Why do turtles have shells?

Green turtle by thoughtco.com

Written by Nur Hasya Haziqah

Hasya was a second year student of International Business in Grenoble Ecole de Management, Grenoble, France, when she interned with TCS in 2020. She hoped to acquire great experience and knowledge regarding the world of non-profit organisations during her internship.

21 Feb 2021

Turtles are interesting animals as they are quite different from other quadrupeds such as mammals or reptiles because their shells are a unique feature. Often, we might mistake these shells as their ‘homes’ or even as ‘deadweight’ that restrict their movement and make them sluggish (although, the classic tale of ‘The Tortoise and the Hare’ shall present you with other morals). Today, it is widely accepted that their shells serve as armours that shield them from predators. Armours so incredible that it is nearly impossible to detach it from the body.

Why?

Because the shell is a turtle’s body, or rather its inside out skeleton. Peek inside (or use x-ray vision) and you will see that the shell consists of 60 bones made out of the ribcage, the backbone, the vertebrae, and the breastbone. Consequently, unlike snakes or crustaceans who perform shedding, turtles do not abandon their shells but instead grow with them. These shells not only protect them from predators but also from abrasion injuries. A turtle’s shell is separated into three: its dorsal (upper piece) is called the carapace, the ventral (bottom piece) is known as the plastron, and finally, a bridge that merges the two parts together.

The carapace—which retains the shape from an oval to a heart-shape, depending on the species—has broad and fused ribs and is attached to the spine. As the shell does not expand, turtles are unable to breathe by expanding and contracting their ribs. Alternatively, oxygen is pumped in by using the muscles within the shell through the mouth. Turtles also sometimes use their cloaca—traditionally the opening for digestive, reproductive, and urinary tracts—to breathe out. Scientists too believe that turtles are able to absorb oxygen from the water when they are hibernating as if they have gills. Moreover,  slider turtles (or ‘pond turtles’), are able to switch from aerobic to anaerobic respiration during hibernating seasons. This means that glucose replaces oxygen, helping these sliders to survive in frozen ponds with little oxygen.

Turtles come from the family of lizard and snakes and have since evolved into ancestors named Pappochelys and Eunotosaurus. The former are known for their buoyant bones and the latter for their digging abilities. Far from the turtles we know today, the ribs of these ancestors had slowly broadened and united, growing over the shoulder blades to create shells that are attached with the turtle’s skeleton. As mentioned, broadened ribs hinder breathing by ventilation and to an extent has a negative effect on locomotion, but all these changes were needed for the survival of our present-day turtles. There has been evidence suggesting how the sturdy-handed Eunotosaurus were essentially skilled in burrowing. Fast forward millions of years and we have the descendants, the turtles now no longer using their shells as a means for digging, but rather for protection, a classic case of exaptation and convergent evolution.

This whole piece may have been detailed yet still insufficient as there are many more discussions and debates that are currently being held regarding the nature and the purpose of turtle shells. What is certain, however, is that humans have been hunting and exploding turtles (and also tortoises) for their shells. Over the course of 150 years, studies have shown that these endangered creatures have been killed and traded off in the black market, reducing their numbers by the millions. Indeed, nature and time have allowed their shells to evolve into indestructible shields, yet it is not enough to fend of the most vicious predators: humans.

References:

  1. Business Insider. 2020. A Turtle’s Shell Protects Some Of The Strangest Features In The Animal Kingdom. Here’s What’s Inside.. [online] Available at: <https://www.businessinsider.com/inside-turtle-shell-protects-strangest-animal-features-2019-7?IR=T> [Accessed 16 August 2020].
  2. Childrensmuseum.org. 2020. Why Do Turtles Have Shells? | The Children’s Museum Of Indianapolis. [online] Available at: <https://www.childrensmuseum.org/blog/why-do-turtles-have-shells> [Accessed 16 August 2020].
  3. Daley, J., 2020. Over 150 Years Of Data Sheds Light On Today’s Illegal Tortoiseshell Trade. [online] Smithsonian Magazine. Available at: <https://www.smithsonianmag.com/smart-news/millions-endangered-sea-turtles-were-hunted-their-shells-over-150-years-180971824/> [Accessed 16 August 2020].
  4. Earth Rangers. 2020. Top Ten Facts About Turtle Shells | Earth Rangers: Where Kids Go To Save Animals!. [online] Available at: <https://www.earthrangers.com/top-10/top-ten-facts-about-turtle-shells/> [Accessed 16 August 2020].
  5. en.wikipedia.org. 2020. Cloaca. [online] Available at: <https://en.wikipedia.org/wiki/Cloaca> [Accessed 16 August 2020].
  6. Phys.org. 2020. Researchers Discover Real Reason Why Turtles Have Shells. [online] Available at: <https://phys.org/news/2016-07-real-turtles-shells.html> [Accessed 16 August 2020].
  7. Yong, E., 2020. Why Turtles Evolved Shells: It Wasn’t For Protection. [online] The Atlantic. Available at: <https://www.theatlantic.com/science/archive/2016/07/the-turtle-shell-first-evolved-for-digging-not-defence/491087/> [Accessed 16 August 2020]
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