Eye size of pelagic crustaceans as a function of habitat depth and possession of photophores.

Eye diameter, interommatidial angle, and rhabdom dimensions were measured for a variety of crustacean species differing in habitat depth and bioluminescence ability. Eyes are smaller and eye growth rates are lower at greater depths for species in five of the six families examined, and photophore-bearing species tend to have larger eyes than relatives which lack photophores. Rhabdoms are smaller and interommatidial angles are larger in small eyes, factors which, with reduced aperture size, are generally associated with decreased visual sensitivity and acuity. This suggests that the eyes of many deep-sea crustaceans are poorly suited to a dimly lit environment; however, the small eyes of deep-sea crustaceans may still perceive luminescent sources from appropriate distances because of the much higher contrast at depth between luminescent sources and background light. Smaller eyes also impose a lower energetic burden and are potentially less visible to predators than are large eyes.