Why are sea turtles important?
Sea turtles are important to the ecosystem because they have been around for such a long period of time, around 200 million years! With several species and environmental processes dependent upon sea turtle, they a become a keystone species. When a keystone species is eliminated from its environment, it will change dramatically, meaning turtles contribute a large amount to their surrounding environment, making their role critical for the wellbeing of their habitat.
Sea turtles are particularly special because they are both marine and terrestrial animals, making them important for two separate ecosystems. As many know, sea turtles lay their nests at the back of beaches. Most of the eggs inside the nest will hatch, but those that do not and the egg shells left behind will decompose releasing vital nutrients such as nitrogen and phosphorous, which directly benefits the dune vegetation allowing it to stay vibrant healthy. When the dune system is healthy and intact, the beaches are more stable and less susceptible to storm events. These eggs left behind are also food for any beach scavengers, like crabs.
Sea turtles are highly important to the marine environment as well. They migrate thousands of kilometres in search of optimal foraging grounds and nesting beaches. During this migration, they swim through parts of the sea that are nutrient poor. Most sea turtle species, except the leatherback, have a hard shell, called a carapace. The carapace protects the turtle organs from predators but is also home to over a hundred different commensal species like barnacles and algae. With all these species dwelling on their carapace, loggerheads can eventually become somewhat of a “mobile reef”, a term coined by James Spotila. When migrating through barren areas of the sea, some fish in the area will feed off the algae, barnacles and skeleton shrimp (epibionts) growing on their carapace. Therefore, sea turtles are both nutrient and energy transporters both in the terrestrial and in the marine environment!
While loggerheads are omnivorous, they prefer to eat benthic invertebrates such as crustaceans (e.g. crabs), mollusks (e.g. bivalves like mussels), echinoderms (e.g. starfish) but also organism such as cnidarians (e.g. jellyfish). It is a common misconception that loggerheads diet is largely made up of fish. By foraging on all of the organisms discussed above, sea turtles help maintain a balanced food web. With the global decline of fish stock, the amount of jellyfish is on the rise. Sea turtles act to keep the population of jellyfish at a relatively stable level. In order to forage on such bottom dwellers, like crabs, loggerheads must sift through the sand in search of their prey. As they do this, they stir up and aerate the sand, allowing nutrient distribution of the sediment.
Loggerhead sea turtles (Caretta caretta)
There are seven species of sea turtles that exist around the world. Six of the seven are considered either “endangered” or “vulnerable” due to several globally present threats, the main one being humans. In the Mediterranean Sea, three of the species are present. The loggerheads and green sea turtles are the most prominent species inhabiting these warm waters, whereas it is on a rare occasion that the leatherback will make an appearance. Here in Greece, both the loggerhead and green sea turtles can be seen foraging, but only the loggerhead sea turtle will lay her nests on its beaches. Loggerheads also nest in Turkey, Cyprus and Libya. Green sea turtles share the Turkish and Cypriot nesting grounds with the loggerheads and also nest in Syria.
On occasion loggerhead, sea turtles from the Atlantic Ocean will swim through the Strait of Gibraltar into the Mediterranean Sea. They enjoy foraging and swimming in the warmer saltier seas of the Mediterranean. Just as humans from different places around the world may look a little different, so do sea turtles! Since the loggerheads in Greece have not bred with the Atlantic population since the Holocene Period nearly 12,000 years ago, there is a difference in the size of the loggerhead turtles. Greek sea turtles tend to be smaller than the Atlantic sea turtles. This difference is important because the carapace measurement (curved carapace length – CCL) is used to determine the age of the turtle. If Greece was to use the same standard of measurement that researchers along the east coast of the US do, then the age of Greek turtles would be largely underestimated. This emphasizes the importance of Wildlife Sense conducting their own research because the sea turtles here do not always measure up to those in the Atlantic!
While loggerhead sea turtles are able to live an upwards of around 60 years, they often do not make it to 40 years of age. This is due to the large amount of threats that they encounter, even on a daily basis. One of the greatest threats to sea turtles is fishing. The use of longlines and trawling equipment drowns thousands of turtles every year. Habitat loss is also becoming a problem for sea turtles. They once roamed the waters freely foraging as they please, but their oceanic habitats are slowly disappearing and with global climate change this situation is prone to become more serious. Not only are their oceanic foraging grounds being compromised, but also their nesting beaches. As anthropogenic activity increases on beaches with buildings, like beach front hotels and bars, are being built over primary nesting areas, slowly the optimal areas a sea turtle could nest begin to disappear. They will then be left to nest in parts of the beach that are prone to inundation or poor quality sand.
Plastic ingestion is also cutting the life of sea turtles short and has not received enough attention in the past. Plastics, especially single-use plastics, are everywhere these days, both in our homes and businesses, but also in the environment including the sea. These items are can take over 500 years to decompose and even when plastics are broken into small pieces they become microplastics that are ingested first by lower trophic level organisms and slowly make their way up the food chain until we too are ingesting them. But plastics don’t have to be microscopic to be ingested. Sea turtles often mistake plastics, such as bags, for food and end up dying.
Nests, also called a clutch, in Kefalonia are made up of around 90 eggs. These eggs incubate in the sand with no additional care from their mothers. Each clutch can contain up to 5 different paternities, meaning the mother mated with several different males. A nest incubates on average around 55 days, with incubation periods being shorter the warmer it is and longer the colder it is. Therefore, a nest that is laid in early June will most likely incubate for a shorter period of time (shorter than 55 days) than a nest laid in early August (longer than 55 days). During the middle third of the incubation period, the sex of each egg will be determined based on the temperature of the sand. If the sand is warmer than 29 °C, then the nest will yield female turtles. However, if the sand is cooler than 29 °C the nest will yield male turtles. If the nest incubates at 29 °C, then the nest will have an equal ratio of males and females. Global climate change is slowly shifting the sex ratio of sea turtle nests worldwide to be female biased, making this area of research extremely important to the future of the species.
When one egg within the nest begins to hatch out of its shell its movements cue the others in the nest to begin to hatch out of their shells. This is why loud noises, like hotel and beach bar parties, or vibrations from machinery can cause a nest to hatch early than it is supposed to hatch therein by causing harm to the nest. Once a few of the hatchlings have made their way out of their shells they cooperatively begin to dig out of the nest. It takes a couple nights for them to reach the beach surface. Once near the surface, hatchlings use sand temperature to know when to emerge. When the sand become cool they understand that it is night time and it is safe to emerge out of their nest. Hatchlings rarely emerge out of their nest during the daytime. It can take up to six nights for all of the hatchlings to finish emerging out of the nest because they rest during the day time. Not all the eggs will hatch from the nest. On average, around 80% of the nest will hatch if it hasn’t been severely affected during its incubation period. When storm events occur several times during the incubation period, the eggs may suffocate because the gas exchange between the nest and the sand surface has been compromised. Additionally, the wet and dark environment may spur the growth of microorganisms such as fungi and bacteria within the nest, which can threaten the survivorship of the nest.
Once a hatchling has reached the surface it head toward the brightest horizon and use the beach slope as a guide to the sea. Typically the sea is the brightest horizon, however when human activity, like beach bars and restaurants, line the back of the beach the hatchlings will crawl towards them and away from the sea. Often times these hatchlings become exhausted in their search for the sea and eventually die. Light pollution can be a large problem in areas with human activity, which is why beach use must be under heavy scrutiny.
Once hatchlings reach the sea they will migrate to their foraging grounds using magnetic orientation. Loggerheads in the Atlantic Ocean become entrained in a cyclical current known as a gyre that takes up a large portion of the North Atlantic Ocean. However in the Mediterranean Sea, the gyres are much smaller and can be spontaneous making it difficult to understand where hatchlings migrate to in the Mediterranean Sea. They will forage and swim in areas protected by sargassum and sea vegetation as they grow and develop. When they are around 15 to 25 years old they will be able to reproduce. Once they are ready to reproduce they will be ready to migrate back to the general area from which they hatched. Loggerheads use water temperature as a cue to indicate when they need to swim back to their nesting grounds. When the sea becomes warmer they begin their migration back to their nesting beaches, close to where they originally hatched from. When the loggerheads reach Kefalonia they will mate for around six weeks beginning around May.
Shortly after mating, the adult females will begin to lay their first nest of the season. Water temperature is also used as a cue for when nesting season is to begin. Therefore, the warmer the waters the early the initial nests will be laid. Something to think about with global climate change, eh? When the female comes ashore to nest, she will be very precautious as she is not well-acquainted with being on land. Her vision on land is blurry and myopic because sea turtles need water to help clarify images.
In fact, when she is on land, tears full of salt will be seen coming from her eyes. Sea turtles must drink water just like we do, but sea water is the only available source, meaning they also drink a lot of salt. Too much salt ingested by any organism can be damaging so they must maintain a certain osmotic balance. To do this sea turtles have a large lacrimal gland, also known as a tear duct, that produces tears and excretes excess salt. Did you know that this gland is larger than their brains? This should make it clear just how vital its function is to the sea turtles!
Females look to lay their nests towards the back of the beach. If disturbed, mainly by human activity, or unable to find an optimal nesting ground she will return to the sea and emerge at another point along that beach or find a more suitable beach nearby. Around one in three emergences leads to a nest. Once she finds a suitable area to lay her nest it may take up to an hour for her to complete the nesting process which includes digging an egg chamber, laying the eggs and camouflaging the area to prevent predators from digging the nest.
After she is done camouflaging, she’ll immediately crawl back into the sea. The females will remain close to their nesting beaches until they are ready to nest again in around two weeks. This two-week period of time where the female does not expend a lot of energy is called the internesting period. Females will repeat this process during the nesting season, late May through early August until they have laid three to four clutches of eggs. The males during this time will forage around the area and slowly make their way back to their primary foraging grounds which are the Adriatic Sea and the Gulf of Gabès on the east coast of Tunisia. After the females have laid her last clutch of eggs for the nesting season, she will also slowly make her way back to her primary foraging grounds. Sea turtles do not nest every year. Rather, they nest every two to three years depending on how many and how nutritious the resources she can consume are during the rest of the year.