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Imagine an animal with both mammalian and insect sight. A tiny, recently discovered Caribbean shrimp seems to fit the fantasy: its unique eye is partly ‘compound’, with multiple facets like that of an insect, and partly ‘simple’ like that of humans (Journal of Experimental Biology, vol 189, p 213).

Dan Nilsson of Lund University in Sweden and Richard Modlin at the University of Alabama found the peculiar eye in Dioptromysis paucispinosa, a crustacean which lives in shallow waters off Belize. Dioptromysis is, strictly speaking, not a true shrimp but a member of a related order called ‘opossum shrimps’ because females nurture larvae in a brood pouch.

Adult Dioptromysis are about 5 millimetres long and have roughly spherical eyes at the end of eyestalks. The surface of each eye is a honeycomb of hexagonal facets, except at the back, where a single giant facet peers towards the shrimp’s tail.

Apart from the giant facets in each eye, Dioptromysis’s eyes are similar to those of other opossum shrimps. Each has between 800 and 900 normal facets, and beneath every facet is a barrel-shaped light receptor called a rhabdom. However, Nilsson and Modlin have found that the two giant facets focus light onto bowl-shaped retinas made up of 120 long, narrow, closely packed rhabdoms. In effect, the giant facets form simple, camera-like eyes at the back of the shrimp’s compound eyes.

By studying Dioptromysis larvae, the researchers have shown that these simple eyes evolved through a relatively small change in the development of the compound eyes. In a normal compound eye, each facet and its accompanying rhabdom is formed by a group of cells called an ommatidium. The researchers found that each giant facet and its retina develops from 120 ommatidia. All 120 ommatidia produce rhabdoms but only one completes its development fully to form the overlying facet.

Nilsson and Modlin estimate that the giant facets offer more than six times the resolution of the rest of the eye. In fact, even though Dioptromysis’s eyes are small, its visual acuity rivals that of insects with large eyes, such as dragonflies. These have purely compound eyes and have evolved their sharp sight by increasing the size of hundreds of individual facets. Dioptromysis achieves the same clarity by possessing a simple eye behind a single giant facet.

Although the giant facets normally point backwards, videotapes of the shrimp show that Dioptromysis frequently rotates its eyestalks so that its simple eyes are pointing forwards. Nilsson and Modlin suggest this is the shrimp’s adaptation to a dimly lit environment. The rhabdoms behind the giant facets are narrower than those in the rest of the eye and each captures much less light. As a result, the giant facets are about 15 times less sensitive to light than normal facets. Dioptromysis may point its giant facets backwards so that the blind spots they produce in low light cover nothing more important than the shrimp’s tail.