structure and sensitivity of the eye of different life history stages of the ontogenetic migrator Gnathophausia ingens

The structure and ultrastructure of the photoreceptors of several life history stages of the ontogenetically migrating lophogastrid crustacean Gnathophausia ingens were examined. The younger instars of this species live in a much brighter light field than the older instars, and this difference is reflected in differences in their visual systems. The shallowest free living individuals (instars 3 and 4) possess a superposition eye with almost no clear zone, which minimizes the light shared between ommatidia and reduces the sensitivity of the eye. A progression to superposition optics with a large clear zone, usually associated with night-active or deep-living species, occurs as the animals move deeper in the water column. Regional differences within the eye are also evident, with a largely nonexistent clear zone in the dorsal region and a large clear zone in the ventral region in the eyes of instar 5 animals, the first instar to move to deeper depths. The deepest living instars (10-12) possess superposition optics with a large clear zone throughout the eye, and are significantly more sensitive to light than the younger, shallower instars.