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Types and distribution   

Species diversity

There are 5 species found in Thai Waters, classified in 2 families: the Family Cheloniidae includes green sea turtle, hawksbill sea turtle, loggerhead sea turtle, and olive ridley sea turtle; Family Dermochelyidae includes leatherback sea turtle. The biological information of each turtle are shown in (Table)

Table  General information of sea turtle 

 

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

 

Figure Guidelines for sea turtle identification 

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012  

Distributions of sea turtle

The abundance of sea turtles was relatively high in the past; it has been reported that five species of sea turtles laid their eggs in Thai Waters found in both the Gulf of Thailand and the Andaman Sea. In the Gulf of Thailand and South China Sea, Most adult female sea turtles choose Ko Kram as their nesting grounds; after egg laying, they will swim for 2,500 kilometers and spend over 30 days to travel back to the Su lu Sea which are the main feeding grounds located in the boundaries of Malaysia, the Philippines, and Indonesia. Meanwhile, some adult female sea turtles are feeding in the south of Cambodia, Vietnam and Malaysia (Figure 3).

In the Andaman Sea, after egg laying, most adult female sea turtles leave from Mo Ko Similan and reach to Mu Ko Andaman, India which are the main feeding grounds. They continuously swim for over 650 kilometer with the average speed of 3 km/h and spend about 15 day to reach to the feeding ground (Figure ).

   

Figure Migratory routes of sea turtles in the Gulf of Thailand and South China Sea 

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

Figure Migratory routes of sea turtles in the Andaman Sea

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012  

 

Figure Sea turtle migration tracking devices

   

Sea turtle’s life cycle

 The current advanced revealed that turtles can detect and use the global magnetic field for their migration. Each year, sexually mature sea turtles swim back to the place for mating, located within 10 km from the beach which to be as their nesting ground. After mating, the adult female turtles still swim along that place until the eggs are ready to be nested, and then they reach to the beach for nesting which can be found all year round. However, nesting can be found at night in June – August and November – January for the Gulf of Thailand and the Andaman Sea, respectively.

 During nesting period, each sea turtle can nest many times. The first nesting produces 50-150 eggs and they go back to the sea nearby the nesting grounds about two weeks and wait for the completion of fertilization; after that, they will lay their eggs again.  First of all, the mature female turtle reaches onto quite beach at night to finds suitable place which is not inundated by seawater, and dig the hole (about 50 centimeters deep) before laying their eggs. The eggs are incubated with natural temperature and moisture in the sand, taking about 7-12 weeks to hatch. It can be seen by the sunken sand over the nest. The hatchlings then take about 2-3 days to crawl up to the ground at night and further reach to the sea using natural light of the ocean horizon as their navigator. They further continuously swim out from the shore to the deep sea for several days; during this time, they use the food stored in their body as energy. It’s expected that they consume plankton, algae, or small aquatic animal for living. However, the feeding boundary and relevant knowledge on these hatchlings are still in question. Most studies have focused on the life cycle of sea turtles with the age of 5-10 years (35 – 40 centimeters of carapace size), it was found that these sea turtles (except leatherback turtle) inhabit in shallow coastal water where plants and benthic animals are abundant. Green turtle feeds on seaweeds, seagrasses, and some mangrove fruits that are floating on the sea surface; hawksbill turtle feeds on sponges in coral reefs, and Olive Ridley turtle feeds on small aquatic animals. Most sea turtles also feed on jellyfish as well. On the other hand, leatherback turtle is only one species of sea turtles that live in deeper sea for their entire life and feeds on jelly fish as food. The natural predators of sea turtles in the ocean are sharks, crocodiles and killer whales. It’s estimated that the survival rate of sea turtles is less than 1 percent.

Figure life cycle of sea turtle

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

   

Reproduction of sea turtle

Generally, sea turtle is reached to sexually mature stage when they are 12-16 years old, stimulated by sex hormone. Both mature male and female turtles would swim to mating areas which are believed that it’s not far from the nesting ground. Female turtles may mate with several male turtles, similarly, male turtle may also mate with many female turtles. Female turtles can keep the sperms which were obtained during mating, for fertilization when the yolk is ready. Many yolks are developed from many “follicles”, moved to ovarian duct, and then fertilized with sperms. After fertilization, albumen and egg shell are produced covering the yolk and the eggs are ready for nesting. At this stage, there are still many yolks, which have not been completely developed yet, to be further developed and available for the next nesting within the same nesting period.

 

Nesting behavior of  sea turtle

Generally, sea turtles lay their eggs on quiet beach at night when the tide is reached to the maximum level or the moonrise begins. The mother turtle crawls up on the beach beyond the minimum high tide and choose the suitable place for nesting, she then dig the hole by her flippers. The nest of hawksbill turtle, green turtle, and Olive Ridley turtle are bout 30-50 cm deep, while the nest of leatherback turtle is about 60-80 cm deep. After digging is reached to the desired depth of the hole, the turtle further scoop up more sand to make the nest’s bottom larger until the shape is appropriate.  After that, she starts to nest by depositing about 70-150 eggs for hawksbill, green, and Olive Ridley turtles and 60-130 eggs for leatherback turtle.  It takes about 20-30 minutes to complete nesting each time. The size of sea turtle egg is about 4 centimeters in diameter, except leatherback turtle’s egg which is about 5.5 cm in diameter. After laying, she re-fills the nest with sand and press the nest using her back flippers (Olive Ridley turtles press the nest by rising her chest carapace and dropping it to the nest for many times). In the final stage of nesting, they spread out the sand surface to cover up the real nest. She then returns to the ocean and doesn’t treat the eggs anymore. Each mother sea turtle nest about 1-3 times a year and each time is separated about 2-3 weeks in length and 30 – 40 days for the leatherback turtle. (Schulz, 1975)

 

Nesting Ground

Sea turtles that lay their eggs on the beaches in the Gulf of Thailand include green turtles and hawksbill turtles, while leatherback turtles, green turtles, Olive Ridley turtles and hawksbill turtles are found in the Andaman Sea. The nesting grounds of green turtles have been mostly found followed by hawksbill turtles. Both turtles nest on the beach along the islands. While, the nesting grounds of Olive Ridley turtle and leatherback turtles have been found with smallest number. They nest along the mainland beaches.

The areas that were reported as a nesting ground for sea turtles in the Gulf of Thailand include the islands of Chonburi, Trat, Prachuap Khiri Khan and Nakhon Si Thammarat Province, and the mainland beaches of Pattani and Narathiwat Province. The important nesting grounds in the Gulf of Thailand are in Ko Kram of Chonburi Province where the nests of green and hawksbill turtles are found. Since Ko Kram is strictly administered by Royal Thai Navy, this area is still maintained appropriateness for sea turtle nesting and the illegal collection of sea turtle eggs is also scarcely found. However, the significant threat to sea turtle is entanglement in fishing gears which cause continuous decline of sea turtle population. The numbers of nesting of hawksbill turtles are extremely lower than in the past. Due to its beautiful and expensive carapace, hawksbill turtles were extremely hunted in the past and it’s easy to catch them because they live in shallow water. The nesting ground in the Andaman Sea are found in the beaches along the west coast of Phuket, Phang Nga, and nearby islands. In addition, it can be occasionally found in Trang and Satun Province and the some islands such as Ko Surin, Ko Similan. The important nesting grounds in the Andaman Sea are Mo Ko Prathong, Had Tai Muang of Phang Nga Province and Had Suan Mapraw of Phuket Province.

 

Temperature and the hatchling

Sea turtle’s eggs are incubated using the heat from sunlight and the moisture in sand. In Thailand, the sand temperature ranges from 25-34 degree Celcius (Chantrapornsyl, 1992a; 1994). The cell division of the embryo is developed and attached on the egg shell within the 6-12 hours after nesting observing from the change of color on the upper part of egg shell which is turned to milky white and the color is expanded with more incubation period.  In this period, moving or turning the egg’s position can cause detachment of the embryo from egg shell and death. Hence, safe transportation of sea turtle’s eggs to the nursery areas has to be operated within 3-6 hours after nesting. If the incubation period is more than 6 hours, moving their eggs to the nursery areas must be carefully done and maintaining the egg’s position is strictly required to avoid any impacts to the embryo; the head, eyes, heart, and internal organs can be clearly seen at twelve days of development; appendages, tail, and spines can be recognized at the age of fifteen days; scutes on carapace are developed and a long tail with front and back flippers can be clearly recognized at the age of twenty five days; all organs are fully developed but it is till soft, short tail, and the scutes on carapace are clear at the age of thirty days; finally, a tiny hatchling with completed organs is found on the age of forty days (Chantrapornsyl 1992b).

   

 

Figure Nesting behavior of sea turtles

 

According to Yntema and Mrosovsky (1983) that study the hatchling of the   loggerhead turtle’s eggs (Caretta caretta) under the controlled temperature condition, it was found that the egg that incubated at the temperature of below 28 degree Celsius will hatch as a male turtle, while the egg incubated at the temperature of higher than 32 degree Celsius will hatch as female turtle. The female and male turtles are hatched in equal rates under the incubation temperature of about 30 degree Celsius. The temperature which provides the equal sex hatchling rate is called pivotal temperature that is varied by each sea turtle species and location. According to Mrosovsky (1992), Pivotal temperature of hawksbill sea turtle was about 29.2 degree Celsius (the study was conducted with stable temperature. It can be concluded that the sex of hatchlings is influenced by the temperature of the nest. The male hatchlings are generally produced at lower incubation temperatures, while the female are produced at the higher temperature.

The incubation period of sea turtle’s eggs is about 50-55 days to hatch (60-65 days for leatherback turtles). The hatchling will use their sharp beak to crack the egg shell, and then all hatchlings gradually crawl up on the ground level at night. Naturally, the hatchling rate is about 80-90 percents of all eggs and they instinctively reach towards the sea after reaching to the surface. At the sea, they start to continuously swim to pelagic sea for 3-5 days without stopping. At this stage, the baby turtles use the remaining yolk that stored within their body as a food until it is finished. They stop swimming and float in the floating plant, seaweeds, or floating materials as their shelter and food source. The bigger turtle with the age of more than 1-2 years will inhabit in coastal water. The carapaces of the hatchlings are 6 and 4-4.5 centimeters long for leatherback turtle and other turtles respectively (Chantrapornsyl, 1992).

   

Figure Young sea turtles

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

   

 

Figure Sea turtles are treated in the artificial ponds before releasing into the sea

The clear evidence on development of natural sea turtle is still scarce. However, some studies of the development of Olive Rider and hawksbill sea turtles in artificial pond illustrated that the growth rate of hawksbill sea turtle was higher than which of olive rider sea turtles—the weights of hawksbill and Olive Rider sea turtles are 8 and 4.5 kilometers at the age of 22 months (Uonchit, 2528). Besides, sea turtles reach to the sexual mature stage at the age of  about 8-12 years old and the weight of 35-45 kilograms (as studied from the artificial pond) (Chantrapornsyl, 1994). According to the survey of the natural green turtles in Australia,  it’s about 20-25 years old to reach to the sexually mature stage depending on individual’ s growth rate —   the male turtle has a long tail which is actually the organ used for reproduction.

Because the each nesting ground is specific to each sea turtle species, it is believed that baby turtle can memorize the birth location since they born by using the sensory organ in their nose with olfactory bulb which are very sensitive to detect odors and chemical composition in that environment and memorize it. The turtles will come back to nest in the same pace when they are mature. However, it’s not clear about when the memorization of sea turtles begins between time they are on the sand surface or at the sea. The continuous frenzy swimming is occurred within 3 – 5 days reaching to pelagic sea, this swimming is an instinct for survival of sea turtles because having more distance from the coast provide lesser predators. Besides, much more swimming is the way to develop strength of their muscle to be able to live in nature. Baby sea turtles inhabit in floating algae or materials and move to coastal water when they have 1-3 years old for living and feeding.

 

Feeding behavior

Coastal waters of continents and islands are abundant with sea turtles’ food—consisting of several ecosystems such as seagrass beds, marine algae, coral reefs, and rocky shores. These diverse ecosystems comprised of the diversity and abundance of plant and animals serve as a home for teenage to adult sea turtles; coral reef and seagrass ecosystems are important to hawksbill and green turtle.

Additionally, sea turtles feed on jellyfish or sponges even though it has poison within their body; the reason is that the poison can be accumulated in sea turtle without having any effect. The feeding behavior can reduce a food competition in ecosystem as well since each species of sea turtle feed on different foods—some are carnivores, herbivores, or omnivores. Due to its different feeding behavior, the evolutionary development of sea turtle’s beak is differed by each species as well as the difference on their feeding habitats. For example, the small serration located on the upper and lower beaks of green turtle have been developed for feeding seagrasses and algae. Adult green turtles are herbivores while the baby green turtles feed on both plant and animal. Loggerhead and Olive Ridley turtle have a strong and big beak that use for crushing the shells of shrimp, crab, and mollusk. Aside from those, they still feed on other foods such as jellyfish and various kinds of marine plant. Hawksbill sea turtle has a sharp and curving beak which can be use for finding their food in small cavities within the coral reefs such as sponges, corals, shrimps, squids, barnacles, or even sea snakes. Leatherback turtles are having thinner beak with a rough serration used for feeding on some animals which are not covered by shells like jellyfish. There are many papillae in their mouth and esophagus that can help them to swallow their foods.

 

Adaptation of sea turtle

The dominant characteristics of sea turtle are a thick and heavy carapace that covers on their body in order to protect them from danger but it also causes them with slow movement on land. On the other hand, it can move faster when living in the sea because their body’s shape has been developed by having a streamlined body with limbs which are flippers for swimming. The problem of the heavy weight that presses their body is insignificant when they live in aquatic environment, so marine turtles can develop more body mass than terrestrial turtles. In addition, the bigger body may reduce loss of water and heat from their body because of the lower surface to volume ratio. Sea turtles need freshwater because the water is an important component of sea turtles’ life. They obtain freshwater from foods and drink some seawater but seawater is consisted of many minerals that may affect to turtles. Hence, they have developed the salt gland, located over the eyes, to remove excess salt and minerals in form of fluid. The eye’s vision of sea turtles is good in the water, but they are nearsighted out of water. They have an excellent sense of smell in both on the ground and under the water. The hatchlings will use the sense to detect and memorize the smell and chemical composition for returning to nest in the future. Besides, a large lung laid along the body make the turtle live in the water for 4 – 7 hours. Current progress revealed that sea turtles can detect earth’s magnetic field used for their migration.

 

 

Figure Underwater life of sea turtles

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

 

Figure Sensory system of sea turtles

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012

Figure Salt gland that help remove excess salt located on over the eyes

Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5December 2012  

Figure Shapes of sea turtle  

(a) The X-ray image shows turtle’s hand and finger bones which are connected altogether by tendon and connective tissues. The frontal legs are characterized as flippers that enhance their swimming ability.

(b) A streamlined shape helps reduce water resistance and also increase the lifting force while the turtle is moving forward since the velocity of water over the turtle is higher than the velocity of water under the turtle.  This is similar to a flying mechanism of airplane.

 Source: Database of Marine and Coastal Resources, Department of Marine and Coastal Resources, Ministry of Natural Resources and Environment. Retrieved from http://www.dmcr.go.th/marinecenter/, accessed on 5 December 2012