The eyes of hyperiid amphipods: relations of optical structure to depth
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In many oceanic hyperiid amphipods the eyes are double structures, with a specialized upward-pointing region covering a narrow field of view. Inter-ommatidial angles were measured across the eyes of 10 species from different depth ranges. With increasing depth (decreasing light) there is a trend towards greater dorso-ventral asymmetry resulting in the separation of the two retinae and an increase in the size of the upper eye. The diameters (D) of the dorsal ommatidia increase with depth, and the inter-ommatidial angles (Δφ) decrease. There are no corresponding changes in the lower eyes. These features can be explained on the assumption that the upper eyes are used to detect small opaque targets against the downwelling light from the surface. In contrast to the resolution of grating-like targets, where the productD Δ φ should increase as light intensity decreases (the ‘eye-parameter’; Snyder 1979), the detection of single targets requires that the ratioD/Δ φ should increase in dim light. Amongst hyperiids this ratio does indeed show a 50-fold increase from surface-to deep-living species, whereas the eye-parameter hardly changes.
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