A mysid shrimp carrying a pair of binoculars

Abstract

Like other mysid shrimps, the tropical shallow-water species Dioptromysis paucispinosa possesses compound eyes of the refracting superposition type. In the rear of each eye, pointing backwards, is a single conspicuous facet almost three times the diameter of those in the rest of the eye. Underlying the large facet is an equally enlarged single crystalline cone, projecting an upright image onto a specialized retina of 120 densely packed and extremely narrow rhabdoms. This peculiar arrangement constitutes an acute zone which operates as a simple eye within a compound eye. Calculations of spatial sampling frequency and photon catch indicate that the centre of the acute zone resolves more than six times better than the normal eye, but it requires intensities more than a log unit higher ­ figures not unlike those of modern compact binoculars. The fact that the acute zone covers a visual field of 15­20°, with a large binocular overlap, strengthens the analogy with a pair of binoculars. The resolution of the acute zone is also remarkable in absolute terms: a rhabdom axis separation of 0.64° rivals foveal vision in large insects, although the entire eye of Dioptromysis measures only 0.4 mm. With the normal posture of the shrimp, the acute zone points backwards, about 12° above the horizon. Difficulties in understanding the animal's use of such an acute zone were resolved by the discovery of large coordinated eye movements. The eyestalk can be rotated around its axis by at least 130°. This makes the acute zone aim forwards and upwards, which is a more sensible direction for spotting and pursuing prey. The acute zone is probably held in the rest position aiming backwards only to avoid having an important part of the forward visual field constantly occupied by an eye region of inferior sensitivity.