonest cleaning have not been observed in cleaner goby associations. This led the authors to believe that the cleaner gobies prefer to feed on the ectoparasites rather then the mucus of its clients (and not the other way round as in the Cleaner wrasse), and they performed a series of experiments to test this hypothesis.
The authors conducted their study using the Broadstripe cleaning goby (E. prochilos) collected from Barbados; seven males and six females were caught and maintained in laboratory aquaria for their experiments.
To test the food preference of the gobies, the authors collected ectoparasites (gnathiid isopods and caligid copepods) and mucus from wild-caught reef fishes (mostly parrotfishes).
They then attached the food items to Plexiglas plates using boiled flour and water as a glue and offered the plates to the gobies that had been acclimated to the aquaria. In each trial, the authors randomly placed four ectoparasites (caligid copepods and gnathiid isopods), two items of mucus and two items of boiled flour (serving as a control) on a grey gridded plate. They then used a video camera to record the feeding behaviour of the gobies.
When only the first two items eaten were considered, the authors found that the gobies ate all the items (ectoparasites, mucus and boiled flour) with equal frequency.
However, when the first four items eaten were considered, the authors found that ectoparasites were eaten more often than either mucus or boiled flour. This preference for ectoparasites by the cleaner gobies reduces the potential for conflict with clients and may explain the large differences noted between the two cleaner fish systems in partner control and manipulation behaviours.
Although cleaner gobies do cheat on occasion, they appear to do so only when the supply of ectoparasites on the client fish is exhausted.
An infested client thus faces a low risk of being cheated on at the beginning of the a cleaning session, but the first bite by the cleaner is a reliable indication that the ectoparasites have been cleared and that it is time to leave the cleaning station, write the authors.
For more information, see the paper: Soares, MC, IM Côté, SC Cardoso, RF Oliveira and R Bshary (2010) Caribbean cleaning gobies prefer client ectoparasites over mucus. Ethology 116, pp. 1244–1248.