From left to right images of a Green Sea Turtle, Loggerhead Sea Turtle, Hawksbill Sea Turtle, Kemp's Ridley Sea Turtle, and Leatherback Sea Turtle. USFWS Images. The green turtle (Chelonia mydas) is the largest of the hard-shell sea turtles, growing on average to a length of three feet and a weight of up to 500 pounds. The carapace is oval-shaped and smooth with shades of black, gray, green, brown, and yellow. The plastron is a yellow-white color. The lifespan of the green turtle is unknown, but sexual maturity occurs between 20 to 50 years of age. A green turtle’s diet is unique compared to the other six species of turtles. Adults are primarily herbivores, feeding on seagrasses and algae. A green turtle’s fatty tissues have a greenish hue because of their diet, which is how they take their name. Green turtles are distributed in tropical and warm temperate waters worldwide. Nests are uncommon in the national seashore, but green turtles are present in the coastal waters.