The effects of light and temperature on the structural organization of the eye of the Antarctic amphipod Orchomene plebs (Crustacea)

Abstract

The Antarctic amphipod Orchomene plebs lives in an environment which is characterized by stable temperatures of — 2 to 0 °C, and low ambient light intensities. The microanatomy of the eye of dark adapted and light-adapted animals, kept at 0 °C, was studied by light and electron micro­scopy and compared with that of individuals kept at + 10 °C for 7 h. The eye of a 19 mm (total body length) specimen consists of approxi­mately 260 ommatidia, each measuring 40-50 μm across. The cornea is smooth and external facets are not developed. The crystalline cones, which are the intracellularly secreted products of two cone cells per ommatidium, possess cores that stain more intensely than their peripheral regions. The ultrastructure of the core reveals a regular lattice of 30 nm particles, which are interpreted as glycogen granules. The rhabdom, whose microvilli measure 85-100 nm in diameter, is spindle-shaped and of the centrally fused type. The ratio of rhabdom length: width is 3.2 in light- adapted and 2.8 in dark-adapted animals. There are five retinula cells per omma, one of them being consistently smaller than the rest. Spherical (0.4 μm) granules of screening pigment, present only in the five retinula cells, exhibit migrations that are not controlled by an internal rhythm but are the direct response to fluctuations in the ambient light intensity. In the light-adapted state the pigment grains form a sleeve around the entire rhabdom, but in the dark-adapted eye they cluster around the proximal ends of the cones. The nuclei of the retinula cells are found below the basement membrane, but those of the cells occupying the spaces between ommatidial groups are located above it. These ‘inter­stitial’ cells lack pigment granules, but contain many vesicular organelles, which in the dark-adapted state only resemble miniature sea-urchins of 0.4 μm diameter. Heat-stressed dark-adapted animals exhibit grotesquely deformed rhabdoms with microvilli whose membranes are severely disrupted. The position of the screening pigment is similar to that of light adapted material, but rhabdom dimensions as well as the contents of the inter­stitial cells are indicative of dark-adapted eyes. When heat-stressed ani­mals after a stay of 7 h at + 10 °C are returned to water of 0 °C and examined 7 h later, their photoreceptors (based on five eyes each) show a remarkable recovery: the visual membranes have reorganized, microvillus patterns have reformed and the rhabdom structure has regenerated. The screening pigments, too, have begun to migrate into the characteristic position of the dark-adapted condition. The experiments show that light and temperature cause separate but intricately related effects that must not be studied in isolation from each other.