JAC: My post on the tuatara parietal eye was short and, for some readers, not informative enough. Where did it come from? What does it look like? (By mistake I published a picture of an iguana and not a tuatara.) Greg answers some of the many questions that have surely been tormenting many of you about this bizarre feature.
by Greg Mayer
The tuatara has long been of interest to us here at WEIT, and just the other day Jerry posted a video of one hatching, along with many interesting notes on their biology, especially on the parietal or ‘third’ eye. Jerry included a picture of the parietal which, as Jon Losos, among others, noted, was not, alas (as a simple google image search indicated), that of a tuatara, but rather that of what looks to me to be a common or green iguana (Iguana iguana— which, if you learn no others, is the one scientific name you should commit to memory). Jon remarked to Jerry that good pictures of a tuatara’s parietal would be hard to find. Well, here’s the best I could find.
This is Plate 20 from Arthur Dendy’s classic 1911 monograph describing the pineal organs (including the parietal eye) of the tuatara. The upper figure is a longitudinal section of the parietal eye, and the lens, retina, and pineal nerve (equivalent to the optic nerve) are readily apparent. The two lower figures provide details of the retina.
Dendy studied both adults and embryos; the above figures are of adults. Dendy, an Englishman, resided in a number of the antipodal parts of the British Empire, and in his monograph records his good fortune in not losing some of his histological sections of tuatara embryos, “… for they were, with most of my Australasian collections, shipwrecked in transmission from New Zealand to South Africa. The boxes containing the sections were, however, salved, and reached me after being soaked for weeks in salt water.”
The following figure, from Angus Bellairs’ still useful Life of Reptiles, is based on Dendy’s top figure, and labels some of the parts for clear identification.
Neither of these pictures, of course, shows the parietal from the outside. I’ve read that the parietal is not externally visible in adult tuatara, but I’ve never checked on the preserved tuatara I’ve seen; Jon has seen tuatara live and up close– perhaps he will stop by again here at WEIT and let us know if he has noticed the eye on the ones he’s seen and held.
The parietal eye is also found in many lizards (which, together with snakes, are the tuatara’s closest living relatives, so the sharing of this features is not anomalous.) In vertebrates, there can be a number of evaginations (together known as the pineal complex) from the region of the brain called the epithalamus. One of these forms the pineal gland or organ, while another forms the parietal organ. Both can be photoreceptive. In lizards and tuatara, the parietal organ can have a lens and a retina, forming the parietal eye. The eye is overlain by a translucent scale, easily visible in many lizards. It cannot, as far as is known, form an image. In lampreys, both the the pineal and parietal can be eye-like, so that some authors refer to them having a pineal eye and a parietal eye (which is why the median eye of lizards and tuatara, though sometimes called the pineal eye, is better called the parietal eye). In lampreys the position of the median eyes is indicated by a whitish, unpigmented, oval on the otherwise dark skin of the middle of the head, In birds and mammals, the parietal organ is absent, and the pineal organ (now called the pineal gland) is buried deep in the head, and has endocrine functions.
The pineal complex was present in some of the earliest fishes, as indicated by the presence of a single median foramen [JAC: small opening in the bone] in the skull of ostracoderms, placoderms, and others. It is most eye-like in the parietal eye of lizards and tuatara, which suggests that a fully eye-like parietal or pineal was not present in early vertebrates, so that the parietal eye did not evolve from a “real” eye.
Bellairs, A. 1970. The Life of Reptiles. 2 vols. Universe Books, New York.
Dendy, A. 1911. On the structure, development and morphological interpretation of the pineal organs and adjacent parts of the brain in the tuatara (Sphenodon punctatus). Philosophical Transactions of the Royal Society of London 201:227-331, pls. 19-31. pdf (Dendy’s interpretations of homology are no longer all accepted, but the morphological and histological work remains fundamental.)