Many aquatic species are known to produce signals by body reflections of ultraviolet (UV) light. Although the function of these signals is unknown, it is believed that they may be involved in mate attraction, the establishment of social hierarchies and group orientation behaviours such as fish schooling. Now, researchers have discovered a novel mode of visual communication in the razorback sucker (Xyrauchen texanus). The findings were published yesterday in the Proceedings of the Royal Society B: Biological Sciences.
The razorback sucker is an endangered species which is unique to the Colorado River basin. While filming its reproductive behaviour, the team of biologists, from Simon Fraser University in British Columbia, the US Geological Survey and the U. S. Fish and Wildlife Service, noticed that the fish produced unusual flashing signals. Intrigued, they decided to investigate further. They first measured the wavelength of the light used in the signals, then carried out behavioural experiments in fish tanks to try and determine the significance of the signals.
To simulate the natural habitat of the fish, the tanks were lit by direct sunlight and filled with fresh river water every day; the bottom of the tanks was also covered in rocks and gravel from the Colorado River. The researcher used remotely-controlled model fish equipped with white light-emitting diodes to generate signals, and filmed the responses of the razorback suckers to them.
In each trial, groups of 5 razorbacks were placed in a tank, which contained one model at each end. For periods of up to 30 minutes, the models were made to emit flashes of the appropriate wavelength. The flashes were emitted every 30 seconds and lasted 0.5 seconds. It was found that, when both models flashed alternately, males swam closer to the non-flashing model than when neither of the models flashed, suggesting that, in the wild, male razorback suckers emit the flashes to signal territoriality to one another.
In the Colorado river, male razorback suckers rest on the river bed at well-defined distances from one another, while females roam freely through the males’ territories. Males resting on the riverbed are barely visible to roving males or females of the same species (conspecifics). When a resting male spots a roving conspecific male approaching its territory, it quickly rolls its eyes. This exposes the whites of the eyes, which reflect downwelling light (that is, light travelling downwards from the water surface), to produce flashes that signal the presence of the resting male to the rover. The approaching male is thus deterred from wandering any further closer to the resting male’s territory.
A resting male razorback sucker (bottom left) emits a flashing signal to deter a roving conspecific male (bottom right).
In field observations, the flashing signals were produced only when conspecific males, but not females, approached resting males. The flashes have the highest contrast against the background at a wavelength of approximately 380 nanometres (nm, billionths of a metre), which corresponds to the ‘near’ UV region of the electromagnetic spectrum. They are intense, but very short-lived, and therefore maintain communication between rival males while at the same time minimizing the likelihood of detection by nearby predators.
Because the flashing signals are produced by males resting on the river bed, they are visible only from above, and over short distances. And the retina of the razorback sucker is perfectly adapted to detecting these signals. The UV-sensitive cones (the photo- receptor cells) are most sensitive to light with a wavelength of 380 nm, and their distribution is restricted to the dorsal (top) region of the retina; this is where light entering the eye from below would strike the retina. Furthermore, the contrast of the UV signal is enhanced by the adaptation of other cone cells to light reflected by the background and by the head of the fish emitting the flashing signals.
The detection of flashing signals is likely to play an important role in the mating behaviour of the razorback sucker. During the spring runoff, water from the river flows onto the land. This increases the sediment content of the water, making it cloudier, so that the dissolved particles scatter light of UV wavelengths, diminishing the flashing signals. Razorback suckers therefore spawn just prior to the spring runoff; during spawning season, males dwell on the river bed in shallow waters to find females. At other times of the year, however, they remain at depths of more than 20 metres, where there is insufficient UV light for communicative purposes. Thus, it appears that the UV cone system has been preserved throughout the razorback sucker’s evolution solely for the detection of territorial signals during spawning season.
Salmon and goldfish are known to have a similar distribution of UV cones in the retina, and eye roll flashes like those observed here in the razorback sucker have been observed in the turtle. It is, therefore, possible that these species, and perhaps others, use the same mode of visual communication. In some of these species, the UV cone system may be involved in detecting predators which attack from below, and whose skin reflects UV light.
Novalis Flamarique, I., et al (2007). Communication using eye roll reflective signalling. Proc. R. Soc. B. DOI: 10.1098/rspb.2006.0246