One fish pretty much looks like another to many humans, especially if they are all the same drab colour, but to the fishes themselves, the differences can be as distinct as night and day. The trick, according to a recent study, is to use ultraviolet vision.
Ulrike Siebeck and colleagues, who published their findings in a recent issue of the journal Current Biology, conducted a series of experiments in which they showed that the Ambon damselfish (Pomacentrus amboinensis) was able to use ultraviolet-reflective facial patterns to distinguish between conspecifics and the Lemon damselfish (P. moluccensis).
Both the Ambon and lemon damselfishes are very similar in colour pattern, but have distinctly different ultraviolet-reflective facial patterns.
Damselfish are territorial and both species are found in overlapping habitats on the reef. The authors observed that conspecifics are attacked more strongly than heterospecifics, presumably because of a higher level of competition for food and mates.
In the first experiment, two similar-sized male fish from each species were placed into plastic tubes and presented simultaneously to a territory owner (male Ambon damselfish) in its natural habitat on the reef.
Using four pairings of ultraviolet-transparent and ultraviolet-opaque filter tubes tested in random order, the authors found that the territory owners attacked their preferred intruder (usually a conspecific) more frequently than the nonpreferred intruder (usually a heterospecific) if the ultraviolet patterning was visible, but not if it was blocked by the ultraviolet-opaque tube lining.
The second experiment tested whether or not the damselfishes were able to use the ultraviolet-reflective facial patterns alone in distinguishing between species. Groups of Ambon damselfishes were trained on test images containing nothing but the facial patterns of Ambon and lemon damselfish faces.
The fishes were trained to nudge either a conspecific or heterospecific test image for a food reward, and then offered a choice of facial patterns. The fishes selected the correct image (the one they were trained on) at least 75% of the time.
The results of the study provide indirect support for the hypothesis that the ultraviolet waveband is used as a "secret communication channel" among some reef fishes.
In the case of the Ambon and Lemon damselfishes, the yellow body colour of both species is relatively well camouflaged when viewed against the brown-dominated background of their habitat, meaning that the facial ultraviolet patterns provide a mechanism for communication without compromising this camouflage.
For more information, see the paper: Siebeck, UE, AN Parker, D Sprenger, LM Mäthger and G Wallis (2010) A species of reef fish that uses ultraviolet patterns for covert face recognition. Current Biology 20, pp. 407–410.