The ommatidia of Arca noae: a three-tier structure with a central light-guiding element for the receptor cell.

The compound eyes of ark clams appear to function as an optical system to trigger shell closure against predators. We have analyzed the structure of the ommatidia of Arca noae by thin section electron microscopy and serial sectioning, Concanavalin A-gold labeling and acid phosphatase cytochemistry. Our results demonstrate that the ommatidia are a three-tier structure composed of a central single receptor cell, surrounded and covered by proximal pigment cells followed by rows of distal pigment cells. The receptor cells of Arca noae have no lens and the disks of their receptive segment are derived from sensory cilia. The distal mitochondrial segment in the cytoplasm between the nucleus and the receptive segment is surrounded by a mass of Concanavalin A-reactive glycogen particles. Although both, proximal and distal pigment cells have numerous microvilli, only those of the proximal pigment cells form a well-aligned brush border. The microvilli of the latter are ≈9-11 μm long and have a diameter of ≈70-80 nm. Numerous microlamellar bodies cover them. The microlamellar bodies are stored in acid phosphatase-negative secretory granules of the pigment granule-free apical cytoplasm of proximal pigment cells before their secretion. Observation of living compound eyes indicated that the apex of proximal pigment cells transmitted significantly more light than the surrounding distal pigment cells. Hence, the regular geometry of the brush border seems to be a light-guiding structure for receptor cells similar to an optical fiber.