Sea turtle, seen off the coast of Maui, Hawaii.
Photographs by Art Ogden
Which turtle species lays the greatest number of eggs? Which turtle may migrate over 1000 miles to nest and may fast for over four months? Do you know which turtle navigates by magnetic cues and is the second largest turtle species in the world? Which species of turtle may lay a single clutch of eggs that were fertilized by multiple males? Can you name a turtle that is found in Africa, South America, Asia, North America and Hawaii and belongs to one of the oldest fossil turtle groups? Which turtle has been called the world's most valuable reptile?
If your answer to all of the above questions was the green sea turtle (Chelonia mydas), you can pat yourself on the back. The green sea turtle is one of seven species of sea turtles and the largest of the Family Chelonidae that includes the loggerhead sea turtle (Caretta caretta), the hawksbill turtle (Eretmochelys imbricata), the Kemp's or Atlantic ridley (Lepidochelys kempii), the olive or Pacific ridley turtle (Lepidochelys olivacea), and the Australian flatback (Natator depressa). The Family Dermochelyidae is composed of a single species, the leatherback sea turtle (Dermochelys coriacea).
Although the green turtle has been a very important source of meat, eggs, and ornamentation for many maritime communities for centuries, only recently (circa 1950's) have we taken the time to study the natural history of this animal. Our latent interest in investigating the biology of this animal has been due, in part, to the logistical difficulties of studying a marine animal that may range over many hundreds of miles of ocean and across many international boundaries. Although studying sea turtle populations is not easy, a more important factor influencing our low priority assigned to sea turtle studies has been the optimistic attitude that "there will always be more sea turtles." We are now painfully aware that this is not going to be the case with the endangered green turtle. As a result, many governments and private marine turtle research projects are now underway along every continent in both hemispheres.
Following the pioneering work of Dr. Archie Carr in the mid 1950's, a great deal of information has been gained about the unique natural history of the green sea turtle. Life for this curious creature begins more than two feet below the warm beach sand as the entire clutch of more than 100 eggs begins to hatch after about two months of incubation. After spending up to 48 hours absorbing their yolk sack, the activity of the clutch increases as they struggle to reach the top of the nest and free themselves of the sand. Their activity continues unless the sand temperature is too warm at which time they overheat and become torpid. This is why most nestling emergence occurs in the evening or on overcast days when the sand temperature is cool enough to allow the hatchlings to escape from their nest.
As amazing as the emergence of the hatchlings from the nest is, even more astonishing events have occurred deep in the nest prior to hatching. While the gender of most vertebrate animals is determined at conception by the presence of sex determining chromosomes, sea turtle sex is determined by the temperature of the sand during incubation. Warm temperatures (>29° C) produce more females while cooler temperatures (<29° C) produce more males.
After emergence, the hatchlings scramble down the beach apparently unaware of the many predators that await them on the beach and in the water. Hatchlings display little if any predator avoidance behavior as they frantically try to race across the beach and into open water.
Those hatchlings that are successful in their journey to the open sea will spend the next several years in a planktonic-like lifestyle in which currents will have an important influence on where they will find themselves. During this time, the hatchlings most likely feed upon both plant and animal material to gain size and weight rapidly. This rapid weight gain may decrease their chance of falling prey to predators. Although the estimates vary widely, it is likely that less than 1% of the hatchlings will ever reach sexual maturity. After several years, the young turtles will move into one of many inshore developmental habitats where they will spend the majority of their hours feeding on sea-grasses and algae with the occasional jellyfish snack.
Some individuals may spend more than 10 years in their developmental habitats before they arrive at, or possibly choose, their adult feeding ground where they will most likely spend the remainder of their adult life feeding until they are large enough to breed.