A MYSID SHRIMP CARRYING A PAIR OF BINOCULARS

Like other mysid shrimps, the tropical shallow-water species Dioptromy sis paucispinosa possesses compound eyes of the refracting superpositi on type. In the rear of each eye, pointing backwards, is a single cons picuous facet almost three times the diameter of those in the rest of the eye. Underlying the large facet is an equally enlarged single crys talline cone, projecting an upright image onto a specialized retina of 120 densely packed and extremely narrow rhabdoms. This peculiar arran gement 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 s ix times better than the normal eye, but it requires intensities more than a log unit higher-figures not unlike those of modern compact bino culars. The fact that the acute zone covers a visual field of 15-20 de grees, with a large binocular overlap, strengthens the analogy with a pair of binoculars. The resolution of the acute zone is also remarkabl e in absolute terms: a rhabdom axis separation of 0.64 degrees rivals foveal vision in large insects, although the entire eye of Dioptromysi s measures only 0.4 mm. With the normal posture of the shrimp, the acu te zone points backwards, about 12 degrees above the horizon. Difficul ties in understanding the animal's use of such an acute zone were reso lved by the discovery of large coordinated eye movements. The eyestalk can be rotated around its axis by at least 130 degrees. This makes th e acute zone aim forwards and upwards, which is a more sensible direct ion 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.