The eye of the New Zealand freshwater crabHalicarcinus lacustris, and some eco‐physiological predictions based on eye anatomy

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Abstract
In specimens of Halicarcinus lacustries of 6 mm body length and width, the two compound eyes each comprise about 400 ommatidia, and anatomically resemble those of other brachyuran crabs. All ommatidia are of the apposition type with short, proximally tapering, quadripartite crystalline cones as well as a cornea (maximum thickness 18 μm) with con vexly‐curved inner surfaces. The mean distance from cornea to basement membrane is 125 μm, and interommatidial angles average 9.5°. Each ommatidium possesses a horizontally‐banded rhabdom, 35–40 μm long and 2.5 μm wide, with seven contributing retinula cells. An eighth axon 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 polarized light, but have poor spatial and temporal resolution. High densities of screening pigment granules throughout the eye, a decrease in microvillus diameter from an average of 61.1 nm distally to 51.4 nm proximally, and an abundance of tiny, spherical vesicles along the edge of the rhabdom, collectively indicate that the eyes of H. lacustris may easily be damaged by excessive radiation. Therefore, despite the apposition anatomy of its eye, H. lacustris must avoid sunny places if it is to perform optimally. The main roles of the eye are likely to involve predator detection, orientation, and synchronization of activities between individuals.