The following article ‘Turtle Tails – The Biology of Sea Turtles’ was originally published in issue 2/2011 of Asian Diver magazine:
Turtle Tails – The Biology of Sea Turtles By Richard Smith (© Richard Smith 2011)
Sea turtles can be found from tropical coral reefs and sub-tropical algal beds to 0˚C sub-polar seas and depths of 1,200 metres. They have a history that can be traced back over 100 million years but despite this they share very similar natural histories. Unfortunately, they are threatened with extinction and conservation initiatives are vital if they are to survive.
There are seven species of sea turtles, all but the leatherback belonging to the family Cheloniidae. Leatherbacks are the only members of the family Dermochelyidae. Worryingly, six of the seven species are listed by the World Conservation Union as vulnerable, endangered or critically endangered. The single exception, the flatback turtle, is so poorly known it’s conservation status has yet to be assessed. Research on sea turtle biology is beginning to shed light on these elusive reptiles but still more is needed to assist conservation efforts being carried out across the world.
Anatomy –
The physical characteristics shared by sea turtles are remarkably similar given the millions of years since the species separated. They are characterised by a hard shell that protects delicate internal organs, a hard beak with strong jaw muscles to bite off chunks of food and large paddle-like limbs. Unusually, for such an ancient lineage, sea turtles are so similar that natural hybrids have been found between virtually all the species, except the leatherback (a hybrid is the juvenile created when two distinct species interbreed, for example a mule being the offspring of a horse and a donkey). Leatherbacks are distinct in having a thick leathery shell rather than the hard plate-like shell present in others.
Behaviour –
Tagging of wild turtles has begun to bring to light some of their natural behaviours and dive profiles. Leatherback turtles have now been recorded diving to a depth of over 1,200 metres with dives of 60 minutes in duration. Sleep patterns have also been recorded in several sea turtle species showing them to conduct mid-water dives where they hang inactive using neutral buoyancy in 10-15 metres of water. Others sleep on the seafloor for periods of up to several hours, especially when cooler waters lower their metabolic rate.
The hard shells of turtles are an important deterrent to all but the most perseverant of predators. Green turtles living in the aptly named Shark Bay of Western Australia however suffer high predation rates by tiger sharks. Predation by tiger sharks has been found to alter their behaviour subtly as the sharks are more common in rich seagrass meadows where the turtle’s food is more nutritious. This has led to those turtles which are already in good physical condition to avoid such areas, suggesting they would rather be hungry than dead!
Physiology –
Sea turtles are cold-blooded, which would be expected to limit their geographic range to tropical and sub-tropical waters. However, leatherbacks are unique among turtles in their ability to inhabit sub-polar regions. They have even been seen swimming around icebergs, an observation which baffled scientists for years given their cold-blooded reptilian heritage. The range of temperatures in which they are found poses a challenge in terms of temperature control: they need to cool down whilst laying their eggs on tropical beaches but also maintain activity in 0 ˚C water. To prevent over heating they are able to increase blood flow through their flippers, helping them to loose heat, much like the large ears of elephants. Conversely, to prevent heat loss they have a layer of blubber-like tissue that insulates them when needed. Their large size also plays an important role in maintaining their body temperature higher than the surrounding water, as large objects retain heat better than smaller ones. Recent research has put to rest the idea of these huge reptiles as warm-blooded by generating heat like a mammal.
Reproduction and Mating –
An observation made early during the study of sea turtle reproduction was that females commonly returned to the same nests from which they were born to lay their own eggs. After reaching sexual maturity they return for a breeding season every 2-5 years but can lay upwards of five clutches in one season.
There are two main stages in the life cycle of sea turtles. Hatchlings emerge from their leathery egg, which are laid by the dozen in a large pit dug on a sandy beach, and make a frenzied dash to the ocean. Those few that make it past a waiting squadron of predators spend some time floating passively in ocean currents. The sole exception to this is the flatback turtle, found only around Australia and neighbouring islands, the young of which remain near to shore. Hatchlings of the other six species spend a few years in the open ocean before switching to a more localised bottom-feeding stage, where they spend around a decade prior to reaching sexual maturity. Leatherback turtles have proven rather elusive in the period before reaching sexual maturity and, disturbingly for conservation efforts, their whereabouts during these years is poorly documented.
The slow reproductive rate of sea turtles has resulted in a huge decline in numbers throughout their range, although some populations have begun to stabilise. Populations of the leatherback turtle appear to be steady in the Atlantic, whilst their numbers are dramatically declining in the Pacific. These differences appear in part to be due to differing reproductive rates in the two oceans. In the Atlantic, females have an interval of two to three years between returning to their nesting beaches compared to four to five years in the Pacific. This phenomenon has been explained by there being more food for the turtles in the Atlantic, enabling them to build up reserves faster and breed more frequently.
Feeding –
Sea turtles are outstanding navigators and migrate, often huge distances, between their nesting beaches and feeding grounds. Some species, such as hawksbill and green, search for food in coastal waters and shuttle back and forth between feeding and nesting sites with intervals of several years. Others, including leatherback and olive ridley, feed in the open ocean and can be swept thousands of kilometres by ocean currents. For these pelagic species, finding highly concentrated patches of zooplankton, such as jellyfish, are vital as they may constitute a large proportion of their yearly food intake in a relatively short time period. Whether they are truly ocean wanderers or able to locate food hotspots is currently a focus of research.
In a Nutshell – Sea Turtle Facts and Information:
Common Name: Green Turtle
Scientific Name: Chelonia mydas
Conservation Status (according to IUCN): Endangered
Maximum Shell Length: 1.5 metres
Maximum Weight: 200 kg
Life Expectancy (estimated): 80 years
Diet: Herbivorous feeding on seagrass and algae but carnivorous as a juvenile feeding on jellies, crustaceans and sponges
Habitat: Generally coastal in rich seagrass and algae areas, but undertake complex migrations crossing thousands of kilometres of ocean.
Distribution: Worldwide throughout tropics and to a lesser extent subtropical oceans.
Common Name: Hawksbill Turtle
Scientific Name: Eretmochelys imbricata
Conservation Status: Critically Endangered
Maximum Shell Length: 1.1 metres
Maximum Weight: 65 kg
Life Expectancy (estimated): 50 years
Diet: Primarily eat sponges in the Caribbean but feed on a wide variety of algae, soft corals and sponges in
Indo-Pacific.
Habitat: Variety of habitats: coral reefs, sea grass, algal beds and mangrove areas
Distribution: Worldwide throughout tropics and to a lesser extent subtropics.
Common Name: Loggerhead Turtle
Scientific Name: Caretta caretta
Conservation Status: Endangered
Maximum Shell Length: 1 metre
Maximum Weight: 150 kg
Life Expectancy (estimated): 50 years
Diet: Carnivorous, feeding on a variety of molluscs, crustaceans, urchins and jellyfish
Habitat: Coastal and oceanic depending on size
Distribution: Worldwide throughout temperate, subtropics and tropical waters
Common Name: Flatback Turtle
Scientific Name: Natator depressus
Conservation Status: Data Deficient
Maximum Shell Length: 1 metre
Maximum Weight: 125 kg
Life Expectancy (estimated): Yet to be assessed
Diet: Carnivorous, feeding on soft corals, jellyfish and sea cucumbers
Habitat: Appears to prefer shallow, inshore and turbid waters over the Australian continental shelf
Distribution: Restricted to Australia, which is the only location it is known to nest, but also present in New Guinea and adjacent oceans
Common Name: Kemp’s Ridley Turtle
Scientific Name: Lepidochelys kempii
Conservation Status: Critically Endangered
Maximum Shell Length: 65 cm
Maximum Weight: 45 kg
Life Expectancy (estimated): 50 years
Diet: Carnivorous, feeding primarily on crustaceans
Habitat: Inshore as adult, foraging to 50 metres depth
Distribution: Nests only on the east coast of Mexico. Adults found throughout Gulf Of Mexico and Caribbean but individuals occasionally caught in the Gulf Stream current and found across to West Africa and the Mediterranean.
Common Name: Olive Ridley Turtle
Scientific Name: Lepidochelys olivacea
Conservation Status: Vulnerable but the most abundant sea turtle species in the world
Maximum Shell Length: 70 cm
Maximum Weight: 45 kg
Life Expectancy (estimated): 50 years
Diet: Omnivorous feeding on jellyfish, crustaceans and occasionally algae
Habitat: Pelagic, primarily living in the open ocean
Distribution: Worldwide throughout tropics with migrations spanning whole oceans
Common Name: Leatherback Turtle
Scientific Name: Dermochelys coriacea
Conservation Status: Critically Endangered
Maximum Shell Length: 1.8 metres
Maximum Weight: 500 kg
Life Expectancy (estimated): 45 years but yet to be confirmed
Diet: Feeds primarily on soft-bodied pelagic animals such as jelly fish and squid
Habitat: Pelagic, living in the open ocean
Distribution: Worldwide distribution from tropical to sub-polar regions.