In sensory neurophysiology, things are seldom simple. What is really occurring in the Mytilus PRCs will be better understood when intracellular recordings are made simultaneously from a PRC and a pigment cell, and optic nerve spikes are observed. The fact that PRC and pigment cell membranes are in intimate contact is no coincidence; function follows form, and vice versa. Unfortunately, Mytilus eye cells are small and soft, and are not easily recorded from.

In closing, one might speculate why Mytilus has evolved an OFF photoreceptor with 37 PRCs. Why so many PRCs? Redundancy? Certainly no image is possible in the absence of a focusing apparatus, and the eye deep in the tissues between the gill tissues. Are they the vestigial remnant of a divergent evolutionary pathway taken by the Mytilidae? LaCourse tested the theory that Mytilus eyes served to warn the mollusk that danger is near (a shadow passing over the eye causing dimming), so the valves close for protection. He destroyed the eyes, then found that the shells still closed when the light was dimmed. He hypothesized that Mytilus has other single-cell PRs located on the gills, mantle, or foot that mediate this action. Then, what are the eyes used for? Do they sense photoperiod and thereby regulate feeding and/or the reproductive cycle? Perhaps increased light indicates low tide, and triggers appropriate behavior, such as spinning more byssus threads.

The moral is that even apparently simple organisms are far more complex at the physiological level than one can imagine. Good research always raises new questions, and suggests new models.

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