Swimming to shore, Honaunau, Hawai’i (photograph by Jeffrey Levinton)
On beach, Heron Island, Australia (photograph by Jeffrey Levinton)
Biology of a Global Voyager
I have traveled far and wide, and a constant of my beach and diving experiences has been encounters with sea turtles. In the Caribbean, from Florida to Jamaica to Bonaire, green turtles, Chelonia mydas, can be seen gliding majestically underwater, hauling out onto beaches and gravel bars, females digging nests, laying eggs, and tiny hatchlings leaving their shells and scrambling to sea. In Heron Island, Australia, one can see platoons of black-tipped sharks waiting in the shallows, catching so many of the newborn, before they begin their amazing sea journey. Every hatchling must get to sea, eat meals, rise to the surface to breathe, grow, and females must eventually get back to a suitable beach to haul out, make an egg nest and complete the amazing cycle. They are air breathers, but one cannot forget seeing them swimming below the surface, and even resting on the sea floor. Green turtles can remain below the surface for five hours!
Green turtle resting on a coral reef, Maui, depth of ~12 m. Photograph by Jeffrey Levinton
To start thinking about how amazing this is, let’s start big. Just look at the geographic range of this amazing sea turtle, one of seven known through the global ocean. They are in every major ocean where there are subtropical and tropical waters. Let me jump ahead! No don’t think that every turtle travels the whole distance. But the facts are amazing enough, so much so that people doubted the great pioneers who figured out the amazing journey that a single female completes, again and again. Let’s get to that just after some basic facts of ecology and biology.
Geographic range of the green sea turtle, Chelonia mydas.
Some Simple Facts of Ecology
Where are they and what do they eat?
So what do you see if you snorkel and watch these creatures. Where do you find them? If you are an average snorkeler or diver, you will see adults in shallow water munching sea grass. This is already interesting. They are more or less cold-blooded and really large, reaching masses of 160 kg (350 lb), so the adults are more or less like cows or buffaloes. Green turtles reach the age of 70 years and don’t become sexually mature until 25-35 years old. They are the only species of the total of seven sea turtles that are true herbivores. More on this now.
Green turtle eating sea grass
Sea grasses are found throughout the very shallow waters of the tropical to cold temperature ocean. They are true grasses, as are found on land. There might be just one common species in my New York area, but you will find about 15 species in the shallow Caribbean, and there are about 70 species worldwide. One of the most common species in the Caribbean is called, surprise surprise, turtle grass (Thalassia testudinum). Yes they have flowers! Yes they have pollen. And, yes they are tough to eat, like land grasses. So are green turtles like cows? Well, yes, sort of! The turtles can be seen munching on the grass, just like cows. But one day, a scientist named Tom Fenchel, was on a boat where a green turtle was purchased from a fisherman and brought on board. The turtle was not in great shape, and Fenchel decided to follow a very interesting intuition. If sea turtles ate grass, which is literally roughage and very hard to digest, maybe green turtles are just like cows. And they were. The hindgut was a specialized site where fermentation aided by symbiotic gut microbes facilitated the digestion and assimilation of digestive products. Subsequent DNA studies demonstrated that the hindgut bacterial flora of green turtles consisted of several bacterial groups known to be crucial in helping terrestrial grass eaters to digest and allow assimilation of cellulose grass sources. So green turtles really are the cows of the sea.
Where do they mate?
Green turtles have separate sexes. So they must mate to produced fertilized eggs. But where do they mate? We now have to introduce the center of all green turtle life: the beach! Females come to a beach, haul out, dig a nest, lay eggs, the eggs hatch, and hatchlings scramble to the sea. But where do the males come in? They arrive during the reproductive-egg laying season, but before the females. So mating must take place offshore as the females return to the nesting beach. Have we observed this? Yes, but much is known from aquaria observations, although tourists in some places are lucky enough to observe this directly. We know most from observation of captive turtles, where males approach females, climbing on to her back and biting her neck and flippers. The male uses his-tail cloaca to guide the penis to the female’s cloaca to inseminate the female. Males must sometimes actively fight off competing males. Mating and egg laying is very seasonal.
Females haul out on the beach, dig a nest and lay eggs. The hatching and scramble to the sea is dangerous for the hatchlngs.
Females now move to the beach, crawl to above the highest tide level, and dig a deep nest with their hind legs. The digging can take hours, and a female green turtle may lay about 100 eggs in a nest, move back to the water by morning, and may return about every two weeks for several times, before the egg-laying season is over. The season lasts 2-3 months. In the Caribbean, the egg-laying season is in late spring to early summer. At Heron Island, Australia, the egg-laying season begins in November, and egg hatching reaches a peak in January, which attracts large numbers of tourists. Eggs hatch 90 days to two months after egg laying, and then scramble out of the burrow along the sand to the water’s edge. The short scramble is dangerous, since shorebirds prey on them, and small predatory sharks await in the shallow as the hatchlings scramble into the water. A female won’t return to mate and lay eggs for 2-5 years.
Green turtle digging her nest, Heron Island, Australia
Hatchling scrambling along sand to the sea, Heron Island, Australia
Green turtle mom returns to the sea, Heron Island, Australia (photograph by Jeffrey Levinton)
The Amazing Navigators
What do the females do after leaving the beach? The amazing journey and their repeated homing to nesting beaches.
A good nesting beach is not necessarily near good feeding grounds with sea grass. Green turtles have a problem. They come back every 2-5 years. But where to they eat and how do they find their way between the beach nesting and sea grass feeding sites. The pioneering sea turtle biologist Archie Carr (1909-1987) had some wild ideas, which he developed by learning to identify green turtle females by characteristic markings. He worked in Tortuguero in Costa Rica, and gradually was able to recognize that females were nesting in Costa Rica, but were feeding all over the Caribbean including Florida, But he concluded that females returned repeatedly to the same beach. This conclusion has been confirmed by tagging and genetic data and had amazing implications. First, the turtles were navigating somehow, often distances of hundreds of kilometers. Second, the return from so many places to Tortuguera in Costa Rica showed that protection of crucial beaches were central in preserving green turtles, and likely all other sea turtle species, which had to return to beaches of their own.
Here is one amazing example of homing. Many green turtles are known to feed in seagrass beds off the tropical coast of Brazil. Work by Koch et al. (1969, J. Theoret. Biol. 22:163-`79) showed that green turtles fed in coastal shallows of eastern Brazil, but females migrated to one of several specific beaches on Ascension Island, over 2000 km away!
Some green turtle populations migrate between coastal seagrass habitats in eastern Brazil, and return repeatedly to specific beaches on Ascension Island, in the middle of the Atlantic about 2000 km away (after Koch et al. 1969)
Satellite traces of five female green turtles, migrating between the eastern coast of Brazil and Ascension Island (from Luschi et al. 1998, Proc. Roy. Soc. 265: 2279-2284).
How do they do it? The amazing navigation is complex.
Keep in mind our evidence is only for females, who can be located repeatedly on the same nesting beach. It is generally suspected that the location of the beach is imprinted at birth and during the initial scrambling to the water. Other factors such as sunlight patterns also contribute, as must the smell of the local beach. But it took the work of Kenneth Lohmann and colleagues to show that the long-term migrations of other sea turtle species were guided by the earth’s magnetic field (see Lohmann and Lohmann 1996 Nature 380, 69-61). This subject is quite complex and some information is here. Because sea turtles must return to the surface, many studies have been done on migration patterns using satellite tracking. But Archie Carr’s amazing hypothesis has been confirmed. Read his wonderful book. World Sea Turtle Day is celebrated on his birthday, June 16.
Connecting All the Dots: Green Turtles are Like Pacific Human Navigators
It is well known that the Pacific Ocean’s vast array of islands was colonized gradually by a series of Pacific peoples. The Hawaiian Islands were colonized by Polynesians in more than one wave, coming from Islands as distant as Tahiti. The first arrival is believed to be in the 4th Century A.D. But how did they find the islands. Was it purposeful? Polynesians could have followed migrations of sea birds. Albatrosses fly hundreds of miles between feeding and nesting areas. Wave patterns might have indicated routes between islands. Celestial fixed points, like stars, must have provided constant directions, and variable movements of the moon and planets may have aided direction. Polynesians used small double-hulled outrigger vessels with sails, but the details are obscured by history and likely secrecy. On the smaller scale, Polynesians, like all fishers, must have moved about to follow fish caught for food. So maybe fishing was the initial driver of sailing. But eventually, a glorious migration occurred, spreading humans across the Pacific. In the context of the broader migrations, are well known shorter navigations between distant islands, aided by stars and currents.
Green turtles can be seen in the same light. Adults need to feed in seagrass beds, which are all in very shallow water, rarely deeper than 5-10 meters, and therefore near shore. Females laid their eggs on beaches and some must have been more suitable than others. This led to a migration pattern, driven by the movement of females, and males which mated with females near beach sites. But turtles must have occasionally expanded their sites of feeding and egg-laying, which resulted in the gigantic range we now see for all sea turtle species, but especially the green turtle, which has been as successful as humans in a magnificent expansion throughout the globe.