Turtles are unique creatures that carry their homes on their backs. Their upper shells provide protection, but what about the bottom part? If you’re wondering what the underside of a turtle’s shell is called, read on for a deep dive into turtle anatomy.
In short, the bottom part of a turtle’s shell is called the plastron. The plastron forms the belly-side of the turtle’s shell.
Anatomy of a Turtle Shell
The Carapace (Upper Shell)
The top part of a turtle’s shell is called the carapace. This curved dome primarily consists of bones covered in keratin, the same protein that makes up human fingernails. Some turtle species, like leatherbacks, have a rubbery carapace made mostly of tough, leathery skin with an underlying layer of small bones.
This flexibility allows them to be more hydrodynamic when swimming. The shapes and textures of carapace scutes (plates) are unique to different turtle species. Their color patterns and shapes act as camouflage from predators like birds of prey.
The carapace’s interlocking bones are attached to the turtle’s rib cage and spine, meaning it grows right along with the turtle. New material is continually added underneath the scutes as the turtle ages. This is why the characteristic scute rings can indicate a turtle’s age, much like tree rings.
The Plastron (Bottom Shell)
The bottom shell is called the plastron. It also contains bone but tends to be a big flatter and more flexibly jointed to the carapace compared to the top. This allows for respiration, egg-laying, mating, and other vital motions turtles need to carry out.
The plastron features a large central opening called the forelobe. This is the soft area between the front shoulders where the turtle’s head, legs, and tail can emerge from the shell. The forelobe varies in size and shape considerably across species based on their behavioral and habitat needs.
Aquatic turtles usually have the smallest openings compared to more terrestrial types.
Bridges That Connect the Two Halves
The carapace and plastron are joined together by bony structures called bridges. Bridges occur in front (where the turtle’s neck pokes out) and in back (near the tail). They allow enough limited motion for breathing, walking, mating while still enclosing and protecting the turtle’s vulnerable head, legs, and tail when retracted.
Sea turtles have the most well-developed bridges to seal up tightly inside their shell. Their streamlined shells and flippers make them powerful swimmers able to migrate across oceans. But this comes at a cost of mobility while on land, where they shuffle and drag themselves with difficulty.
The Structure and Purpose of the Plastron
Made Up of Bony Plates
The plastron is the belly shell of a turtle, made up of several large bony plates that are fused together. The plastron is smooth and flat, in contrast to the curved carapace or top shell. The number and arrangement of these scutes or plates varies between species of turtle.
For example, sea turtles generally have a simplified plastron with only 9 bones, while land and freshwater turtles have more complex plastrons with 12 or 13 bones for greater structural support. These bones provide sturdiness and protection to the turtle’s underside.
Protects Organs and Provides Structure
The plastron serves several important functions for the turtle. Firstly, it acts as armor to protect the internal organs, including the heart, lungs, and liver. The solid bony composition helps shield these vital structures from predators or injury.
At the same time, the plastron provides structural support andreinforcement to the turtle’s body shape. Without this belly armor, turtles would be much more vulnerable to impacts or crushing injuries to their underside when navigating land or water.
Some fun turtle trivia – the plastron actually contains within it a series of chambers and sinuses that allow adult sea turtles to adjust their buoyancy and dive deeper or shallower in the water as needed!
Land turtles don’t have this adaptation as they don’t need to dive and float like their marine cousins.
Allows for Some Flexibility of the Body
Although the plastron provides sturdiness, it still allows for some flexibility of the turtle’s body. The bony plates are connected by soft tissue that permits movement. This is important to allow the turtle to bend its neck, legs and tail.
Male sea turtles also have a specialized loose joint in the center of the plastron, which gives them greater flexibility for mating. Female turtles do not have this anatomical feature.
Variations in Plastron Shape Among Turtle Species
Aquatic Turtles Tend to Have Smaller Plastrons
Aquatic turtles like painted turtles and red-eared sliders have plastrons that are much smaller compared to the overall size of their shells. Their plastrons only take up around 30% of their underside. This allows their limb and tail movements to be less restricted for swimming and diving underwater.
Smaller plastrons mean more streamlined turtle bodies and greater agility in water. For aquatic turtles, swimming ability is more crucial for survival than protecting their underside.
Terrestrial Turtles Have Larger Plastrons
On the other hand, land dwelling turtles like box turtles and tortoises have much larger plastrons that make up 50% or more of their underside. Their plastrons are wide and flat which gives them better stability and weight distribution on land.
The larger plastron also helps protect their limbs and vital organs from predator attacks or crushing blows. For terrestrial turtles, the ability to withstand impacts and avoid damage is more important than swimming speed.
Unique Shapes Fit Different Lifestyles
Beyond size differences, some turtle species also have uniquely shaped plastrons adapted to their habitat and lifestyle. For example, sea turtles have two small openings in their plastron called fenestrae. These allow sea turtles to be more flexible and maneuver better in water while swimming.
Their plastrons are also streamlined and hydrodynamic. On the other hand, box turtles have a hinged plastron that can be entirely closed up and locked when they tuck in their head and limbs. This provides maximum protection like a shielded fortress when faced with threats.
The Plastron as an Identification Tool
Distinct Markings Help Identify Species
Like fingerprints in humans, each turtle species has unique plastron markings that can help identify the particular species. The distinct markings are very useful for herpetologists studying turtles in the wild or captivity to determine what species they are observing.
Characteristics of markings like their shape, color patterns, and location on the plastron can provide clues to species identification. Some turtle species even have specific names for common markings, such as the hognose cooters’ characteristic yellow centers with dark margins on their plastra known as “penciling.”
Size and Shape Also Provide Clues
In addition to distinctive markings, the size and shape of a turtle’s plastron can provide further clues to determine the species. For example, common tortoises tend to have much higher, domed plastra compared to stinkpots, with their oblong, angled shells.
The width of the plastron also varies among species — some box turtles have very wide plastrons spanning almost the entire length of their shells, while other species like musk turtles have narrow plastrons covering less of their underside.
The size of the scutes or scales on the plastron is another distinguishing characteristic, with some species like Galápagos tortoises exhibiting very large scutes compared to other turtle species. Measuring details like plastron length and width as well as scute sizes provides quantitative data to narrow down what species a particular turtle is likely to be.
Examining Plastron Wear and Markings
All turtles accumulate wear and tear on their shells and plastra over their lifetime, with distinct patterns that reflect species environment and habits. For instance, turtles that dig more tend to exhibit more plastron abrasions from frequent burrowing than some aquatic species.
Many terrestrial species like Greek tortoises also show scrape mark clues on their scutes, evidence of run-ins over rocky terrain! I’ve even seen a two-headed turtle in the wild! 😊
In addition to environmental wear, the plastron also grows along with the turtle, recording growth lines and markings at different stages of development. By examining the markings, experienced herpetologists can even identify rough age ranges of turtles based on the scute patterns and number of growth ring-like lines.
| Turtle Species | Distinctive Plastron Characteristics |
|---|---|
| Cooters | Yellow “penciling” markings |
| Common Tortoises | High-domed, spherical plastron shape |
| Box Turtles | Very wide plastron, covering nearly the whole underside |
Is the Plastron Really Necessary?
The plastron is the lower shell of a turtle, but not all turtle species have a complete plastron. Some have only a partial plastron or lack one entirely. Though the plastron offers vital protection, it also limits flexibility – some turtles have adapted to life without a full plastron for increased agility.
Some Turtle Species Lack a Full Plastron
Many softshell turtles like the Spiny Softshell Turtle have a small, rudimentary plastron that leaves their undersides exposed. Sea turtles like the Loggerhead Turtle have a reduced plastron that covers only the front of their bodies.
Side-necked turtles in the genus Phrynops have a highly reduced plastron with large fontanelles – openings in the shell. And pig-nosed turtles in the genus Carettochelys lack a plastron completely.
Offers Less Protection But More Flexibility
The plastron provides crucial protection for a turtle’s delicate underside organs and limbs. Turtles without a full plastron are certainly more vulnerable to predators. However, an incomplete plastron allows for greater maneuverability.
Softshell turtles can tuck their long necks and legs into their shell openings, letting them move with ease through mud and sand. Sea turtles use their reduced plastrons to power through ocean waves. And the flexibility helps Carettochelys pig-nosed turtles capture fish and mollusks in Australia’s rivers.
Example Species Without a Full Plastron
- Spiny Softshell Turtle – Only 25% of underside covered
- Loggerhead Sea Turtle – Plastron covers only the front
- Fly River Turtle (Carettochelys insculpta) – No plastron at all
- Matamata (Phrynops hilarii) – Large openings in plastron
Though a full plastron offers the best protection, some turtles have adapted to survive and thrive with partial or no underside shell. The reduced plastron provides flexibility that aids these species in their specific environments, demonstrating the incredible adaptability of turtles.
Conclusion
In summary, the bottom shell of a turtle is called the plastron. This bony structure protects a turtle’s organs while still allowing for some flexibility. The size, shape and markings of the plastron can vary greatly depending on the turtle’s habitat and lifestyle.
So next time you see a turtle, take a closer look at its belly armor – the intricate plastron that helps make turtles such successful survivors.
