Turtles, Beyond the Liquid Sea.


“Turtles poured out of the surf in wave after wave through the darkness. Heaving, huffing, gasping turtles plowed the coarse black sand with their noses, laboring onto shore. On this rain-soaked October night possibly 30,000 Olive Ridley sea turtles were converging on a half mile of Pacific Beach at Ostional, Costa Rica, in a biological extravaganza called la arribada - the arrival.”
Rudloe A & J

Biologists divide marine habitats vertically into two zones, the pelagic and benthic. The pelagic zone consists of the open water that extends from shore to shore. Pelagic organisms float or swim, and include animals that swim against the currents, such as fish, squid, sea turtles, whales, dolphins.

Form and function

The unmistakable appearance of turtles results from their characteristic shell, which encases the entire body apart from the protruding limbs, tail and head. In most species the shell is a fairly rigid structure, although there are a few cases where the shells are softer, for example the Leatherback. They travel enormous distances and can dive up 3000 ft deep. This is one reason why this species have the softer leathery shell, so they do not implode as their go deeper.

The shell is divided into two main parts, the upper surface being known as the carapace and the lower described as the plastron. These two structures are linked together by bridges located at each side of the body between the front and hind limbs. The strength of the shell is the result of several layered plates, which in turn are covered by shields, made of keratin and usually called ‘scutes’. The pattern of the two layers overlap, consequently providing the shell with reinforcement.


'Hippie' a Hawksbill turtle.
Location : Yongala wreck, Australia.

In young turtles the bony plates are often not yet fused. They gradually grow together over time, joining in a zigzag pattern. Once fusing occurs the growth of the shell slows down, although new bone may continue to be deposited around the edge of the plates where the sutures themselves have become completely ossified. It is at this point that the shell has reached its final size.

The vertebral column within turtle is greatly modified. They posses 8 cervical vertebrae which are extremely flexible in comparison to the vertebral columns found in mammals. The central part of the vertebrae is fused and provides the carapace with support by being joined to the neural bones of the shell. The limb bones are also significantly different to other organisms. The shoulder blade is attached to the carapace and provides an anchor for the limbs to position themselves by.

Living in salt water

Marine turtles have developed to be capable of surviving exclusively in the oceans, adapting totally to their environment by means of a special tear gland, which provides the major route for excretion of sodium and potassium. These glands assist the kidneys, which would be unable to cope with the digestion of so much salt from both food and seawater. The function of the glands is under the hormonal influence of the adrenal cortex ensuring that the turtle does not become dehydrated. Sea turtles can thus ingest salt water and convert it to freshwater to stay hydrated.

Life cycle:

Laying

   
The laying 'ceremony' of the Leatherback turtle.
Location : Trinidad

All sea turtles come ashore to lay eggs, but for most it is a relatively exhausting affair. Watching as dauntless turtles break from the night surf, it is hard to remember that sea turtles are in serious trouble.

Leatherbacks can grow up to 1.8m (6ft) and weigh 680kg. These gentle giants may spent most of their life in distant places such as the coasts of Africa, or the Indian Ocean but when it is time to lay their eggs they come to their 'home' beach to lay 60-80 eggs every two weeks for about 4 months.

Their expedition to the beach can be strenuous and very exhausting for them. Usually you will only see them during the night hours as it makes it possible to avoid predators - some as early as right after sunset. These large and slow creatures can take up to 90 minutes to drag themselves up the beach, dig a hole, lay the eggs and then cover everything back up. It is extraordinary to see them dig a 2-3 ft deep home for their eggs with their hind fins. Only if it is deep enough without the sand falling back into it will she start dropping her eggs. Sometimes it takes the female three or four attempts to dig a hole she is satisfied with. The actual laying only takes 10-20 minutes.

At that time they are concentrating so hard they do not notice anything around them. You could probably run by with a big truck and they would not move! Once the eggs are all in, the turtle starts to cover them and camouflage the spot. Completely worn out they start their return trip to the ocean. It takes them probably another 8-10 Minutes to cover the 30-50 ft back to the water, where they dive as soon as they have gasped a couple of lungs full of air. In 10 days it starts all over again. Eight weeks later up to 100 little turtles crawl out of the nest and try to reach the ocean as fast as they can. It is a run for life. Big seagulls, terns and boobie birds are waiting to pick them up. Once they reach the ocean they are only a little safer. Statistically only 1 out 80 makes it to adulthood and come back to their beach. Life is unbelievable.

The green turtle is another reproductive migrant. When the time for laying eggs draws near, female green turtles swim from their feeding grounds back to the place of their birth. Sometimes up to 4800 km (3000 mi) away. In many cases it is their first return to their place of birth. After their long swim, they haul themselves onto the sandy beaches, scrape out shallow nests, and deposit their eggs. Once this is done, they swim back to their feeding environment. Despite being highly prolific and laying over 1000 eggs in one breeding season green turtles are vulnerable to the effects of predators.

Once the Green turtle begins to lay her eggs she loses all caution and becomes entranced with the process, unconscious of her surroundings. Her eggs are laid in rapid succession and are channeled into the breeding chamber by the hind limbs. The hind limbs are also used to close over the chamber in order to insure it is camouflaged from predators. Unfortunately in some sites where space is limited the females will dig up previously laid nests in order to find a safe incubating site for their own young. These eggs then become subject to predation from birds, which can often be nesting in the same area. The time of laying also aids the turtles in protecting their young as they come ashore in the evening and leave by sunrise thus being able to avoid a big display and disguise the path in which they have taken.

The significance of the incubation temperature has always raised an interesting question of male or female? In sea turtles it is amazing to see that the temperature is considered to be a vital ingredient when it comes to the sex of the hatchlings. Male hatchlings often occur when the incubation temperature is kept at 28° Celsius (82° F) where above 30°Celsius (86°F) female young will hatch. In the intervening temperature ranges hatchlings of both sexes can be expected.

Hatching:

“So ponderous on land, sea turtles swim with grace and speed in the waters of every continent except Antarctica. All begin life as tiny hatchlings dashing for the surf. Those that are not eaten by swooping birds and marine predators seem to spend at least a year drifting on the high seas, eating pelagic crustaceans, jellyfish, algae, and insects blown from shore. As juveniles, each species takes up its own niche in the environment.”1


Green turtle hatchling.
Location : Flinders Cay

Prior to actually emerging from the egg the young turtles begin to break down the shell with the use of the ‘Egg Tooth’ (Caruncle) visible as a small protrusion at the tip of the nose. This is a unique tool and once it has served its purpose it regresses. The length of time taken to escape from the shell can vary and through this period the turtles own shell starts to straighten and take on the appearance we are familiar with. The yolk, which serves as nourishment for the young, will only last a few days and then the turtle’s will need to feed.

Where do the baby hatchlings go? Until recently this was an unanswered question which has had many biologist puzzled. One piece of the puzzle has been found. Off the shore there are patches of sargassum weed (algae) that floats on the surface and is carried through the ocean via currents. Turtles have been found to be taking refuge in these sargassum patches and feed from them. This discovery was affirmed after examining the stomach contents of turtles found in the sargassum. Later as adults they will return to the beaches to mate and lay their young and thus continue the circle of life.

“From Florida beaches the hatchlings swim about 25 miles in 30 hours to take shelter and feed in sargassum, a bushy floating seaweed. Currents draw them farther out, where many are picked up by the Gulf Stream and carried across the Atlantic. The next time anybody sees these little loggerheads, they are at least four inches long and living near the Azores.”1

Courtship and Mating

Courtship and mating behaviors are often triggered by physiological changes set in motion by changes in the environment. For instance, longer daylight hours, warmer temperatures, and other environmental cues in the spring may trigger multiple hormonal fluctuations. These varying hormone levels may produce territoriality in males or nesting behavior in females.

Physiological changes linked with courtship and mating help guarantee that mating occurs during the limited porthole of time called estrus, the brief stage in the female’s reproductive cycle when eggs are released and she is receptive to mating. The frequency and timing of estrus varies with animal species. Courtship is the collection of ritualized behaviors unique to each species that leads up to and enables animals to successfully mate.

Mating is the coalescence of sperm, the male sex cells, in the vicinity of eggs, the female sex cells. Animals use a multiplicity of methods to mate, all of which encourage fertilization, or fusion of sperm and egg. Fertilization initiates growth of a new organism, the establishment of the next generation. Union of sperm and egg, known as reproduction, is the predominant means of reproducing on earth. It results in an collection of hereditarily diverse progeny (young) that are better able to survive a changing environment, thus increasing the potential for preservation of the species.

Marine turtles often face a difficult task when attempting to perform the act of reproduction. The male has to mount the female from behind and latch on in order to be able to copulate inside the female, Some species are forced to adopt a vertical stance during mating as their tail is short and being on the shell of the female makes things difficult. Most males secure themselves by hooking in between the female’s carapace and the plastral hinge. The female in turn assists by providing additional support using her hind limbs.

Once the male is in position the tail is used to establish contact with the female and dilate the cloaca so that the penis can penetrate with ease. Once penetration is accomplished the male holds the tip of his tail under the females plastron, this helps him anchor in place. Mating can be a lingering process, which can sometimes continue for an hour. 

Man and The Turtle

   
Catching turtles in Rotuma Island.
Preparing and cooking them on Ngulu Atoll.

All eight species of sea turtles are endangered or threatened. They are killed for leather and meat; their eggs are taken for food. Their nesting sites for development, poisoned by pollution, strangled by trash, run over by pleasure boats, and drowned by fishing line and net. The green turtle is probably the most badly effected by the influences of man, as they are a popular food both traditionally as seen in Micronesian islands and further a field in restaurants as a delicacy in soup.

While STARSHIP has been traveling around the world we have been able to view these magnificent creatures in many different ways. We have been fortunate to watch them lay their eggs on beaches and later watch the young hatch. As young adults we have watched them swim beneath the cobalt blue waters of many different seas. We have also seen how traditional cultures capture and feast on turtles when they have cause for celebration. In addition evidence found in the Trobriands in PNG many cultures use the turtle shell to make jewelry. 

“It was beauty that all but killed the hawksbill. Polished and carved, the intricate black-and-yellow plates on its back were long sought for tortoiseshell jewelry and combs. Now the hawksbill sits with Kemp's ridley on the edge of extinction.”1

We visited Ngulu Atoll in Micronesia and the crew from STARSHIP watched on as the village men pulled from the ocean some large green sea turtles. The turtle where taken to their village and the chief then began the ritual of preparing them for feast. The turtles are first killed with a sharp blow to the head then the process of removing the internal organs along with the eggs begins.

The Chief of the village laid the turtle on it shell and cut away the flesh from between the two front limbs. From here he could reach inside and remove the organs, which would be kept and eaten as well as the eggs. This at least suggests that nothing is wasted. The turtle is dragged to a shallow fire pit where it is covered in palms and cooked. The cooking process takes only 10 to 15 minutes and then the flesh is ready for eating.

It is true that in most societies this practice is seen as cruel and inhumane. But what we must not forget is a way of life for these people and is the equivalent of a western culture slaughtering a cow in order to feed on it. The people of Ngulu told us that it is not something they do everyday but only for special ceremonies.

A more distressing activity that seems to be occurring is the influence of cultures such as own encouraging the slaughter of the turtles so they can hang the prize shell from their mantle, or the increase in turtle jewelry on the shelves in shops like we saw in a small store in Koror, Palau. If we provide people with a market for this type of product the needless slaughter will continue. Education plays a unique role in the protection of wildlife and can only be encouraged.

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A Mission Statement:

A nearly full moon in the sky and the initial site of the baby hatchlings as they emerge from the sand, taking their first look at the big wide world. On the beach you can sit and watch these ephemeral visitors as they take their first steps towards life. Battling the shore break they materialize triumphant and endeavor to discover what lies beyond the shores of their birth. One can marvel at the beauty of these animals as they slip for the first time into the liquid world of the sea.

I can only hope that this reflection will imprint in our minds and allow a generation to become in awe of these wondrous creatures of the liquid sea.

Louise Oliver

SPECIES :

Common Name Green turtle
Family Cheloniidae
Genus Chelonia
Species mydas
Common Name Hawksbill turtle
Family Cheloniidae
Genus Eretmochelys
Species imbricata
Common Name Leatherback turtle
Family Dermochelyidae
Genus Dermaochelys
Species coriacea

REFERENCES :
Rudloe A & J. "Sea Turtles Race for Survival", Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.
Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.
Alderton D. 1998 Turtles and Tortoises of the World, Blandford Press, London.
Allen GR & Steene R, ‘Indo-Pacific Coral Reef Guide’ 1999, Tropical Reef Research;