Visual Rhythms in Stomatopod Crustaceans Observed in the Pseudopupil

Many aspects of visual function in animals are influenced by the operation of endogenous rhythms. Using techniques of intracellular optical physiology, I investigated visual rhythms in two species of stomatopod crustaceans (mantis shrimps): Squilla empusa, a species active throughout the day and night, and Gonodactylus oerstedii, which is strictly diurnal. Reflectance from within the deep pseudopupil of the compound eyes and its change upon stimulation with light were monitored in individual animals in constant conditions for up to two weeks. Both species expressed circadian rhythms in visual function. In S. empusa, the pupillary response was much stronger during subjective night; little or no response could be elicited during subjective day. In this species, an endogenous rhythm caused pupillary reflectance to increase during subjective day. Rhythms in G. oerstedii were of lower amplitude than in S. empusa and were more difficult to detect. The differences between these species, together with the results of other comparative research on visual rhythms in arthropods, suggest that circadian, rhythmic processes are involved in optimizing nocturnal eyes for maximum sensitivity and dynamic range.