Summary: Are tuatara eyes like every other reptile’s eyes? Maybe. Join Kiersten to find out what makes tuatara eye so special.
For my hearing impaired followers, a complete transcript of this podcast follows the show notes on Podbean
Show Notes:
“The lonely eye,” by I R Schwab and G R O’Connor. Br J Ophthalmic. 2025 Mar:89(3):256. Doi:10.1136/bio.2004.059105
“Reptilian Eyes and Orbital Structures,” Jeanette Wyneken. 2012 Proceedings Association of Reptiles and Amphibian Veterinarians. https://cdn.ymaws.com
Parietal Eye, https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/parietal-eye
Music written and performed by Katherine Camp
Transcript
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Kiersten - This is Ten Things I Like About…a ten minute, ten episode podcast about unknown or misunderstood wildlife.
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Kiersten - Welcome to Ten Things I Like About… This is a podcast about misunderstood or unknown creatures in nature. Some we’ll find right out side our doors and some are continents away but all are fascinating.
This podcast will focus ten, ten minute episodes on different animals and their amazing characteristics. Please join me on this extraordinary journey, you won’t regret it.
As I begin episode four, I want to remind you of the question I posed at the beginning of the third episode. Is the tuatara a lizard or not a lizard? It is definitely lizard like. This episode, episode four, we will be talking about an attribute that might shed some light on the answer to this question. The fourth thing I like about the tuatara is their eyes, all three of them.
To start, let’s look at the anatomy of the general reptile eye. Reptilian eyes are similar to other vertebrate eyes in the fact that they are layered, filled with fluid, and have a lens that focuses light on a retina. The structure of the eye includes three chambers. The anterior chamber is the fluid-filled space inside the eye between the iris and the cornea’s innermost surface. The posterior chamber is a small space directly behind the iris, close to the lens, and bordered by the ciliary muscles. The anterior and posterior chambers are filled with aqueous humour. The third chamber is the vitreous chamber located between the lens and the retina and filled with vitreous humour.
Tuatara have two lateral eyes, one on each side of the head, that rest in a bony orbit. They are separated by a cartilaginous interorbital septum. A periorbital membrane lines the orbital bones and septum that connects to the orbital membranes and the internal parts of the upper and lower eye lids. Whoa! We got a little scientific for a moment there, but anatomy can do that!
These two lateral eyes are placed on the opposites sides of the head so they can have a wide perspective of their environment. Typically we see this eye position in prey animals and not all lizards are prey animals, but many of them are. Tuatara, as full grown adults, don’t have to worry too much about being eaten, but this eye placement is very helpful, none the less.
Most diurnal reptiles can see some amount of color, whether they see color the same way other vertebrates see color is still being studied, but the presence of cones is proof that they can see colors. Cones are useful in bright light, so reptiles active during the day will have more cones that those that are crepuscular, active at dawn and dusk, and those that are nocturnal. More rods are present in the eyes of nocturnal animals and these help pick up light and dark shades. Tuatara are active during the day and night, but are most active after dark; therefore, they most likely have more rods than cones.
They do have both upper and lower eyelids and are capable of closing their lids. They do not appear to have a nictitating membrane, which is another protective lens that can open and close over they eye, that some other reptiles have. They also have pupils that can contract and expand to allow various levels of light into the eye. When looking at close up photos of the tuatara’s eye the pupil is circular or oval along the vertical horizon. Interestingly, pupil shape in reptile eyes varies with behavior of the animal. Diurnal reptiles tend to have to have round pupils while nocturnal hunters have slits. Reptiles with slit pupils will have a pupil that is perpendicular to their orientation because that offers the best focus. The shape of the pupil has a profound influence on the retinal image because of the way light is allowed into the eye. Who knew the anatomy of the reptilian eye could be so fascinating.
Okay, let’s get to that third eye I mentioned. There are actually two orders of reptiles that have a third eye, Order Squamata, which includes lizards and snakes, and Order Rhynchocephlia, which includes only the Tuatara. Lizards and the tuatara are the only reptiles that we know of that have third eyes. The third eye is also called the parietal eye and is found on the dorsal portion of the skull. It is smack dab in the middle of the top of the skull, because where else would a third eye be found?
Anatomically speaking the parietal eye cannot focus on images like lateral eyes, so the third eye is not used for vision. At least not the parietal eyes on the current living tuatara. But this eye is more similar to lateral eyes than you might expect. The parietal eye is ventral to a parietal plug that is very similar to a cornea. Immediately below the plug is a lens that is remarkably similar to the lens in the lateral eyes. Below the lens are layers of pigment and photoreceptors. Photoreceptors detect light. Now we see where we are going with this whole third eye anatomy, maybe.
For a long time we thought that the parietal eye was used to detect light. It helped the animal tell when it was day and when it was night, and probably helped set animal’s the circadian rhythm. It helps the tuatara determine the changing of the seasons based on differences in the light cycle. But is that all there is to it? Some researchers believe that the third eye might be included in melatonin synthesis was well as other hormones. What it truly does is still a mystery.
So….does this help answer our question about whether the tuatara is a lizard or not? Some lizards do have third eyes, just like the tuatara so leaning towards yes? Before you comment though, let me through out one more thing about the tuatara’s third eye. When they are newly hatched the third eye is exposed and looks just like their lateral eyes. I’m not joking, search for tuatara hatchling third eye and you’ll see what I’m talking about. It’s very cool! As the tuatara ages the third eye is covered over with a clear scale obscuring it from view.
I hope you have enjoyed this discussion about tuatara eyes because it’s my fourth favorite thing about these interesting reptiles.
If you're enjoying this podcast please recommend me to friends and family and take a moment to give me a rating on whatever platform your listening. It will help me reach more listeners and give the animals I talk about an even better chance at change.
Join me next week for another exciting episode about the tuatara.
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This has been an episode of Ten Things I like About with Kiersten and Company. Original music written and performed by Katherine Camp, piano extraordinaire.
