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

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 distall...