Eyes on the edge: How archerfish see in and out of water

Archerfish rock.

These little sharpshooters are famous for being able to spit water at an insect, not on the surface of the water, but a good ways above it. And these insects are often camouflaged to boot. Then, they have to catch the insect when it hits the water before other fish get it, or it gets swept away by any water currents.

In other words, archerfish have to calculate, perform precision maneuvers, and anticipate the outcomes of their actions.

This paper, though, looks mainly at the visual problem. Anyone has probably noticed that light behaves differently when it moves through water than when it moves through air. The famous example that every kid has probably asked his parents about is why a spoon in a glass of water looks like the two halves are broken apart.

The authors here looked at the properties of the photoreceptors in the archerfish eyes. Something to remember is that the top of the retina looks down, which for the archerfish means into the water, and the bottom part of the retina looks up, which for the archerfish means looking up and out of the water. Now, it gets a bit sticky because fish have more complicated eyes that we primates do. We have rods and three kinds of cones. The archerfish has rods, single cones and double cones.

First, they found that the three general areas of the eye they looked at, the light-sensing cells of the ventral part of the retina had rather different light absorbing properties than the rest. This correlates with the fish’s visual task: the bottom part of the eye looks up, out of water.

As light goes through the water, the colours change. At the top of the water, the light tends to have shorter wavelengths than the light being reflected from the bottom of the water. The sensitivity of the photoreceptors in those regions matched quite well with the different kinds of light these animals would be seeing.

The highest density of cones is also right in the “sweet spot” where the fish will be looking at its target. The authors don’t use the term “fovea,” but it seems to me that is exactly what it is. And the resolution the archerfish is probably capable of is estimated to be about 8 minutes of arc; by comparison, arthropods eyes tend to be only able to resolve several degrees of arc, and I think humans in fractions of a second of arc.

They interpret these differences across the retina as adaptations to living at the interface, which is perfectly reasonable. The authors suggest that it something that other fish living near the surface have, although one might expect it to be particularly enhanced in the archerfish.

Next steps for this group is to start doing a bucket of behavioural tests to see how well the fish is able to discriminate different colours, shades, and so on. Since the fish has a nice behaviour of spitting at things they see, it should be possible to train them to spit at objects they can see, then tweak the visual stimuli until they can’t do the task any more.

Reference

Temple, S., Hart, N., Marshall, N., & Collin, S. (2010). A spitting image: specializations in archerfish eyes for vision at the interface between air and water Proceedings of the Royal Society B: Biological Sciences, 277 (1694), 2607-2615 DOI: 10.1098/rspb.2010.0345