3.5. Sea turtle biology

Sea turtles are ancient reptiles that inhabit all marine environments. They have remained unchanged for over 150 million years, since the age of the dinosaurs. Along with other turtles and tortoises, they belong to the order Testudines. Although turtles are cold-blooded, which means their body temperature varies with ambient temperature, they regulate their body temperature by controlling the blood flow in the skin and fins. Moreover, a layer of blubber is protecting them from cold water and they can also sunbathe on the surface to warm up. In the winter months, when the sea is coldest, sea turtles are resting on muddy sea bottom. During this resting condition, called hibernation, they are expending minimal energy and rarely emerge to the surface to breathe. They have lungs for breathing, just like any other vertebrate, but they only spend five percent of their time on the sea surface.

Sea turtles have modified appendages that have become flipper-like, allowing them to move in water with ease. The front flippers are used to propel the sea turtle through the water, while the rear flippers are used for steering and stabilising the body. The rear flippers are also used by females to dig holes in which eggs are laid. Turtles are the only reptiles with hard shells. The turtle shell is comprised of two parts – the plastron on the ventral side and carapace on the dorsal side. It is built out of a bony layer and a keratin layer. The bony layer provides support and is composed of broadened ribs and other bones fused with the vertebral column. The upper layer of the carapace is made out of keratinous plates called scutes. The number of scutes differs between species. The leatherback sea turtle is the only species having waxy skin and embedded dermal ossicles instead of keratinous scutes. Sea turtles have a hydrodynamic shell allowing for easier movement through water. The shell is flattened so there is no room left for the head and limbs to be retracted.

Photo 3.5.1. Sea turtle. Blue World Institute

The diet of adult sea turtles is not unique for all species and their jaw structure is adapted to the specific feeding habits. There are no teeth in the mouth of a sea turtle which is why it is called a beak. Its biting force is considered to be one of the most powerful among all animals. The loggerhead sea turtle is equipped with large, strong jaws able to break through bivalve and snail shells. The green sea turtle is the only exclusively herbivorous species grazing on seagrass and algae, which is why its jaws are serrated. The leatherback sea turtle has a distinctly shaped beak, allowing it to catch jellyfish more effectively. Sea turtles ingest salt water when feeding and need to have an efficient way of excreting excess salt. They have salt glands secreting a salty liquid through tear ducts which has a salt concentration twice as high as the surrounding water.

Sea turtles can travel for very long distances between feeding and mating areas throughout their lifetime. Upon hatching they immediately leave the beaches and swim to areas of open sea that are kilometers away. All of them use the same paths as previous generations. They are able to detect the Earth’s magnetic field and use this to instinctively orient themselves in such a way as to remain in favourable currents, which is especially important for hatchlings. Adult sea turtles can learn some aspects of areas they passed through and use this experience in later orientation. This mechanism is not yet completely understood, but it is believed that turtles use the magnetic field and their sense of smell to find their way back to the beaches they hatched on. Olfaction is the most developed sense in sea turtles. Hatchlings take in and „learn“ the scent of the beach where they surfaced. After reaching sexual maturity, females use it to find the beach they hatched on. They choose the right one by following the scent when they approach the shore. They do this successfully even after decades of growing in areas thousands of kilometres away.

Sea turtles migrate from habitats used primarily for feeding to areas where they mate, most often close to nesting beaches. Females mate with different males on several occasions in the breeding season. The females are able to nest up to seven times in two-week intervals. However, they do not breed every year and the inter-nesting period is called the remigration period. It is different for each species and can last for two to seven years. Females come ashore sandy beaches in the summer, and lay their eggs in holes they dig in the warm sand. This is the process that can last throughout the night. Females will almost exclusively nest on beaches they hatched on themselves. This is called philopatry.

Photo 3.5.2. Sea turtle eggs. Blue World Institute

Females lay around hundred eggs with an incubation (development) period of around six weeks. It is interesting to note that the gender of the turtles hatching from the fertilised eggs is not yet determined. It depends upon ambient temperature in the nest. Sand temperature will ultimately affect the sex ratio of hatchlings. Provided the eggs are surrounded by sand at a temperature greater than 29°C, the hatchlings will be female. Males will hatch at lower temperatures. The same number of males and females will be attained if the ambient temperature is exactly 29°C. Hatchlings dig through the sand in a combined effort and surface on the beach. This takes place mainly at night in order to avoid predators. Upon surfacing, they instinctively rush towards the sea and swim out as far as they can.

Photo 3.5.3. Sea turtle hatchling. Blue World Institute

Juvenile turtles remain at open sea for ten years on average. In this period called the pelagic phase (pelagic zone stands for open sea), young turtles are unable to dive to the bottom so they feed on plankton, most often jellyfish. The treat from predators is not as great as it would be near the shore, where these turtles migrate upon measuring thirty centimeters or more. The neritic phase begins upon arrival to the littoral zone and lasts for as long as the sea turtles are alive. In this phase turtles are large enough to be able to dive to the seabed and prey upon bottom-dwelling prey. Although predators in this area are more numerous, the number of species posing a potential threat to the turtles diminishes as they grow. On average, they reach sexual maturity at the age of thirty, when they mate and find sandy beaches to lay eggs.

3.5.1 Sea turtles - cold-blooded reptiles

Turtles, are ectotherm repties (cold blooded) which means that their internal temperature varies according to their ambient environment. Therefore, they depend on external sources of heat to determine their body temperature. For that reason, in cold water they do not have the ability to warm themselves, and must instead migrate to warmer waters.
In other words, they are hot when their environment is hot and cold when their environment is cold. In hot environments, cold blooded animals such as the turtle have blood that is much warmer than warm blooded animals. Cold blooded animals are also much more active in warm environments and are very sluggish in cold environments. This is due to the fact that their muscle activity is dependent upon chemical reactions that run quickly when they are hot and slowly when they are cold. Cold blooded creatures can convert much more of its food into body mass compared to warm blooded creatures. Turtles of different species prefer to have their blood at a certain temperature. Some can get by with their blood at around 60°F, others like their blood at around 80°F, and some like it at around 90°F. Note that none of these are actually “cold”, unless during “hibernation” when the blood can be at around 40°F or near freezing; Sea turtles are commonly found in waters off the Mid-Atlantic and Northeast U.S. during the summer and early fall. They usually begin to migrate south by late October. It is largely unknown why some sea turtle do not migrate south prior to the drop in water temperatures. Loggerhead sea turtles and green sea turtles are often affected by cold stunning. These species are all found to have similar reactions to the cold water temperatures.

3.5.2 Sea turtles vs tortoises: What is the difference?

Most people people know what sea turtle and tortoises are, but struggle to define the difference between the two animals. First of all, they are both arranged at the order “Testudines” but tortoises are classified in the family of “Testudinidae’’ and sea turtles in numerous families, like “Carettochelyidae” (pig-nosed turtle), “Dermatemydidae” (Central American river turtles), Emydidae (pond/water turtles), etc. Unlike tortoises, which are terrible swimmers and live solely on land, sea turtles only live at water apart from when they come to land to lay eggs and only for some minutes since the mother quickly abandons her young and returns back to the water, leaving the hatchlings alone. In contrast with the sea turtles, when the mother tortoise lays her eggs (on the ground), she protects her young for the first 80 days. Because of the difference of their natural habitat, they also differentiate in their structure, so sea turtles’ flat shells are streamlined and lighter (than tortoises’ dome-shaped heavy and well-protecting shell), which helps sea turtles to glide easier. Also sea turtles’ webbed feet have fins and long talons, which helps them getting a good grip and climbing onto surfaces, instead of tortoises’ short and sturdy feet. As far as lifespan is concerned, sea turtles live is about 60 to 70 years, with 40 to 50 years of that required to reach maturity. Although tortoises can live around 60-80 years, some have been known to live for over 150 years, whereas experts say it is possible for most of them to live for over 100 years in captivity (it is sometimes reported that tortoises have lived for about 200 years in captivity), but living beyond that would require "carefully controlled, nurturing environments". Finally, sea turtles are primarily found in tropical and semi-tropical climates like the once in the waters around of Africa and America, meanwhile tortoises mostly chose Asia and Africa.

3.5.3 Sea turtle shell: Tell me more!

Sea turtle shells depending on species. The adult carapace ranges in shape from oval to heart-shaped. In all species except the leather back, the bony shell is composed by broadened, fused ribs, and the backbone is attached to the carapace (the dorsal (top) side of the shell). Most turtles can hide their head in their shells but some species, like the sea turtle, can’t. The bottom side of the shell is called the plastron. A turtle’s shell is attached to their body. Turtles do not look for a bigger shell as they get bigger, instead the shell grows with the turtle. The inner layer of a turtle’s shell is made up of 60 bones, including their backbone, breastbone and ribs. A layer of thousands of small dermal bones lies just below the leathery skin. A sea turtle's large, bony shell provides protection from predation and abrasion. On the top of the shell have bones turtle’s carapace, or upper shell, is flatter to help them swim while a tortoises’ carapace is higher and dome-shaped. Depending on the species, sea turtles shells range in color they can be olive-green, yellow, greenish-brown, reddish-brown, or even black in color. The green sea turtle gets its name from the color of its body fat. Additionally, outside of the shell is a single nuchal bone, a series of twelve paired periphals then extend along each side. The ventral surface is called the plastron, these are joined by an area called the bridge. The skeletal elements of the plastron are also largely in pairs. These make up the front half of the plastron and the hyoplastron contains the anterior bridge strut. Overlying the bone elements are a series of scutes, which are made of keratin and are a lot like horn or nail tissue.

3.5.4 What do sea turtles eat?

Have you ever wondered what food do sea turtles it? Well, in this text we will discuss this specific subject! Firstly, we must note that the food sea turtles eat depends mainly on factors like the type of the turtle, their environment, their physical features and in general, their ability on consuming some specific types of sea life! Their jaw structure is adjusted for their diet and that’s one of the reasons why some are carnivorous (meat eating), some herbivorous (plant eating) and some others are omnivorous (eating both meat and plants). For example, a hawksbill has a narrow head with jaws, made for getting food from crevices in coral reefs. Loggerheads' and ridleys’ jaws are adapted for crushing and grinding. Loggerheads have a huge head and very strong jaws which allow them to crush hard-shelled prey. However, there are some turtles who change their food as they age. The diet of hatchlings, varies from that of adults. Hatchlings eat a variety of prey items including things like pelagic molluscs and crustaceans, hydrozoans (like jellies and corals), fish eggs, seaweed, and jellies. Green hatchlings, are omnivores unlike the herbivore adult and as young turtles they eat a variety of prey as mentioned before. Loggerhead adults on the other hand, are carnivores which means that they usually eat crabs, conchs, whelks, and horseshoe crabs. Loggerhead hatchlings are omnivores, eating both plant and animal material. Leatherback turtles eat jellyfish, tunicates, and other soft-bodied animals because hard substances can damage their jaws since they are fragile and scissor-shaped. Their mouth cavity and throat have backward pointing spoke, which assist in swallowing. These are just a few things all about this subject that give us a little taste of how amazing sea life is!

3.5.5 Migrations: How do sea turtles orient themselves?

It is well known that many animals (mammals, fishes, birds, insects etc. ) during their life use to immigrate, by traveling thousands of kilometers looking for food and protection from weather conditions in order to survive.
One of the most extraordinary animal thought is the sea turtle.
The sea turtle after being born on a coast somewhere in our planet, is driven to the sea by instinct. In the sea is travelling thousands of kilometers across the oceans facing a lot of enemies and other dangers. Nature has awarded the turtles everything necessary to make them excellent swimmers.
After fifteen to twenty years of adventure the sea turtles become adults and they are ready to give birth to their eggs. To do this thought, they return to the same beach and coast that they were born. The sea turtles remember perfectly this place although a lot of years have passed since their born. Derivation of offspring from the sea turtles occurs exactly in the same place.
For many years scientists were searching for reasons to explain this fact. How is it possible to find their way after of so many years travelling in the oceans; How they remember their way back to the same place they were born;
Scientists have long known that the turtles, like many animals, navigate at sea by sensing the invisible lines of the magnetic field, similar to how sailors use latitude and longitude. But they didn't know how the turtles were able to return to the very spot where they were born.
After a lot of research, they finally manage to solve the mystery. They discovered that sea turtles also rely on Earth's magnetic field to find their way home. That's because each part of the coastline has its own magnetic signature, which the animals remember and later use as an internal compass.

3.5.6 Sea turtles nesting: Find out more!

Female sea turtles come ashore on a sandy beach to nest a few weeks after mating. They have to build nests and lay their eggs. They usually nest during the warmest months of the year from May to October, but there are many exceptions. The exception is the leatherback turtle, which nests in fall and winter. Leatherback turtles start in February, and depending on water temperature, hatchlings emerge well into the winter months.The female turtle emerges from the sea at night and ascends the beach, searching for a suitable nesting site (somewhere dark and quiet).Sea turtles use beaches and the lower dunes to nest and lay their eggs. Once at the chosen nesting site, she begins to dig a body pit by using all four flippers. Using their front flippers they sweep away sand and start digging a hole that will take them many hours into the night to complete. After digging for a while with their front flippers, they switch to use their hind flippers curling them to scoop out sand to make a deep hole. Next they lay the eggs into the deep hole and cover it back over before dragging themselves down the beach and back into the water before sunrise.The number of eggs in a nest, called a clutch, varies by species. In addition, sea turtles may lay more than one clutch during a nesting seasonSea turtles deposit an average of about 100 eggs in each nest and lay between 3 and 7 nests during the nesting season. In Cyprus sea turtles favour the Akamas peninsula which is a national park, as well as the Akrotiri region where hundreds of turtles lay their eggs every season..

3.5.7 What causes a sea turtle to be born male or female?

In most species, gender is determined during fertilization. Nevertheless, the factor that is actually responsible for the sex of most of the turtles, other reptiles and teleost fish is determined after fertilization. The temperature of the developing eggs, particularly during the middle one-third of embryonic development, is what decides whether the offspring will be male or female. This is called the temperature-dependent sex determination or TSD. Research shows that if turtles' eggs incubate in temperatures below 81,86° Fahrenheight, the turtle hatchlings will be male. On the other hand, if the eggs hatch above 87,8° Fahrenheit, the hatchlings will be female. Temperatures that fluctuate between the two extreme values will produce a mix of male and female baby turtles. The critical period of incubation is known as the the thermosensitive period. The most serious problem researchers have pointed out is that the warmer the sand the higher the ratio of female turtles. The Earth experiences climate change and consequently the temperatures are increasing gradually. These could result in skewed and even lethal incubation conditions, which would affect turtle species and other reptiles. For instance, density, pH and environmental background colour are also observed to alter sex ratio, which could be classified either as temperature-dependent sex determination or as temperature-dependent sex differentiation, depending on the involved mechmanisms. All of the above, mentioned could lead to reconsidering the status of these turtle species that are claimed to have TSD when submitted to extreme temperatures instead of the temperature experienced during development in the wild, since changes in sex ratio with temperature variation are ecologically and evolutionally relevant. Furthermore, the cortisol-mediated pathway and epigenetic regulatory are thought to be the potential mechanisms involved to influence.