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Thursday, April 5, 2012

Lizards of glass

I love snakes, but I am also partial to other limbless animals, especially limbless vertebrates. Among the salamanders, sirens and amphiumas are some of my favorites. Caecilians are also pretty cool. And in the sister group to the snakes, it’s hard to pass up the many species of legless lizard. Snakes are pretty much just legless lizards too, since they evolved from lizard ancestors, but they have undergone a spectacular evolutionary radiation into over 3,000 species in over 20 families. Elsewhere in the lizard family tree, limblessness has evolved many times, usually in concordance with a fossorial or burrowing habit.

The southeastern US is home to four species of legless lizard, called glass lizards, in the genus Ophisaurus, which means snake-lizard (ophi for snake, and saurus for lizard, as in dinosaur). They are not closely related to snakes, nor to any other species of southeastern lizard, but rather to the alligator lizards of the Pacific northwest, genus Elgaria. They can be differentiated from snakes by several characteristics. Chief among these are the presence of external ear openings and of moveable eyelids, both of which snakes lack. Instead, the eyes of snakes are covered by a hard, clear scale known as a spectacle. Snakes cannot hear, although they are very sensitive to vibrations of the ground and air, so they can detect most airborne sounds almost as if they are feeling them. Our sense of hearing (and the glass lizards’) is a mechanical sense also, so it’s not really that different.

In addition to these subtle features, the glass lizards possess a longitudinal groove that runs down each side of their body, which snakes lack. This groove allows the body wall to expand and contract as they breathe, digest food, and reproduce. Snakes have solved this problem in two different ways. One is to reduce or stagger the internal organs so that they fit better into a cylindrical body. Many of the paired organs, such as kidneys and gonads, are situated one in front of the other, and some are no longer paired, such as the single left lung. Snakes also have extremely stretchy skin that allows considerable distortion of the body after they have eaten a large prey item, but glass lizards have two lungs and are quite rigid. You can feel the stiffness of a glass lizard when you pick it up – but when you do, be careful! They aren’t called glass lizards for nothing. Like many lizards, glass lizards can break off, or autotomize, their tails, which can serve to distract a predator in pursuit, which may choose to attack the writhing tail while the body slinks away inconspicuously. In some glass lizards, the tail may be as long as or longer than the body, so the effect can be quite dramatic.

This week in Florida, I have been lucky enough to stumble upon two of the four species of glass lizard. The four species can be hard to distinguish, and until the 1950s were lumped together into a single species. Juvenile glass lizards can be particularly tricky, but luckily for me both of the lizards I saw were adults. The first was caught by friends the day before I arrived, when it was found swimming in an estuary of the Matanzas River. This is unusual habitat for a glass lizard, which are usually found in pine flatwoods and other grassy uplands. However, we determined that this species was the Island Glass Lizard, Ophisaurus compressus. As you might imagine, these are often found on islands and in coastal dunes, and they must get there somehow.

Island Glass Lizard - Ophisaurus compressus
You can tell this species from the others in several ways. One is to closely scrutinize the scales of the head. All glass lizards have a ring of small scales around the eye. In the island glass lizard, these are directly adjacent to the upper labial (lip) scales, whereas in the other common species of glass lizard, they are separated by an additional row of scales, called the lorilabials. A smaller species, the mimic glass lizard, also has scalation similar to the island glass lizard, but we are outside of its range and it does not grow as large as this individual. It is called the mimic glass lizard because it is so similar in appearance to the island glass lizard that it can only be reliably distinguished by size, range, and scale counts. It was only described in 1987, when a curator at the North Carolina State Museum of Natural Sciences noticed that some small glass lizards in his collection were subtly different from the three described species. The mimic glass lizard is found in recently burned pitcher plant bogs and other mesic flatwoods.

Closeup of Island Glass Lizard, showing lack of lorilabials
Closeup of Eastern Glass Lizard,
showing lorilabials between perioculars and supralabials
The other glass lizard was found on a small island. This was an Eastern Glass Lizard, Ophisaurus ventralis. The lorilabials tell us that this is not the island glass lizard, but there is still a fourth species, the slender glass lizard, that we must differentiate. The color pattern can help us with this identification. Eastern glass lizards are checkered over the entire dorsal side of the body (above the groove), whereas slender glass lizards are striped both above and below the lateral groove. This species is a habitat generalist that can be found widely throughout the coastal plains of the Atlantic and Gulf coasts, from North Carolina to Louisiana. Slender glass lizards in the southeast are more closely associated with the imperiled dry uplands dominated by turkey oak and longleaf pine, ever more rare habitats that support a great variety of supremely interesting herpetofauna.

Eastern Glass Lizard - Ophisaurus ventralis
Unfortunately for the eastern glass lizard, its tail broke as I was photographing it. I was able to recover both the tail and the body of the lizard, and you can see how the muscles of the broken segment of tail are pulled out of the tail segment attached to the body, leaving deep cavities. The tail has a store of ATP, so it can thrash about for a few minutes. In the time between the break and the video I took, above, I was able to capture the body of the glass lizard, put it into a bag and tie the bag shut, and switch my camera to video mode, so the thrashing had already decreased in vigor by about half, believe it or not.

Broken tail segment with muscles protruding

Cavities left in tail segment still attached to body
Kind of sweet, kind of disgusting.

So there is your quick guide to identifying the glass lizards of the southeast. It can get tricky with the juveniles, as I said, so pay close attention to detail and take lots of photos of ones that you find. Handle them carefully! Feel free to post them here for aid in identification.

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.


Debi said...

Thank you so much for the information. My dog just chased a snake exactly like this one. The snake didn't even try to bite him. The snake was trying to get away from him. I tried to pull him away from the snake but he took off with a squirmy tail in his mouth. I though how gross why would he bite off a snake's tail. The snake didn't move so I thought he was dead. When I finally caught up with my dog he had dropped the tail and was playing with it. The tail is still trashing about in my yard. I was very upset in thinking that he had killed the snake. A few moments later I saw the snake slowly slither away. (The snake is or was about 15 inches long. The tail is still trashing around the yard and it's about 2-3 inches long). Thank you for making me feel better about the encounter in that the snake will survive another day.

Andrew Durso said...

Hi Debi, I'm very glad that my article helped you feel better. One point of clarification—glass lizards are not snakes, they just look like them. I realize this is a technicality, especially as far as your dog's or the lizard's health and safety is concerned, but it's important to me.

Unknown said...

Thanks you so so so so so much! I am researching glass lizards for a competition and this help me so much!!!!!! Amazing source of information! :)

Andrew Durso said...

I'm so glad it was helpful to you Angela! Good luck on your competition.

Michele Reyes said...

Fantastic post! I came across an Eastern Glass Lizard in the Ocala National Forest this morning.

Andrew Durso said...

Very cool! Thank you. Please consider sharing your find on HerpMapper.org

Britestar said...

We encountered a young Island Glass Lizard this evening on our walk. Knew it was different. So happy to have found this site. Is it possible to post a picture ?

Andrew Durso said...

Very cool! I don't think it's possible to post a photo here, but you could put it on iNaturalist.org or HerpMapper.org!