Sea turtle
Sea turtles (superfamily Chelonioidea), sometimes called marine turtles,[3] are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are the flatback, green, hawksbill, leatherback, loggerhead, Kemp's ridley, and olive ridley.[4][5] Six of the seven sea turtle species, all but the flatback, are present in U.S. waters, and are listed as endangered and/or threatened under the Endangered Species Act.[6] All but the flatback turtle are listed as threatened with extinction globally on the IUCN Red List of Threatened Species. The flatback turtle is found only in the waters of Australia, Papua New Guinea, and Indonesia.[7][6]
Sea turtles can be categorized as hard-shelled (cheloniid) or leathery-shelled (dermochelyid).[8] The only dermochelyid species of sea turtle is the leatherback.[8]
Description
For each of the seven species of sea turtles, females and males are the same size. As adults, it is possible to tell male turtles from female turtles by their long tails with a cloacal opening near the tip. Adult female sea turtles have shorter tails, with a cloacal opening near the base. Hatchling and sub-adult turtles do not exhibit sexual dimorphism; it is not possible to determine their sex by looking at them.[9]
In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. This tapering at both ends reduces volume and means that sea turtles cannot retract their head and limbs into their shells for protection, unlike many other turtles and tortoises.[10] However, the streamlined body plan reduces friction and drag in the water and allows sea turtles to swim more easily and swiftly.
The leatherback sea turtle is the largest sea turtle, reaching 1.4 to more than 1.8 m (4.6 to 5.9 ft) in length and weighing between 300 and 640 kg (661 to 1,411 lbs).[11] Other sea turtle species are smaller, ranging from as little as 60 cm (2 ft) long in the case of the Kemp's ridley, which is the smallest sea turtle species, to 120 cm (3.9 ft) long in the case of the green turtle, the second largest.[5][12]
The skulls of sea turtles have cheek regions that are enclosed in bone.[13][14] Although this condition appears to resemble that found in the earliest known fossil reptiles (anapsids), it is possible it is a more recently evolved trait in sea turtles, placing them outside the anapsids.[15][13]
Distribution and habitat
Sea turtles can be found in all oceans except for the polar regions. The flatback sea turtle is found solely on the northern coast of Australia. The Kemp's ridley sea turtle is found solely in the Gulf of Mexico and along the East Coast of the United States.[30]
Sea turtles are generally found in the waters over continental shelves. During the first three to five years of life, sea turtles spend most of their time in the pelagic zone floating in seaweed mats. Green sea turtles in particular are often found in Sargassum mats, in which they find food, shelter and water.[31] Once the sea turtle has reached adulthood it moves closer to the shore.[32] Females will come ashore to lay their eggs on sandy beaches during the nesting season.[33]
Sea turtles migrate to reach their spawning beaches, which are limited in numbers. Living in the ocean therefore means they usually migrate over large distances. All sea turtles have large body sizes, which is helpful for moving large distances. Large body sizes also offer good protection against the large predators (notably sharks) found in the ocean.[34]
In 2020, diminished human activity resulting from the COVID-19 virus caused an increase in sea turtle nesting. Some areas in Thailand saw an abnormally high number of nests, and Florida experienced a similar phenomenon. Less plastic and light pollution could explain these observations.[35]
Physiology
Osmoregulation
Sea turtles maintain an internal environment that is hypotonic to the ocean. To maintain hypotonicity they must excrete excess salt ions.[50] Like other marine reptiles, sea turtles rely on a specialized gland to rid the body of excess salt, because reptilian kidneys cannot produce urine with a higher ion concentration than sea water.[51] All species of sea turtles have a lachrymal gland in the orbital cavity, capable of producing tears with a higher salt concentration than sea water.[52]
Leatherback sea turtles face an increased osmotic challenge compared to other species of sea turtle, since their primary prey are jellyfish and other gelatinous plankton, whose fluids have the same concentration of salts as sea water. The much larger lachrymal gland found in leatherback sea turtles may have evolved to cope with the higher intake of salts from their prey. A constant output of concentrated salty tears may be required to balance the input of salts from regular feeding, even considering leatherback sea turtle tears can have a salt ion concentration almost twice that of other species of sea turtle.[53]
Ecology
Diet
The loggerhead, Kemp's ridley, olive ridley, and hawksbill sea turtles are omnivorous their entire life. Omnivorous turtles may eat a wide variety of plant and animal life including decapods, seagrasses, seaweed, sponges, mollusks, cnidarians, Echinoderms, worms and fish.[72][73][74][75] However, some species specialize on certain prey.
The diet of green sea turtles changes with age.[76] Juveniles are omnivorous, but as they mature they become exclusively herbivorous.[73][76] This diet shift has an effect on the green sea turtle's morphology.[77][78] Green sea turtles have a serrated jaw that is used to eat sea grass and algae.[79]
Leatherback sea turtles feed almost exclusively on jellyfish and help control jellyfish populations.[80][81]
Hawksbill sea turtles principally eat sponges, which constitute 70–95 % of their diets in the Caribbean.[82]
Larynx mechanisms
There was little information regarding the sea turtle's larynx. Sea turtles, like other turtle species, lack an epiglottis to cover the larynx entrance. Key findings from an experiment reveal the following in regards to the larynx morphology: a close apposition between the linguolaryngeal cleft's smooth mucosal walls and the laryngeal folds, a dorsal part of the glottis, the glottal mucosa attached to the arytenoid cartilage, and the way the hyoid sling is arranged and the relationship between the compressor laryngis muscle and cricoid cartilage. The glottal opening and closing mechanisms have been examined. During the opening stage, two abductor artytenoideae muscles swing arytenoid cartilages and the glottis walls. As a result, the glottis profile is transformed from a slit to a triangle. In the closing stage, the tongue is drawn posteriorly due to the close apposition of the glottis walls and linguolaryngeal cleft walls and hyoglossal sling contractions.[83]
Relationship with humans
Sea turtles are caught worldwide, although it is illegal to hunt most species in many countries.[84][85] A great deal of intentional sea turtle harvests worldwide are for food. Many parts of the world have long considered sea turtles to be fine dining. In England during the 1700s, Sea Turtles were consumed as a delicacy to near extinction, often as turtle soup.[86] Ancient Chinese texts dating to the 5th century B.C.E. describe sea turtles as exotic delicacies.[87] Many coastal communities around the world depend on sea turtles as a source of protein, often harvesting several sea turtles at once and keeping them alive on their backs until needed. Coastal peoples gather sea turtle eggs for consumption.[88]