What are sea turtles?

Sea turtles are marine reptiles, which have been on Earth for around 120 million years. Sea turtles evolved from terrestrial turtles. They differ from tortoises and semi-aquatic turtles in a number of anatomical ways. One obvious feature is the flippers, which are feet whose digits have been fused, so that the flippers act as powerful paddles for swimming; they also have streamlined shells (carapaces).

Sea turtles don't withdraw their heads and flippers into the shell, but instead have a hard skull, which compensates for protection. There are 7 species of sea turtle on Earth today, each with its particular adaptations for its preferred habitat and diet.

Sea turtles rise to the water surface to breathe as they have lungs but are able to spend long periods under water, even sleeping under water for several hours! They are able to do this by storing oxygen in their brain and tissues and slowing down their metabolic rate.

Turtles are also able to dive down to great depths, one species, the leatherback has been recorded at depths of up to a 1000 metres! It is able to do this because it has a flexible leathery carapace and hence it is able to avoid crushing of the carapace by pressure at these depths.

Sea turtles have glands in the eyes for excreting excess salt enabling them to cope with their salty environment.

Which sea turtles are found in Watamu?

1. Green turtle (Chelonia mydas): it is classed as endangered, but the most abundant of turtles found in Kenya and forms most of our nesting turtles. It is the largest hard-shelled marine turtle and grows to a carapace length of up to 1.2m and an adult can weigh up to 200kgs . It feeds mostly on sea grasses and marine algae, resulting in a green colouration of the layer of fat (calipee) under the shell hence it’s name. It is this calipee that is used to make turtle soup. It is highly prized for it’s meat and eggs and is known as the good eating turtle. They lay on average 100-130 eggs.
2. Hawksbill turtle ( Eretmochelys imbricata ): this turtle gets its name from it’s narrow snouted hawk like head. It is critically endangered. Adults grow to around 90cm in carapace length, weighing up to 80kgs . The carapace is very beautiful in colour and patterning and it is this turtle that has been hunted for its shell, which is used to make 'tortoise shell'. It is omnivorous when mature and feeds mostly on sponges, molluscs and soft corals, for which its hawk like beak has developed in order to pick these prey out from between hard corals. They lay 110 –180 eggs.
3. Olive ridley (Lepidochelys olivacea): is an endangered species, that spends time both in deep and shallow waters. It is a smaller turtle with a heart shaped carapace that is olive in colour. It grows to on average 70cm in shell length and weighs 35-50kgs. They are carnivores feeding on a mixed diet of crustaceans, fish and molluscs. WTW identified the first nesting of this species on the whole of mainland East Africa. Olive ridleys besides the normal individual nesting also participate in a phenomenon called 'arribadas' in 3 countries. It is a mass nesting strategy to reduce the impacts of predation. The highest olive ridley arribada on record was in 1968 at Escobilla, Mexico and involved 80,000 turtles.
4. Loggerhead (Caretta caretta): this turtle gets its name from its large, broad head housing powerful jaw muscles to crush its favoured prey of crustaceans. It is classed as an endangered. It has a carapace growing to an average one metre in length and weighs up to 160kgs. The carapace is reddish brown and they lay 100-125 eggs per clutch.
5. Leatherback (Dermochelys coriacea): is critically endangered, it is markedly different physiologically from the rest of the turtles, hence in its own family, the Dermochelyidae. The most obvious difference is its huge barrel shaped body without a hard shell, but instead has a leathery scaleless skin. The carapace is black in colour with the flippers dotted with white spots. The carapace length ranges between1.4 - 2m, with the turtle weighing 260-600 kgs. The largest ever recorded leatherback weighed 916 kgs and measured 3m in length. They feed almost exclusively on a diet of jellyfish and lay up to 90 eggs.

Life cycle & migration

They spend nearly all their time at sea with only females leaving the sea to nest on land during the breeding season. One of the most remarkable things about turtles is their migration patterns and scientists still have much to learn about sea turtles navigational ability. This begins with their first 36-48 hours out of the nest when they swim as hatchlings into the deep sea, they then drift hundreds and sometimes thousands of kilometres in the deep ocean currents.

Little is known of this life phase of sea turtles and the first years of sea turtle’s life are referred to as the ‘lost years’. They re-emerge in inshore waters after a few years, about the size of a dinner plate and begin their normal feeding patterns, but still migrating over various distances to different feeding areas, sometimes thousands of kilometres.

Turtles spend most of their juvenile and adult time in their own distinctive habitat, most species spend their time in the warmer, shallower waters except the olive ridley and the leatherback, which forage in the open ocean. Food varies according to species, area and according to life stage of the turtle.

In the deep ocean, in its early years, the young turtles are omnivores, as they mature they adopt their species specific diet. Green turtles are herbivores, while the other species have particular carnivore diets that include jellyfish, sponges, crustaceans, molluscs and small fishes.

After 10-50 years, depending on the species, the sea turtle reaches sexual maturity and they migrate what can be thousands of kilometres back to their natal beach (birth place) for breeding. How turtles are able to navigate thousands of kilometres exactly to the beach they were born on, but left up to 50 years before is still not fully understood by scientists. There are various theories including navigation using the earth’s magnetic field, by currents, or by ocean smells, all imprinted in their brains from the turtle’s youth.

Mating & nesting

The mating process can take several hours and periods of up to 9 hours have been recorded. Females often mate with several males storing the sperm to fertilize several batches of eggs that they will lay at intervals over the nesting season. Each nest therefore may have mixed parenthood.

During the nesting season, depending on species, the female can lay up to 12 nests at an interval of approximately every 10-15 days. Most turtles though lay 2-6 nests in a nesting season. The female does not mate again but uses stored sperm from the earlier mating, staying offshore while the next batch of eggs develops inside of her. After the nesting season is finished the female returns back to her foraging waters to build up body fat again for the next long journey and egg production feats, this may take 2-7 years. Clutch size can vary between 60 and 200 eggs depending on species and individual, though an average of around a 100 is normal.

Each egg is round, with a flexible shell and about the same size as a ping pong ball, with the leatherback egg the size of a tennis ball.

Most turtles lay eggs at night on undisturbed quiet beaches. They crawl up the beach and dig large body pits around themselves, they then use their rear flippers to dig a nest chamber which is flask shaped and has an average depth of 50cm beneath the surface. Once the eggs are laid the nest is then carefully covered with sand. The turtle then uses its powerful front flippers to pull sand over the whole area of the body pit to disguise the nest as protection against predators. The whole nesting process can take hours.

Hatching

There is no parental aftercare and the hatchlings hatch after 45-70 days incubation. Temperatures are very important for nest success and sex determination of the hatchlings. A threshold of around 28C determines sex of the hatchling, hotter than this and you get more females, cooler more males.

The hatchlings take between 2-5 days to emerge from the egg to the sand surface. They work together by pulling down the sand from above their heads to beneath them, creating an elevator effect. They emerge from the sand when they sense it is cooler in the evening to avoid dehydration from the day’s sun and the more numerous day predators.

On emergence from the nest they search out the brightest horizon (under natural conditions this is always seaward) and then crawl frantically to the sea. They then start what is called a 'swim frenzy' out to the deep waters to the safety of drifting sargassum weed mats where they rest and start feeding. This swim frenzy period can take around 48hrs, during this phase the hatchling does not eat, but utilizes energy derived from the yolk sac of its egg.

Once its energy reserves are depleted and it has reached safer waters the hatchling will commence feeding on small invertebrates. It will then spend the 'lost years' drifting along with the ocean currents, sometimes in close association with floating rafts of seaweed.

Predators of eggs and hatchlings are numerous: on the beach they include crabs, pigs, dogs, birds, mongeese; while in the water they fall prey to seabirds and all types of predatory fish. After a few years and growth to about 30-50cms their natural predators are restricted to sharks.