Sea turtles are part of the oldest lineage of living reptiles in existence, having been present since the Jurassic period (PRITCHARD, P. 1997; CUBAS, Z. et al, 2020). The origin of these animals is terrestrial, but they have adapted to the marine environment. Among the adaptations that the group developed, we have their locomotor limbs becoming fins, with small claws, in addition to their carapace becoming flattened dorsoventrally, becoming lighter and more hydrodynamic. Another adaptation was the appearance of salt glands, located behind the eyeballs, which have the function of filtering excess sodium from the body (WYNEKEN, J. 2004; CUBAS, Z. et al, 2020).
There are seven species of sea turtle in the world, five of which occur on the Brazilian coast. These animals are divided into two families: Cheloniidae, with four species (the green turtle (Chelonia mydas), the loggerhead turtle (Caretta caretta), the hawksbill turtle (Eretmochelys imbricata) and the olive ridley turtle (Lepidochelys olivacea)); and Dermochelyidae, with only one species (the leatherback turtle (Dermochelys coriacea)). There are also two other species of the Cheloniidae family, the Australian sea turtle (Natator depressus), which only occurs off the coast of Australia, and the Kemp's ridley turtle (Lepidochelys kempii), which occurs in the Atlantic Ocean, mainly off the coast of the Gulf of Mexico. All of them are on the International Union for Conservation of Nature's red list of endangered species (KERSTING, D. et al, 2009). Biometrics and photo-identification methods are therefore used to monitor these species and carry out further research into their conservation.
The identification of sea turtle species is usually done through the characteristics of the skull and the dermal plates of the head and carapace. In addition, the shape of the mandible and the number of digits on the flippers can be used (WYNEKEN, J. 2004). The main dermal plates of the carapace used in this method are the marginal, costal, vertebral and nuchal plates (Figure 1). In the hawksbill turtle (Eretmochelys imbricata), the inframarginal shields are also used. The plastron also has its own characteristics, but is used to locate the animal's internal structures.
The green turtle (Chelonia mydas) has four pairs of lateral plates, and the plates are juxtaposed. The head has a pair of prefrontal plates (or shields) and four pairs of post-orbital shields (MÁRQUEZ, M.R. 1990). The coloration of the carapace is greyish-green (Figure 2. A), but the name green turtle comes from the fact that the lipid tissue of these animals is green due to their diet of seaweed (SANTOS, A. et al. 2011). This species has an average length and weight of 110cm and 145kg (INDIAN OCEAN TUNA COMMISSION et al. 2011). The loggerhead turtle (Caretta caretta) has a larger head than the others, with 2 pairs of prefrontal plates (or shields) and 3 post-orbital pairs (MÁRQUEZ, M.R. 1990). The carapace is yellowish brown (Figure 2.B) and has 5 pairs of lateral plates, the plates being juxtaposed. This species has an average length and weight of 100cm and 100 to 180kg (INDIAN OCEAN TUNA COMMISSION et al. 2011). Its diet is omnivorous, consisting of fish, crustaceans, jellyfish and gastropods (CUBAS, Z. et al, 2020). The hawksbill turtle (Eretmochelys imbricata) has 4 pairs of lateral plates, with the plates overlapping. The head has 2 pairs of prefrontal plates (or shields) and 3 pairs of postorbital plates (MÁRQUEZ, M.R. 1990). The carapace is brown (Figure 2.C). This species is identified by its pointed horny beak and has an average length and weight of 90cm and 60kg (SANTOS, A. et al. 2011; INDIAN OCEAN TUNA COMMISSION et al. 2011). Its diet is omnivorous, but due to the shape of its beak it is predominantly spongivorous (CUBAS, Z. et al, 2020). The olive ridley turtle (Lepidochelys olivacea) has 5 to 9 pairs (usually 6) of lateral plates, and the plates are asymmetrical. The head has 2 pairs of prefrontal plates (or shields) and 3 post-orbital pairs (MÁRQUEZ, M.R. 1990). The coloration of the carapace is olive green (Figure 2. D). This species has an average length and weight of 70cm and 45kg (SANTOS, A. et al. 2011; INDIAN OCEAN TUNA COMMISSION et al. 2011). Its diet is omnivorous, including fish, crabs, oysters, urchins, shrimp and algae (CUBAS, Z. et al, 2020). The leatherback turtle (Dermochelys coriacea) has 7 longitudinal keels, unlike the other species, it does not have plates. In addition, the head and fins are covered in skin, also without plates or shields (MÁRQUEZ, M.R. 1990). The carapace is black with white, bluish and pinkish spots (Figure 3). This species is the largest of all, with an average length of 170cm and an average weight of 450kg (SANTOS, A. et al. 2011; INDIAN OCEAN TUNA COMMISSION et al. 2011). Its diet is omnivorous, including cnidarians (jellyfish) and tunicates (CUBAS, Z. et al, 2020).
The other two species are easily identified because they occur in endemic regions. The Australian sea turtle (Natator depressus) has an average length and weight of 76 to 96 cm and 70 to 90 kg. Its diet is omnivorous, including jellyfish, sea cucumbers, squid, octopus and other soft invertebrates. Unlike the others, the hatchlings of this species prefer coastal waters rather than dispersing into the ocean. In addition, they are the fastest sea turtles because their carapace is flatter and more developed for hydrodynamics. The Kemp's ridley turtle (Lepidochelys kempii) has an average length and weight of up to 0.6m and up to 45kg. Their diet is carnivorous, feeding on molluscs and crustaceans. The turtles of the Lepidochelys genus (olive ridley turtle and Kemp's ridley turtle) are the only ones that have the behavior of mass reproduction. This phenomenon is known as arribada and is when 10,000 female turtles of this genus meet to lay their eggs together over a period of 1-3 days.
Identification methods for sea turtles off the Brazilian coast are being improved as more research is carried out. In Brazil, much of what we know about sea turtles comes from data collected by organizations working to conserve these species, such as the TAMAR project. The use of molecular markers is an innovative method that has also been used to identify species and possible hybridization processes, especially between the species of green turtle (Chelonia mydas) and hawksbill turtle (Eretmochelys imbricata) (DA SILVA et al, 2021). In short, it is necessary to emphasize the importance of these species for the marine ecosystem. These animals have achieved a high level of evolutionary stability within their niche on the planet. These species control countless aquatic organisms due to their feeding patterns. In addition, they are part of the food chain of countless other marine species. The study of sea turtles is fundamental to understanding life in the ocean environment. As such, a great deal of research has been and still is being carried out into microbiology, virology, parasitology and pathology, among other subjects applied to these animals.
Because they have a varied diet, they accidentally ingest plastic and other garbage floating in the oceans and because they are unable to metabolize this waste, these animals strand themselves very weak or, more often than not, dead. In view of this, it is necessary to carry out campaigns on behalf of sea turtles, including all niches of society, especially residents of coastal regions, as they are in greater contact with this reality. In this sense, such campaigns could generate mass awareness of the importance of conserving these seven species for the balance of the planet's overall ecosystem.
Figure 1: Dermal plates of the carapace of a sea turtle.
Source adapted from: MÁRQUEZ, M.R., 1990.
Figure 2: Image A: Green turtle (Chelonia mydas). Image B: Loggerhead turtle (Caretta caretta). Image C: Hawksbill turtle (Eretmochelys imbricata). Image D: Olive ridley turtle (Lepidochelys olivacea).
Source adapted: MÁRQUEZ, M.R., 1990.
Figure 3: Leatherback turtle (Dermochelys coriacea).
Source adapted from: MÁRQUEZ, M.R., 1990.
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