What the shell?

Hello y’all! It’s Ben from the Herpetology Team, and I’m here to discuss with y’all the strangest and most notable part of all turtles, their shell! The shells of many Testudines come with a variety of sizes, shapes, forms, and functions.  So, let’s get into the basics!

In my opinion, the turtle’s shell is one of the most interesting of all organs in the animal kingdom. The shell is composed of two main sections, the dorsal (or top) section, called the carapace, and the ventral (or bottom) section, called the plastron. These two sections are fused together and composed in a mosaic of epidermal, dermal, and skeletal tissue. The carapace and plastron are usually comprised of scutes and underlying bony plates. The scutes and bony plates overlap one another in order to provide the shell with increased structural integrity.  

The anatomical origin of the shell is fascinating. The shell is the rib cage, sternum, and vertebrae of the turtle that have evolved over 200 million years to form one cohesive unit. The shell has a variety of functions within and across the order of Testudines. Protection is likely the most common and obvious function when one thinks of a shell. This fact is undeniable, for example, that the shells of the eastern box turtle, Terrapin carolina, have shown evidence of surviving attacks from predators and even fires. But the shell serves so many more functions than protection. Let us examine a few examples.

The shell of the African leopard tortoise, Stigmocheyles pardalis, serves to help with camouflage. Researchers have observed that populations of this tortoise found in dryer habitats have shells that are paler in coloration than populations found in wetter environments with darker soils. Another example is the gular projection found on the plastron of the male African spurred tortoise, Centrochelys sulcata, which functions to injure other male sulcata tortoises during breeding season combats. In case of the green sea turtle, Chelonia mydas, and a number of other aquatic turtles, their shells are dorsal-ventrally flattened. This more flattened shape is advantageous because it is more hydrodynamic within their aquatic environment.

Clearly, there are many advantages to having a shell. They can provide protection, camouflage, serve as weapons during breeding season, or help a turtle swim faster. In my opinion, the “disadvantages” of having a shell are just as fascinating as the advantages. An obvious example is the reputation that tortoises have for being slow, and a clear explanation for this phenomenon is that their shell limits their limb mobility. But I assure you that any vertebrate would likely have limited mobility if both their shoulder and hip girdles were somehow located within their ribcage. Within the turtle’s evolutionary adaptations, there is a clear trade-off between losing mobility in exchange for gaining durability.

Let’s move on to my favorite evolutionary “disadvantage” of having a shell. The ribcage of most vertebrates assists in that animal’s respiration. But as an animal’s ribcage becomes fused, its ribcage loses the ability to move and consequentially aid in breathing. So, the Testudines have had to develop with some clever adaptations in order to obtain, preserve, and use oxygen. One adaptation is similar to how we breathe, by creating negative pressure internally, which inflates the lungs, then pumping that air out and deflating the lungs. Additionally, many aquatic turtles can have a small amount of gas exchange occur through their skin. The common musk turtle, Sternotherus odoratus, has another interesting adaptation and this species can introduce oxygen into their system with buccopharyngeal breathing, where oxygen-rich water is pumped through threadlike structures in their mouth where gas exchange can occur efficiently. Functionally, this is much more similar to how fish obtain oxygen than how we breathe. A similar process has been noted in a number of species, including the Fitzroy river turtle, Rheodytes leukops, where they pump water into their cloaca to perform gas exchange. So, therefore it is accurate to say that turtles breathe with their “butts!”

Lastly, my favorite adaptation to this respiration problem that the development of a shell initially caused is remedied by the shell itself! Let me explain … the painted turtle, Chrysemys picta, can survive without breathing air longer than any other air-breathing vertebrate. While I can’t hold my breath for more than a couple minutes underwater, these turtles have been documented not breathing air for five months! They can achieve this incredible feat using two methods; the first is to slow their metabolism to a standstill. In an almost freezing environment, their heart beats about once every five minutes. This slowing effect means that anaerobic respiration will occur at a much slower rate. A byproduct of this anaerobic respiration is lactic acid. At high enough concentrations lactic acid can become very toxic. But the shell of the painted turtle, Chrysemys picta, neutralizes this toxic build-up of lactic acid by releasing calcium carbonates. These minerals are used to buffer the lactic acid produced in the absence of oxygen. The released calcium carbonate and lactic acid subsequently bind to form the neutral and non-toxic calcium lactate. This calcium lactate is then deposited directly in the skeleton of the turtle. Once the turtle has access to oxygen and can undergo aerobic respiration, the calcium lactate is slowly released from their shell and skeletal system back into the bloodstream and expelled from the turtles’ system at a safe rate. So, while some turtles can breathe with their “butts,” others use their shells to hold their breath!

References

Turtles, Tortoises, and Terrapins: A Natural History
By Ronald Orenstein

Turtles of the World
By Franck Bonin, Bernard Devaux, and Alain Dupre
Translated by Peter C.H. Pritchard

Druzisky, K., & Brainerd, E. (2001). Buccal oscillation and lung ventilation in a semi-aquatic turtle, Platysternon megacephalum. Zoology, 104(2), 143-152. doi: 10.1078/0944-2006-00015

Ben M.
Keeper I – Herpetology