Abstract
The structural organization of the eye of Gammarus oceanicus conforms to that of other gammaridean species. Individual facets and a compartmentalization of the eye cuticle do not occur, though internally ommatidial groups are clearly separated from one another. The cones are bipartite; the rhabdoms are centrally fused and reach a maximum length and width of 30 μm and 10 μm, respectively. Four large retinula cells and one small retinula cell per ommatidium are developed with nuclei located below the fenestrated membrane. All eyes, though fixed at 0045, displayed the light-adapted condition with screening pigment granules surrounding the entire rhabdom and insulating the latter from the reflecting vesicles of auxiliary cells. With regard to quality and quantity of cell organelles, the retinula cells proved extraordinarily richly endowed: there were photic vesicles of 0.04 μm diameter along the edges of the rhabdom, multivesicular bodies with and without limiting membranes, two types of electron-dense spherical organelles (i.e. screening pigment grains and carotenoid-containing structures), mitochondria, microtubules, and unusual multilamellate bodies. The latter, surrounded by a membrane and associated with a grainy ground substance, are thought to be somehow related to the continuous light of the Arctic summer that G. oceanicus is subjected to. It is hypothesized that the unusual lamellate bodies are either involved in the task of meeting extra energy requirements during times of illumination or represent one stage in the conversion of shed microvillar material to residual bodies.
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