THE EYE OF THE NEW-ZEALAND FRESH-WATER CRAB HALICARCINUS-LACUSTRIS, AND SOME ECOPHYSIOLOGICAL PREDICTIONS BASED ON EYE ANATOMY

In specimens of Halicarcinus lacustries of 6 mm body length and width, the two compound eyes each comprise about 400 ommatidia, and anatomic ally resemble those of other brachyuran crabs. All ommatidia are of th e apposition type with short, proximally tapering, quadripartite cryst alline cones as well as a cornea (maximum thickness 18 mum) with conve xly-curved inner surfaces. The mean distance from cornea to basement m embrane is 125 mum, and interommatidial angles average 9.5-degrees. Ea ch ommatidium possesses a horizontally-banded rhabdom, 35-40 mum long and 2.5 mum wide, with seven contributing retinula cells. An eighth ax on can be traced to a small distally-placed retinula cell. We conclude that the photoreceptors of H. lacustris are useful in detecting small environmental changes in brightness, as well as the plane of polarize d light, but have poor spatial and temporal resolution. High densities of screening pigment granules throughout the eye, a decrease in micro villus diameter from an average of 61.1 nm distally to 51.4 nm proxima lly, and an abundance of tiny, spherical vesicles along the edge of th e rhabdom, collectively indicate that the eyes of H. lacustris may eas ily be damaged by excessive radiation. Therefore, despite the appositi on anatomy of its eye, H. lacustris must avoid sunny places if it is t o perform optimally. The main roles of the eye are likely to involve p redator detection, orientation, and synchronization of activities betw een individuals.