Archerfish eyes must cope with the light spectrum of two very different media. These fish live among mangroves and in rivers, and are renowned for their ability to bring down an insect target from overhanging foliage by spitting a stream of water at their prey, as pictured here. Shelby Temple and colleagues have investigated the visual pigments and spatial resolving power of archerfish eyes, and present the results in the context of the requirements for vision at the water–air interface (S. Temple et al. Proc. R. Soc. Lond. B doi:10.1098/rspb.2010.0345; 2010).
Using microspectrophotometry, the authors find that — as is known in other species — the pigments and spectral tuning vary between different parts of the retina (in this case, subdivided into dorsal, ventro-nasal and ventro-temporal regions). They interpret these variations in terms of the tasks the eye has to perform when operating along three visual axes; that is, three directions in which the eye might look.
A simplified description of their conclusions is that the dorsal retina has maximum spectral sensitivity at 454 and 570 nanometres, a combination that Temple et al. consider is well suited for discriminating between shades of brown, and for identifying objects beneath them. The values for the ventro-nasal retina (visual axis up and behind) are 502 nm and 620 nm. The 502-nm peak is tuned, the authors suggest, for detecting dark images against a background of bright sky, such as the silhouette of an aerial predator.
The ventro-temporal retina presents peak sensitivities at 453 nm, 535 nm and 565 nm. This combination is possibly used for colour vision along the visual axis appropriate for sighting prey against a background of foliage. From video recordings, Temple et al. conclude that this part of the retina aligns with spitting angles, and they estimate that the visual resolution allows an archerfish to tell the difference between two objects 2 mm apart at a range of 550 mm.
Archerfish can be trained to spit at coloured targets. That, say Temple et al., makes them excellent subjects for investigating further aspects of the function of intra-retinal differences.