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Ocean Watch
By Susan Scott
Monday, October 9, 2000
The surf is big here on the North Shore, my wandering tattler friend is preening on the beach outside my window and I have interesting mail on my desk. Turtles don’t
shed their shellsLife is good.
Life is also apparently good for reader Ted Farm, who for years has written me interesting letters. Here's his latest:
"We sit in my grass hut at Ewa Beach every afternoon with an animated view of waves, surfers, ocean traffic, fishermen, and of course, turtles. The question came up, how does the turtle grow? The turtle is a reptile with its skeleton is on the outside (we think). Crustaceans have their skeletons on the outside and must get rid of them to grow. But we do not find any turtle shells that indicate a turtle is changing its shell. How does he do it? We also wonder why snakes shed their skin. Is it to grow?"
It's true that crustaceans have outer skeletons that they must shed to grow. Turtles, however, have skeletons on the inside of their bodies, complete with limb bones, neck bones and vertebral columns.
The shell, although connected to the skeleton in some places, is actually the animal's lifelong suit of armor. It never falls off, but instead grows with the turtle.
In general, turtle shells consist of an inner layer of fused bones and an outer layer of tough skin called keratin. Hawaii's greens, and most turtles in the world, have this kind of shell structure, but there are exceptions.
The most unusual is the huge leatherback turtle, which has a shell type all its own. Its inner layer consists of interwoven bones rather than plates, and its outer layer is entirely cartilage instead of keratin.
We all know keratin by other names. In its soft form, this skin protein is hair, fur and wool. Hard keratin makes scales, feathers, hooves, horns, claws, nails -- and the exterior of turtle shells.
In turtles, keratin grows in shields called scutes, which contain the distinct colors and patterns of the species.
Interestingly, the edges of the bones and scutes in a shell's two layers don't match up. In some places, one scute covers several fused bones. In other places, two scutes cover one big bone. This mismatch strengthens the shell and provides more protection for the soft parts inside.
As a turtle grows, new bone grows around the edges of the shell's bony plates. Scutes grow in a similar manner with keratin growing around their edges.
But again, there are exceptions. Some freshwater turtles shed their old scutes as they grow and make new ones. The old scutes drift in the water like big rubbery scales.
When injured, turtle shells bleed, and the wound likely hurts, since shells contain nerves. Damaged shells can heal, but the process is slow.
Sometimes, veterinarians can repair broken turtle shells with polymer resin and fiberglass.
As turtles grow, the scaly skin on their limbs and head flakes off in patches as does that of lizards and geckos.
Snakes, however, have skin cells that tend to hold together when shed. So when a snake's body grows, it sheds its skin either whole or in large pieces.
Most land snakes do this three to five times per year, but the yellow-bellied sea snake, Hawaii's only species (but rare), sheds its skin more often, sometimes as often as every two weeks.
To get rid of its old skin, this snake turns itself into tight knots, then swims through them, rubbing off its skin.
Ted, your hut seems as much fun as my job. Thanks for taking the time to share it with us.
Marine science writer Susan Scott's Ocean Watch column
appears Mondays in the Star-Bulletin. Contact her at honu@aloha.net.
