Turtles are reptiles and possess skin, though its structure is highly specialized across their body. Confusion often arises because the shell is not a separate house but an inseparable part of its anatomy. This shell is a complex, composite structure that integrates bone, living tissue, and a modified skin layer for protection. The integument ranges from the softer, flexible skin on exposed parts to the hard, keratinous plates forming the shell’s exterior.
Skin on Limbs, Head, and Tail
The skin on the exposed parts of a turtle—the head, neck, limbs, and tail—is typically scaly, leathery, or wrinkled soft tissue. This skin is not fused to the shell, allowing the flexibility needed to move, eat, and retract the head and limbs for defense.
This exposed skin provides sensory perception, protection, and water regulation. It acts as a protective barrier against abrasions and pathogens, and helps prevent dehydration. Like other reptiles, turtles periodically shed this outer layer of skin as they grow.
The Shell Structure: Bone and Dermal Layers
The shell is composed of two main parts: the convex upper shell, called the carapace, and the flatter lower shell, known as the plastron. The inner part of this structure is bone, formed from the fusion of the turtle’s ribs, vertebrae, and parts of the shoulder girdle with dermal bone plates.
This bony structure is covered by a dermal layer, a living tissue that provides blood and nutrients to the entire shell complex. The spinal cord runs directly through the carapace, confirming the shell is a living part of the animal. This dermal tissue is the foundation upon which the shell’s outermost layer is produced.
The Scutes: Keratin Shields and Skin Renewal
The outermost layer of the shell consists of large, plate-like scales called scutes, which are essentially highly modified epidermal skin scales. Scutes are made of keratin, the same fibrous structural protein found in human fingernails, hair, and the scales of other reptiles.
The scutes protect the underlying bony plates from scrapes, bruises, and environmental damage. Because the scutes are dead keratin tissue, they do not contain nerves, but the living tissue underneath is sensitive, which is why a turtle can feel a gentle touch on its shell.
The process of growth and renewal of scutes, known as ecdysis or shedding, varies significantly between species. For many aquatic turtles, the outer layers of the scutes are shed annually in thin, translucent sheets to accommodate growth. Terrestrial tortoises, however, often retain their scutes, with new, larger layers growing underneath the old ones, sometimes leading to visible growth rings.