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When I was a child, I remember my dad telling me that tuatara had three eyes, with the third one on top of its forehead.  I can still see the way my dad pointed to his forehead when he said that.  I didn’t know what to make of it then:  My dad often told outlandish stories.  Was this one really true?  Over the years when I saw tuatara at zoos, I would look at the top of their foreheads searching for that third eye.  I couldn’t see it, and so I became a skeptic.

When I became a guide on Tiritiri Matangi a few years ago I remember some other guides mentioning the third eye of tuatara.  They said it in a way that made it seem like it was a unique oddity of a unique living fossil as if it were something only tuatara had.  It all seemed fanciful so I set about finding out for myself and what I discovered is that the truth is even stranger than the hype.

First, then, some basic facts. Tuatara do indeed have a third eye—complete with a lens and retina (i.e. with cells that detect light).  This eye is positioned in the middle of the forehead, behind a small hole in the skull.   My dad was right after all!  Tuatara really do have a third eye in the middle of the forehead.  It’s called the parietal eye (pronounced pa-rye-e-tal) (1).  So why can’t we see this eye?  It turns out the parietal eye is covered from birth by a layer of translucent skin, plus possibly some small scales.  Scales grow as the tuatara ages and hide the presence of the eye.

But there’s more.  The nerve from the parietal eye connects to a gland deep within the tuatara’s brain called the pineal gland.  This gland also has cells that detect light.  What is the purpose of that you may well ask?  What’s the point of being able to detect light directly deep within the brain?  Indeed, what’s the point of an eye if it’s covered up with scales and skin?  And what’s the point of the eye and gland—collectively known as the pineal complex—in the first place, irrespective of whether it’s covered up or not?

This is where the truth becomes stranger than the hype.  It turns out that the tuatara is far from unique in having a parietal eye.  Many other vertebrates (2) have a parietal eye as well!  Examples include most lizards, frogs, sharks, and some fish (see photos below).  In tuatara the parietal eye is particularly well developed, but even that’s not unique.  The parietal eye in some lizards (3) is at least as well developed as that of tuatara, if not more so.  In truth there is actually a continuous spectrum amongst vertebrates from no parietal eye at all, to a well-developed one.  And even if a vertebrate doesn’t have a parietal eye like the tuatara, it tends to have some form of pineal complex, although this may comprise the pineal gland only without the added eye (4).  The latter is the case in birds and mammals (including humans).

Perhaps you’ve heard of melatonin?  It’s a hormone in the body that helps regulate patterns of sleep and our general daily body-clock.  This hormone is produced by the pineal gland and it’s produced in response to darkness, i.e. the absence of light.  In vertebrates such as mammals and birds without a parietal eye, the pineal gland receives the signal for light (or darkness) from the brain, which of course it receives in turn from the normal eyes.  In those vertebrates with a parietal eye, such as tuatara, signals for light can come directly from that eye instead.  This eye doesn’t need to be good enough to form an image—and indeed it isn’t.  All it needs to be able to do is sense whether or not there is light around.  It would seem that it doesn’t matter if the parietal eye is somewhat covered.  Try putting your hand over your eyes and you’ll see what I mean. You’ll still be able to tell whether it’s daytime or not.

So is the parietal eye in tuatara there to help regulate the tuatara’s body-clock?  Possibly; probably.  The thing is, no-one has yet managed to confirm this, or indeed confirm much about tuatara’s parietal eye at all (5).  What is certain is that the parietal eye in some lizards helps regulate how they bask, i.e. how much time and where they bask in order to adjust their body temperature.  It is likely that the tuatara’s parietal eye has a similar function.  It might even have the function of helping a tuatara find its burrow again as well.  Again this has been observed in some lizards. (6)

So, to summarise:  Yes, the tuatara has a third eye but then so do lots of other vertebrates.  It’s nothing unusual, it’s just one manifestation of a complex of structures found in most vertebrates that tend to help regulate body-clocks.  The tuatara’s parietal eye is particularly well-developed, but it is also well-developed in some lizards as well, so that’s nothing unusual either.  The eye doesn’t form complex images, only used to detect the presence or absence of light. To put this into even broader context, many invertebrates, such as insects, have different sets of eyes as well.  Like the parietal eye in vertebrates, some of these eyes are also there just to detect light in general for a variety of reasons, rather than to form images.  At the end of the day, it could be argued that the system of just having eyes for normal “sight”, such as in birds and mammals, is the unusual thing rather than the tuatara’s extra eye!

So is the tuatara a three-eyed monster?  Well, it’s definitely three-eyed, but is it a monster?  Definitely not, if by monster we mean something strange or fantastical.

1. Or pineal eye.
2. Loosely speaking, vertebrates are those creatures most people would call animals. They comprise mammals, reptiles, amphibians, birds, and the various forms of fish (including sharks and rays).
3. But not snakes.
4. Interestingly crocodiles (and their kin, eg.g. alligators) don’t have a pineal gland at all.
5. It hasn’t even been confirmed that the tuatara’s pineal gland produces melatonin, although tuatara certainly do produce melatonin somewhere in their bodies, and it’s reasonable to assume that it’s in the pineal gland, like other vertebrates, such as lizards (the closest living relatives of tuatara).
6. The mechanism here seems to be by detecting the direction of polarised light.

References:
Cree, A. 2014. Tuatara: Biology and conservation of a venerable survivor. Canterbury University Press, Christchurch. pp. 424–426.

Kardong, K.V. 2019. Vertebrates: Comparative Anatomy, Function, Evolution (8th ed.). McGraw Hill Education, New York. pp. 610–611, 691–692.

Diagrams from Wikipedia (shared through Creative Common licensing):

https://commons.wikimedia.org/wiki/File:Frog_parietal_eye.JPG

https://commons.wikimedia.org/wiki/File:Anolis_carolinensis_parietal_eye.JPG