Why Are Green Sea Turtles Called Green? Not Their Shells

Where the name comes from

The green sea turtle (Chelonia mydas) got its English name from people who hunted it, not from naturalists admiring it at sea. For centuries green turtles were caught for food, and when they were butchered, sailors and cooks noticed that the layer of fat between the shell and the body was a distinct green color.

That fat, together with the cartilage known as calipee, was the key ingredient of green turtle soup, a famous and expensive dish in 18th- and 19th-century Britain and America. So the name “green turtle” was in common use long before anyone worried about protecting the species.

The scientific name does not mention the color. The genus Chelonia comes from the Greek word for a tortoise or turtle, and Carl Linnaeus first described the species in 1758 as Testudo mydas.

Why is green turtle fat green?

The green fat is linked to the green turtle’s unusual diet. It is the only sea turtle that becomes a herbivore as an adult, grazing on seagrass meadows and on seaweeds (marine algae). Plant pigments from this diet, thought to be mainly yellow-green carotenoids, build up in the fat stored in the turtle’s body.

You can see a similar effect in other animals: the color of egg yolks, for example, depends partly on what the hens eat, and flamingos get their pink color from pigments in their food. Young green turtles that live in the open ocean eat mostly small animals, and it is the plant diet of adult life near the coast that makes the species distinctive. Learn more about their menu in what do sea turtles eat?

What color is a green sea turtle’s shell?

The upper shell, or carapace, of an adult green turtle is usually olive, brown or grayish. Many have a pattern of radiating streaks, spots or marbling in darker brown, yellow or black on each shell plate. The lower shell, or plastron, is pale yellow to white, and the skin of the head and flippers is dark with pale edges to the scales.

Color changes with age. Hatchlings are mostly black on top with a white underside and white edges to the flippers. As they grow, the shell turns dark brown to olive, and patterns become more varied in adults.

The shell can look greener in the wild than it really is. Algae often grow on the shells of slow-moving turtles, and in sunlit, shallow water the blue-green light can give the whole animal a greenish tint. Clean, dry shells on nesting beaches are usually clearly brown or olive.

The black turtle of the eastern Pacific

In the eastern Pacific, from Baja California and the Gulf of California south to the Galápagos Islands and Peru, many green turtles are much darker, with a gray to almost black shell and skin. They are also usually smaller and have a narrower, more domed shell than green turtles elsewhere. These turtles are known as black turtles, or in Spanish as tortuga prieta or tortuga negra.

For a long time black turtles were treated as a separate species, Chelonia agassizii, or as a subspecies of the green turtle. Genetic studies have shown that they are not distinct enough to be a separate species, and most scientists now treat them as a regional form of Chelonia mydas. Even so, the name black turtle is still widely used, and eastern Pacific green turtles are managed as their own population.

How to recognize a green sea turtle

Since the color can mislead, divers and snorkelers use other features to tell a green turtle from the hawksbill sea turtle and the loggerhead, which share many of the same waters.

The single pair of large scales between the eyes is the most reliable feature for an adult green turtle. On a nesting beach, green turtles leave broad, symmetrical tracks, because they move both front flippers at the same time, much like doing the butterfly stroke.

Myths and facts about the name

Myth: green turtles are named for their green shells. As shown above, the shell is not green; the name refers to the fat inside the body.

Myth: green turtles get their color from eating green plants and turning green all over. Only the internal fat is tinted. The skin and shell keep their normal brown and olive tones whatever the turtle eats, although algae growing on the shell can make it look greenish.

Fact: the honu of Hawaii is a green turtle. The famous Hawaiian sea turtle, known as honu, belongs to this species. Hawaiian green turtles are known for coming ashore to bask in the sun, which makes them some of the most photographed sea turtles in the world.