The world is filled with polarized light, but unfortunately we're oblivious to it. While we have to accept the limitations of our visual capacities, some birds and insects eagerly exploit their ability to see polarized light. But can any animals actually discriminate between different orientations of polarized light? Martina Mussi, Theodore Haimberger and Craig Hawryshyn set out to test whether damselfish can detect differences in the angle, or e-vector, of UV polarized light(p. 3037).

Hawryshyn and his colleagues trained damselfish to swim towards a particular e-vector orientation (0° or 90°) of UV polarized light. When the team gave the trained fish a choice between two light beams with 0° and 90° e-vector orientations, they found that damselfish chose the`correct' beam over 80% of the time. What's more, fish made the right choice regardless of brightness differences between the two beams, so they don't simply rely on differences in light intensity. The team concludes that damselfish can clearly discriminate between the horizontal and the vertical plane of UV polarized light. But when the team filtered out the light beams'UV wavelengths, the fish chose randomly, suggesting that damselfish need UV light to distinguish between different e-vectors.

How might damselfish use this curious ability? It could give them an edge when they're hunting transparent plankton in their underwater homes; plankton exoskeletons scatter polarized light, so hungry damselfish with an eye for different e-vectors may find it easier to spot their bite-sized meals.

Mussi, M., Haimberger, T. J. and Hawryshyn, C. W.(
). Behavioural discrimination of polarized light in the damselfish Chromis viridis (family Pomacentridae).
J. Exp. Biol.