Cleaner fish are undeniably photogenic. They are colourful little fish that much larger fish will sit still for and allow the cleaner to dart all around them, even into the waiting jaws of their “clients,” while the cleaners feast of many small surface parasites. Cleaner fish provide one of the classic examples of mutualism in the animal kingdom, proving that evolution does not always mean bloody competition.
How do “client” fish recognize cleaner fish from a quick snack, particularly given that there are many different species of cleaners? Karen Cheney and colleagues tackle the problem not just from many different angles, but from many different lighting conditions.
First, they found all cleaner fish had either blue or yellow in their colouration, compared to only about 2/3 of a control group of fishes. They also confirmed a previous finding that all cleaner fish had a long stripe on the side.
But why blue and yellow? Is there any particular reason for these particular colours? It is just evolutionary happenstance, where one random colour became established, and later, all the others without it were selected against?
Cheney and colleagues hypothesized that these colours are used because they stand out strongly against the background. Demonstrating this is tricky, because what we see as a colour in a nice, brightly lit aquarium is not necessarily what another fish will see in the natural habitat, for several reasons. First, fish may not see colour like we do. Second, water filters out different wavelengths of light, so the colours change as you descend. Third, the major light source is overhead, whereas aquaria let in light from all sides.
To test this, they did some computer modeling using the known visual properties of several fish species, and seeing how they responded to different colours in front of an average coral reef background. Blue, of almost any shade, came out as the most conspicuous for all species for fairly long distances. Yellow was particularly conspicuous against black – such as the long black stripes that cleaner fish have. It was also highly conspicuous against blue water, the other major visual element in a coral reef habitat.
To top this all off, Cheney and company took some fish models, painted with different colours and patterns, into coral reefs. One was a realistic representation of a local cleaner fish, and the others had some variation of colour pattern. Just knocking out blue from the model significantly dropped the response of client fish (figure shown; click to enlarge).
Blue alone is not enough to attract clients, as several models with blue colours in the wrong places fared no better than the blue-less but otherwise realistic models. The ever more radical departures from the realistic colour scheme showed even more declines in client responses, but none of the “duds” were significantly different from each other.
The authors suggest that cleaner probably evolved before the conspicuous colouration, which is a sensible hypothesis. Just because something is conspicuous does not guarantee a client’s appropriate response. In fact, it is difficult to imagine a scenario where a highly visible colour preceded the evolution of the cleaning behaviour.
This research does a very clean job of examining the signaling end of partnership between cleaner and client. Hopefully, someone will pick up with this on the receiver end, and start examining the sensory capabilities of the client fish that enable them to recognize cleaners and react appropriately to those signals.
Cheney, K., Grutter, A., Blomberg, S., & Marshall, N. (2009). Blue and Yellow Signal Cleaning Behavior in Coral Reef Fishes Current Biology DOI: 10.1016/j.cub.2009.06.028
Picture: New Scientist gallery