On the functions of double eyes in midwater animals

Midwater predators often have double eyes consisting of a large upward-pointing part with a narrow field of view and high resolution, and a small downward-pointing part with a wide field of view and low resolution. In crustaceans with compound eyes the different eye parts are of basically similar construction, but in fishes the downward-pointing regions may employ unusual optical systems with unknown image-forming capabilities. It has been suggested that the upward-directed parts are used to detect silhouettes of animals against the residual daylight, whereas the lower parts look out for luminescent organisms. Here I calculate the sizes that apposition compound eyes would need to attain in order to fulfil these tasks, and the way that size should vary with depth. It is concluded that silhouette detection is much the more demanding task, and becomes increasingly difficult as light levels decrease. For this reason the upward-pointing parts must increase rapidly with depth. This is not the case with luminescence detectors, where the task is most difficult near the surface because of upwelling background light, and becomes easier with depth. On the whole these predictions fit well with the sizes and shapes of real midwater eyes, especially in the case of the hyperiid amphipods.