Last year, my lab published an article in Bioessays about the evolution of the Tapetum Lucidum, the structure in nocturnal animals that causes reflective “eye shine.” This is a theoretical paper, outlining our argument that this structure evolved in vertebrates to compensate for the backwards orientation of our retinas. Implicit in this argument is the claim that the vertebrate retina is a sub-optimal design compared to the front-facing design of the retina of Cephalopods (octopuses, squids, and nautiloids).
Below is a short and non-technical summary of our argument, but I also wrote a longer version for Skeptical Inquirer if you want more detail.
First, a quick word about our “backwards retinas.” In vertebrates, our photoreceptor cells (the long cells that absorb light and begin the transmission of visual information to our brains) appear to face backwards, that is, away from the incoming light. This is in contrast to the retinas of Cephalopods, whose photoreceptors are very similar, but are installed the more logical way, facing forward.
This has perplexed scientists for a very long time. Oddly, creationists at the Discovery Institute often associate this particular “flawed design” argument with me, even though when I first wrote about it on this blog (2015), and then again in my book Human Errors (2018), it had already been written about by many biologists, perhaps most famously by Richard Dawkins in the Ancestor’s Tale (2004).
In any event, because “bad design” arguments really drive creationists crazy (which is why I had so much fun with them after Human Errors was published), they have tried every which way they can to argue that the inverted design of the vertebrate retina is actually superior. To be sure, vertebrate retinas have sophisticated structural adaptations that Cephalopods don’t. Our eyes are more advanced in many ways. But what creationists won’t admit (or don’t understand) is that a great many of those advanced structures appear to have adapted to compensate for the backwards nature of the retina. Cephalopod retinas are simpler because they don’t need complicated adaptations.
And this is what my latest paper is all about. I argue that the mirror-like tapetum lucidum, located behind the retina, is another such compensatory adaptation, designed to aid the vertebrate retina under dim light conditions.
What is the Tapetum? Many (but not all) nocturnal animals have a structure behind their photoreceptors that acts like a mirror. This is why their eyes “shine” when hit with a strong light. The purpose of this reflective structure is to bounce photons that slip past the photoreceptors back in the forward direction, giving those photons a second chance to collide with a photoreceptor. By doing this, under very dim conditions, the tapetum lucidum enhances the photosensitivity of the retina and gives nocturnal animals a small boost in their night vision.
There are a great many kinds of tapetum among nocturnal animals. It can be located in a couple different tissue layers; it can be made of several different materials; and it can be arranged in several different structural geometries. Some are even “occlusible,” meaning it can be obscured to prevent the eye shine from revealing a hidden predator to its prey (or vice versa). There is a wide diversity among the various tapeta.
Importantly, the various tapeta are so completely different that the structure appears to have evolved, disappeared, and then evolved again many many times throughout the animal kingdom. In our paper, we performed a phylogenetic analysis to show the dozens of times this structure appears to have evolved in various vertebrate lineages. The figure below shows a sample of the various kinds of tapetum and in which animals they appear.
Okay, so the tapetum has evolved countless times in various animal lineages, but what is the evidence that this structure compensates for the backwards nature of the vertebrate retina?
The sheer number of times it has evolved, with different structures, configurations, and compositions, shows that it is a relatively easy structure to evolve and that it offers immediate and powerful benefits. This is called convergent evolution, like the wings of birds, bats, insects, and pterosaurs, which each evolved independently.
So why has the tapetum never evolved in any cephalopod? Cephalopods occupy the same deep and murky waters that Tapetum-possessing fish do. They have the same need for highly sensitive photoreceptors. And Cephalopods have been around for even longer than vertebrates.
Perhaps they have never evolved the tapetum because their forward-facing retinas allow their photoreceptors to be packed as tightly as possible into an unbroken sheet of photosensitive tissue. In other words, the design of the cephalopod retina is as optimal as it can be and the tapetum would offer no further advantage. If this is the case, then the backwards retina of vertebrates truly is a sub-optimal design.
After our paper was published, an arachnologist named Nathan Morehouse informed us of more corroborating evidence for this hypothesis. A class of spiders called jumping spiders have two kinds of eyes, principle and secondary, and these eyes are similar to the camera-like eye of vertebrates and cephalopods. It turns out that the single pair of principle eyes have forward-facing photoreceptors, like cephalopods, while the secondary eyes have backwards-facing retinas, like vertebrates.
And wouldn’t you know it, the secondary eyes of jumping spiders often have tapeta to enhance their photodetection, but the principle eyes never do. As these eyes are found in the very same organism, the fact that the backwards secondary eyes have tapeta and the forward primary ones don’t underscores the point that forward-facing retinas are optimal, at least when it comes to photodetection, while backwards-facing retinas need a little help from the tapetum. Together, Morehouse and I published an update to our article outlining this additional evidence.
In summary, in the retinas of cephalopods and the primary eyes of jumping spiders, the photoreceptors face forward and have no tapetum lucidum, while the retinas of vertebrates and the secondary eyes of spiders face backwards and do have tapeta to assist. We therefore propose that the tapetum lucidum is a compensatory co-adaptation for the sub-optimal nature of inverted retinas.
Experiments to test this hypothesis are challenging but possible. For example, scientists could make direct comparisons of the photosensitivity of isolated cephalopod and vertebrate retinas in situ, with and without tapeta, followed by the grafting of a tapetum onto a cephalopod retina to observe if photodetection is enhanced. These types of experiments require a great deal of care and many controls, but it should be possible to test this hypothesis. The testing of predictions made by hypotheses is the hallmark of science and a key feature that separates scientific ideas from pseudoscientific ones (such as intelligent design creationism).
It is our hope that this hypothesis will provoke robust discussion and future research that will either refute or lend support to this idea. Even if it is proven incorrect, the theoretical and experimental work that this hypothesis inspires will help to illuminate the function and evolution of the vertebrate and cephalopod retinas. If you’ll pardon the pun.
-NHL
The technical article here. Follow up letter here. Skeptical Inquirer article here, discusses both articles above for a non-technical audience.
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