“All tortoises are turtles, but not all turtles are tortoises. All terrapins are turtles, but not all turtles are terrapins. Some turtles are just turtles.” ~The World of Turtles and Crocodiles~
Sea Turtles – Ancient Reptiles
Sea turtles have existed on Earth since 120 million years ago. There are currently seven species (or types) of sea turtles, also called marine turtles, but there were once many more. Turtles belong to the reptile group of animals that also includes snakes, lizards, and crocodiles, all of which share the following characteristics:
- Scaly skin: this helps reptiles conserve moisture as most of them live in dry, sunny and/or salty environments;
- Cold-blooded: their body temperature depends on the temperature of their surroundings;
- Air breathers; all reptiles are born with lungs and use them to get oxygen from the air;
- Oviparous: they lay eggs (although some snakes give birth to live young).
Turtle, Tortoise, Terrapin – What is the Difference?
All reptiles with either a soft or bony shell and a backbone are called turtles. There are approximately 263 species of turtles in the world and they can be divided into three major groups: tortoises, freshwater turtles or terrapins, and marine turtles.
The term tortoise usually refers to any strictly land-dwelling turtle, while the word terrapin means “small turtle” and is usually used when referring to turtles living in fresh and brackish areas, spending their time both on land and in water. The word “turtle” usually refers to those species spending most of their lives in water (either fresh or salty).
Marine turtles can be defined as reptiles with a shell that have adapted to live in the oceans. Some of these adaptations can also be found in terrapins and freshwater turtles, while others are specific to marine animals only.
Sea Turtle Biology – Adapting to a life in the Oceans
Sea turtles have a soft or bony shell, flipper-like limbs, and spend 99% of their lives in the ocean. Their bodies have adapted to life in the ocean; they have a streamlined shell and flippers that allow them to swim quickly; furthermore, they can hold their breath for long periods of time, and they hydrate by drinking salt water and expelling the salt from glands behind their eyes.
Only adult females temporarily leave the water to lay eggs on land to incubate and hatch successfully. This is the only time in their life when they leave the marine environment. There are some exceptions, however, such as the Hawaiian green turtle population, where both males and females come out of the water to bask along the shore.
The Senses of Sea Turtles
Though near-sighted on land, sea turtles have excellent vision underwater. Their vision includes light within the ultraviolet range and they can see colours, shapes and patterns. On land, marine turtles use visual cues to orientate.
Even though sea turtles do not have outer ears, but their inner ears can hear low frequencies and sense vibrations. The ecological role of hearing in turtles is not clear. Experiments conducted both underwater and on land revealed that marine turtles respond to sounds in their hearing range with behavioural or physiological changes. Though they do not use hearing to navigate under water, their ears help detect water pressure changes and vibrations that might indicate that a predator is nearby.
Sea turtles have been observed underwater with their nostrils open while slowly opening and closing their mouths. This behaviour is described as “throat-pumping” and is used by turtles to move water towards the nose. More than “smelling” the water, sea turtles analyze the chemicals in it to find food, or for navigation purposes. This ability is called “chemoreception”.
Very little is known about the sense of taste in sea turtles, but we do know that they are particularly sensitive to touch on the soft parts of their bodies, like their flippers and neck.
Sea Turtles Are Amazing Navigators
Maybe the most fascinating trait of sea turtles is their incredible ability to precisely navigate their way back to the area where they were born every few years. Both males and females have this navigational ability. How do they do it? When turtle hatchlings make their way from their nest to the water, they imprint a type of magnetic signature of that beach into their minds. This signature stays with them for the rest of their lives. Similar to birds, turtles can sense the magnetic field of the earth, which creates a sort of map in their brains. The two species of Ridley turtles nesting together around the same time every year could indicate that they also a sense of time. Beyond that, a sea turtle’s navigational ability still remains a mystery to science.
The Life Cycle of Turtles
All marine turtles share the same general life cycle, with some small differences between species.
- Female turtles come on to the beach at night to lay a nest of 100-200 eggs.
- Nests hatch in 40 to 60 days.
- Baby turtles, or hatchlings, are completely independent at birth and never see their mothers.
- As soon as they come out of the nest, hatchlings crawl as fast as possible to the sea and swim towards the open ocean.
- Young turtles spend several years drifting with the currents (often referred to as the lost years), feeding on small animals living in algae floating in the water.
- After a few years, immature turtles will settle close to shore where it may take them more than 30 years to reach adulthood.
- The time juvenile turtles spend on growing areas varies according to the species and food availability.
- Upon reaching maturity, adult turtles migrate from their developmental areas to their mating grounds.
- Female turtles swim back to the beach on which they were born to lay eggs every 2 to 3 years, while males migrate annually from the mating areas to the feeding grounds.
- Sea turtles have a very low natural survival rate: only 1 in 1,000 turtles will make it to adulthood.
- Marine turtles may display multiple paternities and a single clutch of eggs may have as many as five fathers. This is due to the ability of female turtles to store sperm in their oviducts until ovulation.
The Lost Years Of Sea Turtles
Scientists are still unsure exactly where turtle hatchlings spend the first few months or years of their life. Hatchlings probably enter an oceanic phase after leaving their nesting beach. They find shelter in mats of algae, floating passively in major current systems (or gyres), and feed on the surface on pelagic vegetation and animals. After one to ten years in the oceanic zone, some species of turtles retreat to coastal areas where they forage and grow until maturity.
The 5 Sea Turtles of the Indian Ocean
5 types of sea turtles live in the Indian Ocean, though loggerhead and leatherback turtles are less common than the green, hawksbill and Olive ridley turtles.
Green Sea Turtle
Green turtles are the largest of all the hard-shelled sea turtles. They grow to about 120 cm in length and weigh from 135-160 kg. The green turtle has a smooth, sub-circular to heart-shaped carapace with colour varying from greenish-yellow to greyish-brown. The plastron is yellowish-white in colour. In the Maldives, the colouration of adults is darker than that of green turtles in the western Indian Ocean.
IUCN Red List Status for Green Turtle: Endangered (2004)
Current global Green Turtle Population Estimate: 203,000 breeding females
Hawksbill Sea Turtle
The hawksbill turtle gets its name from its narrow, elongated head that tapers sharply to a V-shaped lower jaw. The hawksbill is one of the smallest species of marine turtles, with adults measuring about 75-90 cm in length and weighing around 70 kg. Indian Ocean turtles tend to be smaller than their Pacific and Atlantic counterparts. Female hawksbills measure 70 cm and weigh 44 kg on average. Males and females tend to be around the same size, but males may have longer claws and brighter colouring. The carapace of the hawksbill is unique amongst the sea turtles as the scutes overlap. It has five central and four pairs of lateral scutes on its carapace. These scutes are streaked and marbled with amber, yellow, black, or brown and the turtle has a yellowish plastron.
IUCN Red List Status for Hawksbill Turtle: Critically Endangered (2008)
Current global Hawksbill Turtle Population Estimate: Approximately 8,000 nesting females
Olive Ridley Sea Turtle
The Olive ridley sea turtle is the smallest and most abundant of all sea turtles, growing on average up to 70 cm and weighing 45 kg. The heart-shaped, olive green coloured carapace gives the name to the Olive ridley. Males and females grow to the same size; however, females have a slightly more rounded carapace. The turtle has 5 to 9 pairs of costal scutes and either one or two claws on each flipper. Indian Ocean Olive ridley turtles are, on average, smaller than individuals found in the Pacific and Atlantic.
IUCN Red List Status for Olive Ridley Turtle: Vulnerable (2007)
Current global Olive Ridley Turtle Population Estimate: Approximately 800,000 nesting adult females
Loggerhead Sea Turtle
Adult loggerhead turtles measure between 75 and 100 cm in length and typically weigh up to 160 kg. The largest recorded loggerhead weighed 545 kg and measured 213 cm in length. Loggerhead turtles are so called due to their exceptionally large heads. Their heart-shaped carapace is orange to reddish-brown with a yellowish-brown plastron. They typically have five vertebral scutes down the carapace’s mid-line and five pairs of costal scutes along the borders. They normally carry many encrusting organisms, such as barnacles, on their head and carapace.
IUCN Red List Status for Loggerhead Turtle: Endangered (1996)
Current global Loggerhead Turtle Population Estimate: 40,000 – 50,000 nesting adult females (2004)
Leatherback Sea Turtle
The largest of all the sea turtle species (and the largest living reptile) – the leatherback turtle – owes its name to its unique shell. Composed of a layer of tough, rubbery skin strengthened by a matrix of thousands of tiny bone plates, it looks almost like a jigsaw puzzle. It is the only marine turtle without a hard carapace and claws on its flippers. The leathery, dark grey to black carapace is mottled with white spots and marked by five ridges; the plastron is cream to black in colour. The leatherback’s body is teardrop-shaped and tapers at the hind end to a blunt point. Mature males and females can grow to over 2 metres in length and weigh up to 900 kg. Leatherbacks mature between 8 and 15 years of age. Their estimated lifespan is around 45 years in the wild, but this is not well documented.
IUCN Red List Status for Leatherback Turtle: Vulnerable (2013)
Current global Leatherback Turtle population estimate: 30-40,000 adult females in 1996
Threats to Sea Turtles
Sea turtles have been around for millions of years and grace the waters of all the world’s oceans except the Arctic. Unfortunately, sea turtles face many threats to their survival. All seven species are listed as endangered by CITES (Convention on International Trade in Endangered Species of Wild Flora and Fauna); six of the seven species of sea turtles are classified as threatened with extinction by the IUCN Red List of Threatened Species (International Union for Conservation of Nature and Natural Resources). One species, the flatback, is not listed due to insufficient data.
Major Threats to Sea Turtles in the Indian Ocean
Currently, the biggest threats to turtles in the Indian Ocean are direct harvest for meat and eggs, the pet trade (hatchlings), marine garbage, including ghost nets, and the destruction of nesting habitats.
Due to the primary method of fishing in the Maldives being live bait pole-and-line, the fisheries in the here pose little threat to sea turtles. However, fishing techniques used elsewhere in the Indian Ocean, including gillnetting, trawling, long lining, and purse seine fishing associated with fish aggregating devices (FADs), all pose serious threats to sea turtles, both as active fishing techniques and in the ghost gear that they generate.
Sources: Hudgins, Ali, and Mancini (2017) “Marine Turtles of the Maldives: A field identification and conduct guide“. An IUCN Publication funded by USAID.