Regionally different ommatidial structure in the compound eye of the water-flea Polyphemus (Cladocera, Crustacea)

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Abstract
The 2 compound eyes of P. pediculus are completely fused in the midline to form a single integrated unit containing 130 ommatidia with 4 different types of rhabdoms. The general features of the eye include a cuticle lacking corneal lenses, crystalline cones composed of 5 cells and the presence of juxtacrystalline cells and distal pigment cells. The rhabdoms are fused and the palisade, when present, is a part of the extracellular space in which the rhabdom is suspended. Four different types of rhabdoms were zonally arranged in the eye; a foveal type, in the dorsofrontal region of the eye, characterized by its long and slender shape (only 5 retinula cells contribute to forming this irregularly layered rhabdom, with the 1st layer composing the distal half of the rhabdom); a 2nd type, located ventrally to the fovea, is conventionally layered and is formed by 6 retinula cells, one of which is aberrant; a dorsolateral type is continuous (unlayered) and formed by 6 retinula cells of which one is aberrant; and a dorsal and ventral edge type that is wide, short and lacking palisade. Six retinula cells contribute to the continuous rhabdom and 2 of these are aberrant with tiny rhabdomeres. The foveal type of rhabdom has a peculiar arrangement of the microvilli, which is thought to depress the sensitivity to vertically polarized light. This mechanism is believed to enhance the ability to detect prey. The zoned eye, with its specialized receptive apparatus is interpreted as an adaptation for coping with a diversity of visual tasks by a very small animal.