Sea Turtles! Part 3: Leatherbacks, Loggerheads, and Greens.

This blog is intended to be a continuation of my sea turtle series, so for basic information about sea turtle biology and behavior you can check out Sea Turtles Part 1 and if you’re interested in the threats that sea turtles face including disease, parasites, predators and human impacts you can go see Sea Turtles Part II. Now that I’ve covered the basic backgrounds of sea turtles in general, I’d like to go into specifics about what makes each species unique. As I’m talking about each individual species, you can refer to my chart that compiles some information about each species that might make it easier to organize in your head.

The Leatherback

The leatherback is a “super turtle” in several aspects. First, as you may have guessed, it doesn’t have a hard shell like that of other sea turtles. Its shell is different – instead of having the ribs fuse and form a bony carapace (the top part of the shell), it has a layer of rubbery skin with a fibrous cartilage layer. This cartilage layer is covered again by membrane bone – called osteoderms. These are tiny bones about the size of a dime or quarter that lock together to form the carapace. This layer is a little flexible and this adaptation allows the leatherback to dive deeply into the water – depths of more than half a mile. It also swims faster, farther, and deeper than all other sea turtles, produces the heaviest and largest eggs, and yet is a gentle giant – eating primarily jellyfish.

One amazing thing about the leatherback is its cardiovascular system, which includes a countercurrent heat exchange system in its limbs. This system allows the leatherback to conserve body heat so that it can live in a wide range of habitats that other sea turtles cannot. The arteries and veins of the leatherback are formed in a tight network in its flippers. Arteries are the blood vessels that flow from the heart into the limbs, and veins flow from the limbs back to the heart.

As the warm blood from the arteries flows past the vein, they transfer heat to the veins before the veins lose their heat to the surface of the skin. With a counter current heat exchange system, the veins don’t carry cold blood back to the heart and lower the core body temperature – when the blood gets back to the lungs and heart it is warm. If the animal is too hot – the heat transfer can be reversed by transferring heat to the arteries so that the veins are cool when they flow back to the heart, thereby lowering the core body temperature.
They also control their body heat with thick layers of fat that surround the muscle. This works as excellent insulation in cold water. In warm water, they have a network of blood vessels that allows the blood to flow past the fat to the outer edges of the skin so that heat can be easily released. This is why nesting females’ throats turn pink.

Another adaptation that leatherbacks and a few other sea turtles have is called esophageal papillae. These prongs are made of cartilage and line the throat of the turtle, and basically grip on to the jellyfish that it’s eating and ensures that it doesn’t slip back out as the turtle uses the muscles in its throat to expel excess salt water.

The esophageal papillae also protect the leatherback from the stinging cells for the jellyfish – as you can imagine, the jellyfish don’t go down without a fight, so these cartilaginous prongs keep the turtle from being injured by its prey. Now, leatherbacks travel vast distances and require a lot of energy to make these migrations possible – but jellyfish are primarily water, a little protein, some vitamins and minerals, and some fat. So – how on earth can a leatherback fuel it’s body with them? Well, they actually have an extremely long esophagus that leads from the mouth to the rear of the body, and then it loops up the side again until it reaches the stomach, which is about a quarter of the way from the front of the turtle. This long esophagus acts as a holding pouch so that the leatherback can continually digest its food – as parts of its meal leaves the stomach digested, new jellyfish are being pushed into the stomach.

I’d like to mention just a couple more interesting things about the anatomy of the leatherback. The ends of leatherback bones contain cartilage laden with blood vessels. This is unique among sea turtles, and is thought to facilitate their rapid growth by allowing access to necessary nutrients.

The Loggerhead

The loggerhead is the most abundant sea turtle in the United States. I love the loggerhead because it is the quintessential “awkward turtle.” They are slower than many other species of sea turtles, and that may be due to the fact that they are the most common turtle to be seen with epibiota on their shells. They are, of course, named the loggerhead because of their wide skull. Hatchling and juvenile loggerheads are pelagic – or live in the open ocean far from any coast. They like to hang out in convergence zones where they can get access to a variety of food sources. Loggerheads are a warm water species, and are rarely seen in water lower than 50ºF. Once they reach juvenile stage, loggerheads are take on a reddish-brown shell which is characteristic of the species and will stay with them throughout their lives.

Loggerheads prefer to eat mollusks and crustaceans, in order to do this their rhamphotheca (beak) is extremely thick, and they have broad, heavy muscles in their jaws for crushing.

As you may have guessed, loggerheads are completely carnivorous. They are opportunistic feeders. They eat mostly invertebrates and insects, but their diet can also include jellyfish, medusae, salps, barnacles, snails, aquirts, anemones, sea cucumbers, marine worms and fish.

Loggerheads reach sexual maturity at 17-33 years of age. Females lay an average of 110 eggs per clutch, with usually 2-3 nests per year, although they can lay up to 7. they wait about 2 weeks between nests. Their eggs will incubate in the nest for 50-75 days, depending on how warm the sand is. Like most sea turtles, females use an alternate gait on land, meaning they move the front left flipper at the same time as the back right, and vice versa. They are documented to have high nest fidelity.

The Green

The green sea turtle is your “poster child” sea turtle, if you will. They are the most commonly recognized species because of their beauty. Surprisingly, their shells and skin are not green as the name might imply – their coloration is more of a brown/yellow/white mixture. They are called green turtles because their fat is green. Green sea turtles are the only species in which the adults are completely herbivorous (vegetarian) and eat primarily sea grass. Green juveniles are carnivorous like other species, though, and their digestive system actually changes as they mature to allow for this dietary shift. Green turtles’ large intestines actually double in length in order to digest the plant material properly. Because of all the vegetation they eat, their FAT is actually green – which is how they got their name. They can be distinguished from other similar looking species by looking at the scales on their head. Between the eyes, they have a pair of scales where other turtles only have one or several. Their scales are well defined and dark. They are also the only species with jagged edges on their lower beak, these act as pruning shears to take bites of grass.

Green turtles play an important role in the “underwater prairies” of their feeding grounds. The role they play is similar to that of the buffalo on the open prairies in the early days of American colonization. They modify the prairies by eating so much of the excess plant material, stimulating growth in the ecosystem. They will also return to the sites that they previously grazed in order to eat the new sprouts that are full of nutrients. It’s estimated that there are only between 3-9% of green sea turtles left from their pre-human numbers. The reduction of green sea turtles can have serious repercussions for the marine grasslands they modify. The marine grasslands are very important to the ocean as a whole because they act as nurseries for many species, and it is one of the most productive ecosystems in the world.

Green turtles become sexually mature at between 12 and 50 years depending on where they feed and what they were able to eat. Their nesting season is between June and September, and they are noted to have high nest fidelity. In other words, they commonly return to the same beach to nest each season that they reproduce. They lay 1-7 clutches/year, and usually have 13 days between each nest.

Green turtle females are said to be more skittish than other types of sea turtle females, and frequently have false crawls. Like the leatherback, green turtles have a unique gait as adults – moving both front limbs at the same time, then both back limbs. The hatchlings, however, use the alternate gait. Hatchlings also have the added advantage of countershading – meaning that they have dark carapaces and light colored plastrons to help camouflage them from both underwater and aerial predators.

As I mentioned in my previous blog, fibropapillomatosis is a debilitating skin disease that particularly affects green sea turtles. In fact, it’s estimated that 40-60% of green sea turtles in Florida are affected by it. However, this disease is a small problem compared to the impacts that humans are causing the species. They are still frequently hunted in some countries for their fat, meat and skin. They are used frequently in green turtle soup. Historically, green turtles actually played an import role in allowing the Spanish colonization of America. They provided a valuable food source to the colonists. Currently, loss of nesting habitat is the biggest threat in the United States.