Are cephalopods better designed than humans?

The answer to the titular question depends on what you think they are designed for; hence, it seems to be a silly question. When it comes to the eyes, however, there is little doubt: human (and other vertebrate) retinas are in backward in that nerves, blood vessels, and other structures are piled on top of the photoreceptors and thus reduce sensitivity, if not resolution. Additionally, the nerves go out through a hole in the retina, which makes us susceptible to glaucoma. Cephalopod retinas, by contrast, face forward, with nothing covering the photoreceptors, and the nerves do not have to pass through a hole in the retina. Cephalopod eyes are thus clearly better designed than human eyes.

I raise this question by way of introducing an article, The night begins to shine: The tapetum lucidum and our backward retinas, by Nathan H. Lents, in the January-February issue of Skeptical Inquirer. The tapetum lucidum, or just tapetum, is a reflective membrane in the retinas of many nocturnal vertebrates and is located just behind the photoreceptors. Its purpose is presumedly to direct the light that the photoreceptors miss back into them and thereby increase the sensitivity. Interestingly, tapetums composed of many different materials appear to have evolved many different times in different creatures. Cephalopods have no tapetum, presumably because their retinas face forward, with the photoreceptors directly facing the light. Prof. Lents explains all this and more in great but easily understandable detail.

Please forgive a physicist’s digression. I once lost a debate as to whether the retina is in backward or not. It is in backward. (In most anatomical drawings, however, the cones appear at least vaguely conical, with the open end facing the incoming light. Thus, the cones themselves are not exactly in backward.)

At night, we are presumably dealing with rod (scotopic) vision, and the rods supposedly trap light due to total internal reflection. If that is so, then the law of refraction prohibits light traveling in the opposite direction (toward the light source) from getting into the rod. How then is the tapetum helpful?

I cannot immediately find an answer to this question, so I will stick my neck out and suggest that the evanescent wave penetrates the rod and is absorbed by the dyes in the rod, similarly to frustrated total internal reflection. If that is so, then it is hard to see how the tapetum can be very effective, but I guess every little bit counts.

Acknowledgements. Thanks to Nathan Lents for discovering what I can only call a silly mistake and to Steven Fliesler of the school of medicine of the University of Buffalo for interesting comments.