On our Galapagos cruise in the islands we are sure to see the green marine turtle, and with some luck possibly the hawksbill marine turtle, orEretmochelys imbricata.
Green turtles, Chelonia mydas, are one of the largest and most widespread of all the marine turtles in tropical and subtropical oceans. Adults can reach up to 370 lbs and are easily recognized by their oval carapaces about 40 inches long which varies from olive to brown, grey and black with bold streaks and blotches The common name is derived from the green color of the fat and connective tissues of this species. The plastron remains a pale yellow or orange throughout life. Overall, males are generally smaller than females, and green turtles differ in appearance from other marine turtles by the possession of a single pair of scales in front of the eyes and a serrated bottom jaw.
In the Eastern Pacific Ocean, including the Galapagos Islands, the green turtle tends to be smaller than its Atlantic cousin with a narrower carapace that may sometimes be completely black, explaining the other common name of 'black turtle' used frequently in Galapagos. For this reason some prefer to use the binomial Chelonia mydas agassizii or Chelonia agassiziiwhen referring to this population, even going so far as to call the population “endemic” to the Galapagos Islands. Research, however, has yet to determine the phylogenetic affinities of the various Pacific populations.
The green turtle is the only species known to nest on the beaches of the Galapagos Islands. As the waters of the archipelago start to warm in the latter part of the calendar year, females and males gather in front of the nesting beaches. At this time on our Galapagos cruise we can see them from shore, or on occasion from the Zodiacs as we tour the shoreline. When the females are ready, they will come ashore at night to dig a nest on their preferred beach. Not every trip ashore results in a successful nest, but over the nesting season, a female could possibly nest two to three times, laying on an average of one hundred eggs each time. She will then not return to her nesting beach for the next two to three years. After roughly fifty-five days, the nestlings hatch and return to the sea. The sex of the hatchlings is determined by the temperature inside the nest, with higher temperatures above 30º C resulting in females, and below 30º C resulting in males. The position of the egg in the nest and the depth at which it lies determines the overall temperature at which the egg will be incubated. Even the year’s climatic conditions will play a role in the sex ratio produced annually (food for thought when considering climate change possibilities). Green turtles reach sexual maturity anywhere from 11 to 59 years of age (apparently food-dependent), and only females return to land, the males forever keeping to the water for their entire lives. Green marine turtles are for the main herbivorous when adults, feeding on a variety of green algae growing in the nutrient-rich waters of the islands. As young and immature individuals, however, they are omnivorous and opportunistic in their eating preferences.
The hawksbill marine turtle is a critically endangered species and has been exploited for thousands of years as the sole source of commercial tortoiseshell. It is cosmopolitan in its range, being found in all tropical oceans of the world. Quite a bit smaller (82 kg or 180 lbs) than the green turtle, the beautiful carapace of the hawksbill is generally streaked and marbled with amber, yellow or brown which can be easily confused with young and immature green turtles. One principal difference used for identifying a hawksbill from other species are the scales (or scutes) of the hawksbill carapace which noticeably overlap. This imbricate nature (overlapping edges in a regular manner) of the scutes gives rise to the Latin species name of 'imbricata'. On their front flippers hawksbills have two claws on each of their forelimbs whereas green turtles have only one, and the narrow head and strongly hooked beak give rise to this turtle's common name.
Hawksbill turtles are omnivores, and feed primarily on sponges. They show a large level of feeding selectivity in that they only eat certain species of sponges, some of which are toxic to other animals. Sea jellies and other coelenterates are also common prey items as are mollusks, fish, marine algae, crustaceans, and other sea plants and animals. They reach sexual maturity at around 3 ½ years old, and possibly live up to 50 years, however that number has not been confirmed in the wild. Hawksbill marine turtles are not known to nest in the Galapagos Islands.
Through radio tagged individuals and rare sightings, we know the leatherback marine turtle, Dermochelys coriacea, visits the northern regions of the Galapagos Islands. They arrive from the Pacific coast of Central America where they nest, yet are almost never seen inside the archipelago, adding much to the mystery of these immense, prehistoric creatures. They feed almost exclusively on jellies, which could explain the little time spent in the Galapagos region as the waters around the archipelago are not known for large densities of these marine organisms.
The last species seen on extremely rare occasions in Galapagos are the Olive Ridley turtles, Lepidochelys olivacea. Pantropical in distribution, they are believed to use primarily the coastal waters of over 80 countries in the tropical to subtropical zones of all the world’s oceans. The Olive Ridley is the smallest living marine turtle with an adult carapace length averaging 65 cm (25 inches) and almost never weighing over 50 kg (110 lbs). They are omnivorous in that they feed on crustaceans and marine invertebrates as well as filamentous algae when no other options are available.
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