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 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, chameleons and lizards give birth to live young).
Turtle, Tortoise, Terrapin – What is the Difference?
All reptiles with a shell consisting of a top half, the carapace, and a bottom half, the plastron, are called turtles. The shell consists of bone as well as keratinous layer on top. Depending on the species, bone and keratinous layer can be developed quite differently, as can be seen in e.g. soft- and hard-shelled turtles. There are over 310 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. Through what is called “chemoreception”, sea turtles analyze the chemicals in the water to find food or to navigate. Chemoreception includes both the senses of taste and smell.
Very little is known about the sense of touch 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 showing synchronized nesting activity, with many thousands of females coming to the beach to nest at the same time, could indicate that turtles also have a precise sense of time. But as many aspects of a sea turtle’s life, this 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: it is estimated that only 1 in 1000 hatchlings might 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.
Sea Turtles of the Indian Ocean
Six types of sea turtles live in Indian Ocean, with abundance of each species differing between countries and oceanic areas. While green turtles and hawksbills are abundant in shallow coastal areas for example in the Maldives and Kenya, loggerheads have their biggest nesting population worldwide in Oman. Olive ridley turtles can be found nesting en masse on the coast of India, while leatherbacks are more commonly found in South Africa, Mozambique and the Andaman and Nicobar Islands. The enigmatic flatback turtle can only be found in Australian waters.
Green Sea Turtle
IUCN Red List Status for Green Turtle: Endangered (2004)
Green turtles are the largest of all the hard-shelled sea turtles. They grow to up to 134 cm in length and weigh from 135-250 kg as fully-grown adults. 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.
Hawksbill Sea Turtle
IUCN Red List Status for Hawksbill Turtle: Critically Endangered (2008)
The hawksbill is one of the smallest species of marine turtles, with adults measuring about 75-95.5 cm in length and weighing around 70-86.2 kg. Indian Ocean turtles tend to be smaller than their Pacific and Atlantic counterparts. 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.
Olive Ridley Sea Turtle
IUCN Red List Status for Olive Ridley Turtle: Vulnerable (2008)
The olive ridley sea turtle is the smallest and most abundant of all sea turtles, growing on average up to 80 cm and weighing less than 50 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. Indian Ocean Olive ridley turtles are, on average, smaller than individuals found in the Pacific and Atlantic.
Loggerhead Sea Turtle
IUCN Red List Status for Loggerhead Turtle: Vulnerable (2015)
Adult loggerhead turtles measure between 65 and 115 cm in length and typically weigh between 40 and 180 kg. The largest reliably recorded loggerhead measured 146.7 cm in length! Loggerheads are named after their exceptionally large head. Their heart-shaped carapace is orange to reddish-brown with a yellowish-brown plastron. They normally carry many encrusting organisms, such as barnacles, on their head and carapace.
Leatherback Sea Turtle
IUCN Red List Status for Leatherback Turtle: Vulnerable (2013)
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. Mature males and females can grow to over 2 metres in length and weigh up to 900 kg.
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.
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