The sandlance Limnichthyes fasciatus is a small predatory fish, but its eyes have astonishing similarities to the eyes of land animals - chameleons in particular. Chameleon eyes are famous for being able to move independently of each other, but there are other remarkable features too, also shared by the sandlance, which John Pettigrew of the University of Queensland, Australia and colleagues have been investigating.
Most eyes have the same design (see background box) - and with nearly all fish eyes, light goes through a flat cornea, and is focussed by a spherical lens. The sandlance, though, has a flattened lens, and uses its cornea to do a lot of the focussing. But why?
Chameleons and sandlances hunt in similar ways: they stay very still and wait for their prey (small insects and small shrimp-like animals respectively) to unwittingly get close enough for an ambush. Chameleons strike with their sticky tongues and sandlances with a lightening-fast attack. This strategy means that obvious movements are a bad idea, potential meals will just see you and keep their distance. This is where clever eyes come in handy.
If you hold one finger out at arms length, and one closer to your head, and then look at your fingers while moving your head from side to side, the closer finger moves much more than the distant one. This effect is called parallax, and by moving their heads, animals can use it as a way to judge distances. But moving your head is a dead giveaway if you are waiting in ambush.
By using the cornea for focussing power, the sandlance eye has a peculiar optical ability. Moving its eyes alone, it can use parallax to gauge how far away prey are - without the need for conspicuous movements.
(Current Biology vol. 9, p421-4)
Background
Eyes work by turning the light from objects into signals that can be sent to the brain. They are quite like cameras - light enters the eye through the cornea at the front, passes through the lens, and hits the retina at the back. The retina is like a photographic film - but it converts light into nerve impulses. Human eyes focus with the lens alone - muscles change its shape when we look at close or distant objects. The cornea is fixed in humans, but with sandlances and chameleons, the cornea does the changing (or ‘accommodating’) - with interesting consequences.