Sea Turtles

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Robinson N.J., and Paladino F.V , Sea Turtles, Reference Module in Earth Systems and Environmental Sciences, Elsevier, 2013. 11-Sep-13 doi: 10.1016/B978-0-12-409548-9.04352-9.

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Sea Turtles☆ NJ Robinson, Purdue University, West Lafayette, IN, USA FV Paladino, Indiana University-Purdue University Fort Wayne, Fort Wayne, IN, USA ã 2013 Elsevier Inc. All rights reserved.

Taxonomy Morphology Behavior and Physiology Reproduction Life History and Distribution Survival Rates and Threats Cheloniidae Genus Chelonia: Chelonia mydas (Linnaeus, 1758) the Green Turtle Genus Lepidochelys: Lepidochelys kempii (Garman, 1880) the Kemp’s Ridley Turtle, Lepidochelys olivacea (Eschsholtz, 1829) the Olive Ridley Turtle Genus Eretmochelys: Eretmochelys imbricata (Linnaeus, 1766), the Hawksbill Turtle Genus Caretta: Caretta caretta (Linnaeus, 1759): the Loggerhead Turtle Genus Natator: Natator depressus (Garman, 1880) the Flatback Turtle Dermochelyidae Genus Dermochelys: Dermochelys coriacea (Vandelli, 1761) the Leatherback Turtle References

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Taxonomy Sea turtles, along with other aquatic turtles and tortoises, are members of the order Testudines. Sea turtles constitute a single radiation that became distinct from the other turtles during the early Cretaceous period approximately 110 million years ago (Hirayama, 1998). It is widely accepted that at least three families evolved from the original sea turtle lineage: the Protostegidae, Cheloniidae, and Dermochelyidae (Kear and Lee, 2006). Today, only two sea turtle families can be found in the world’s ocean: the Cheloniidae and Dermochelyidae. The ‘hard-shelled’ Cheloniidae evolved about 60 million years ago and the ‘soft-shelled’ Dermochelyidae evolved about 90 million years ago (Bowen et al., 1993; Duchene et al., 2012). The Cheloniidae contain six extant species within five genera: the flatback turtle (Natator depressus), the green turtle (Chelonia mydas), the hawksbill turtle (Eretmochelys imbricata), the loggerhead turtle (Caretta caretta), the Kemp’s ridley turtle (Lepidochelys kempii) and the olive ridley turtle (Lepidochelys olivacea). The Dermochelyidae contain a single extant species within a single genus: the leatherback turtle (Dermochelys coriacea).

Morphology A readily identifiable characteristic of any Testudine is the presence of a bony or cartilaginous shell encompassing the torso of the animal and serving to protect the vital organs. The shell is a composite structure derived from the ribs, but it also includes bones from the shoulder girdle and specialized dermal bones. The shell is divided into a dorsal section called the carapace and a ventral section called the plastron. In hard-shelled turtles, the carapace and plastron are comprised of paired bony plates with typical bilateral symmetry as found in all higher vertebrates. In the leatherback turtle, the carapace and plastron are instead comprised of an asymmetrical, interlocking mosaic of cartilaginous osteoderms beneath a layer of leathery skin. At the front of a sea turtle’s skull is a prominent, toothless beak made of hard bone or cartilage and covered in horny keratin. All turtles are anapsids, meaning that there are no openings (fenestra) in the temple region of the skulls behind the eye socket. While it was initially thought that the anapsids were a monophyletic group, recent studies suggest that the anapsid skull of turtles may be due to reversion from diapsid ancestors and not from direct descent (Lyson et al., 2012). Consequently, sea turtles may be more closely related to lizards and snakes than to birds and crocodiles, as previously thought (Lyson et al., 2010). A clear morphological distinction between sea turtles and other Testudines is evident in the shape of their limbs. Non-marine Testudines generally have short, sturdy, and occasionally webbed feet. In contrast, the limbs of sea turtles are broadly flattened to form large paddle-like flippers. The structure of the flippers is provided by greatly elongated digits and, in the front flippers only, a ☆ Change History: May 2013. FV Paladino added keywords, and abstract. Original sections ‘Introduction’ and ‘General Sea Turtle Biology’ broken down into smaller sections, ‘Taxonomy’, ‘Morphology’, ‘Behavior and Physiology’, ‘Reproduction’, ‘Life-History and Distribution’, and ‘Survival Rate and Threats’, and have been reworded, reorganized, and updated. All figures are replaced.

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short, thick radius and ulna. The front flippers are used to propel the turtle through the water using a lift-based propulsion system analogous to bird-wings (Renous et al., 2000). The rear flippers are much shorter than the front flippers and function as a rudder to steer while in the water. While sea turtles are highly efficient swimmers, the increased musculature necessary for swimming means that, unlike other Testudines, sea turtles are unable to retract their limbs or necks into their shells. Sea turtles start their life small, generally less than 10 cm in length and 55 g in weight, but they eventually grow into some of the largest reptiles on this planet. Growth rates of juvenile turtles are rapid until they reach sexual maturity at seven to 40 years of age (Chaloupka and Musick, 1997). Beyond this age, most of the turtle’s available energy is devoted towards reproduction and growth rates slow down rapidly until they are almost negligible (Price et al., 2004). Of the extant sea turtles, the leatherback turtle is by far the largest. Adult leatherback turtles regularly measure over 1.4 m long in curved carapace length (CCL) and weigh between 200 and 1000 kg. The smallest of the sea turtles are the ridleys, but even they can grow to 80 cm CCL and weigh up to 50 kg. For the hard-shelled turtles, species can be differentiated by counting the number and arrangement of keratinized scales (called scutes) on the carapace and head (Figure 1). Of particular importance are the scutes between the eyes (prefrontal scutes), the scutes running down the center of the carapace (vertebral scutes), the scutes adjacent to the vertebral scutes (lateral scutes), and the scutes joining the plastron to the carapace (inframarginal scutes). The leatherback turtle is readily distinguished by the absence of bony scutes and a smooth, leathery carapace with lateral dermal ridges.

Behavior and Physiology It may be more apt to refer to sea turtles as ‘surfacers’ instead of ‘divers’. Indeed, sea turtles spend the majority of their lives at depth and here they feed, mate, and even sleep. Such feats are particularly impressive considering that sea turtles use lungs to breathe air and must hold a single breath between dives. While most turtles spend the majority of their time in the upper 200 m of the water column (Polovina et al., 2003; Sale et al., 2006; Rice and Balazs, 2008), leatherback turtles have been recorded diving to depths of up to 1280 m (Doyle et al., 2008). When a diving turtle descends down the water column, it experiences increasing hydrostatic pressure at a rate of 1 atm for every 10 m depth. As pressure increases, a compressive force is exerted on all gases in the body. This is only a minor issue for animals that exhale before diving, such as whales, but sea turtles actually inhale before diving. Furthermore, if the pressures are not equalized between the air in the lungs and the surrounding water, this can cause significant structural damage to the thoracic region. Sea turtles lungs, however, are able to collapse at depth without pulling apart the visceral and parietal pleura (Berkson, 1967). Not only does this minimize any pressure differentials but as the lungs collapse they shunt all the compressed air into the trachea (Davenport et al., 2009). Here, the potential for gases to diffuse into the blood is reduced. This is beneficial because any gas exchange between the respiratory track and the blood under high hydrostatic pressures can cause nitrogen narcosis on descent and decompression sickness (‘the bends’) on ascent. Not only are sea turtles highly adapted to deep diving, but also they are proficient in diving for prolonged periods of time. Overwintering loggerhead turtles can remain submerged for over seven hours between breaths (Hochscheid et al., 2005). Although during routine dives, most non-overwintering turtles rarely remain submerged for longer than 60 min (Reina et al., 2005; McMahon et al., 2007; Hatase et al., 2007). Enabling sea turtles to remain submerged for prolonged periods of time are a number of adaptations in their respiratory and cardiovascular systems. First, respiratory tidal volumes are relatively large at 1 to 2 l per breath, allowing for rapid uptake of oxygen and washout of carbon dioxide (Lutcavage et al., 1990). Second, even though sea turtles inhale before diving, their lungs collapse at depth and so oxygen inhaled prior to diving is only accessible during shallow dives. Consequently, sea turtles store a large quantity of oxygen attached to respiratory pigments like hemoglobin and myoglobin, which remains accessible at depth. Third, while diving, sea turtles shunt blood flow away from non-essential organs while maintaining blood flow to the heart, brain, central nervous system, and kidneys. This helps to reduce the oxygen consumption rates of these non-essential organs and extends the amount of time a turtle can operate aerobically on a single breath. As a result, sea turtles can spend long periods of time underwater without accruing significant oxygen debt (Berkson, 1966; Bradshaw et al., 2007), which would require longer surface intervals and expose them to predation from below. As a consequence of their environment, everything sea turtles eat or drink contains seawater. If most animals ingest excessive amounts of seawater (which has three times the salt concentrations than vertebrate cytoplasm) their cells dehydrate and die. Thus, sea turtles must be able to cope with an extreme level of salt intake. While the nephrons in reptilian kidneys have short loops of Henle that are unable to fully remove this excess salt load from the blood, marine reptiles have ancillary salt glands. These modified lacrimal (tear) glands can excrete a hyper-saline solution twice as concentrated as seawater and thus, six times as concentrated as typical vertebrate cytoplasm (Nicolson and Lutz, 1989; Reina et al., 2002) (Figure 2).

Reproduction Mating is typically observed near the nesting beaches. Mating may also occur during migration or at the foraging grounds, although evidence of such behavior is sparse. Fertilization is internal and female turtles can store sperm for long-periods of time (Gist and Jones, 1989). When mating, male turtles use the claws on their front flippers to clasp onto the female’s carapace. Competition

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Figure 1 Scute patterns and shell morphology of the 7 sea turtle species. Reproduced from Wyneken, 2001, illustrations by Dawn Witherington.

between males can be fierce and often multiple males often try to mate with a single female at the same time. Sea turtles have been known to be monogamous, polyandrous and polygynous (Crim et al., 2002; Phillips et al., 2013). Sea turtles are oviparous and lay amniotic eggs with protective membranes that allow for complete embryonic development within the egg (Figure 3). Amniotic eggs, however, must incubate in an aerial environment. Consequently, female sea turtles venture out of the water to deposit their eggs on sandy tropical or sub-tropical beaches.

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Figure 2 Close up of the face of a leatherback turtle. The fluid being expelled from the salt glands near the eyes gives the turtle the appearance of crying. Photo taken by Nathan J. Robinson.

Figure 3 Eggs of a hawksbill sea turtle. In the top of the picture is the cloaca, which is the common urogenital opening, though which sea turtles pass their eggs. Photo taken by Nathan J. Robinson.

Sea turtles generally nest at night with the exception of the Kemp’s ridley and flatback turtles. After a gravid female turtle crawls out of the water and above the high-tide line it begins a fixed-routine of behaviors that constitute the nesting process (Multimedia 1). This starts with the synchronized beating of the front flippers and sweeping with the rear flippers to create a ‘body pit’ in the sand. Next the female uses the rear flippers alternately to dig a hole straight into the ground. Once the turtle digs as far as it can reach, which can be up to 80 cm deep for leatherback turtles, it widens the walls at the base of the hole, creating a pear-shaped chamber in which to lay its eggs. A single clutch may contain between 50 and 150 eggs. Sea turtle eggs are 3 to 6 cm in diameter and have soft, leathery shells. With the clutch laid, the turtle uses its rear flippers to sweep sand over the nest. After the next is completely covered, the turtle throws sand with both front and rear flippers to effectively camouflage the area. Subsequently, the turtle returns to the sea and the offspring do not receive any further parental care. Within a single nesting season, a female can nest between one and 14 times. To this extent, the reproductive output of some sea turtle species can total more than 1000 eggs in a single nesting season. The eggs require 45 to 70 days to develop depending on species and beach temperature. Sea turtles have temperature-dependent sex determination with incubation temperatures during the middle trimester determine the sex of the embryos (Morreale et al., 1982). Females are produced at higher temperatures and males are produced at lower temperatures. The temperature at which an equal percentage of males and females are produced (termed the pivotal temperature) ranges from 28 to 31  C (Ackerman, 1997). The exact genetic mechanism that causes sex differentiation is not well known, but it is suspected to be associated with the Lhx9 gene (Bieser et al., 2013). Temperature has an additional influence on the percentage of eggs within each nest that successfully hatch and leave the nest (Santidria´n Tomillo et al., 2009). Specifically, if a nest exceeds 36  C or drops below 24  C for a prolonged period of time this generally proves fatal to the developing embryos (Ackerman, 1997).

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Figure 4 Olive ridley turtles emerging as part of an arribada on Playa Ostional, Costa Rica. Photo taken by Tera C. Dornfeld.

After nesting, individuals migrate to foraging grounds that may be thousands of kilometers away (Schroeder et al., 2003; Shillinger et al., 2008). Here they will amass the necessary energy reserves to nest in a subsequent season. Due to the energetic costs of migrating and nesting, individuals can take many years to acquire the necessary resources (Wallace et al., 2005). Male turtles conduct similar migrations to female turtles (James et al., 2005), but not having to produce eggs means that the energetic cost of reproduction for male turtles is lower. As a result, male turtles are able to return to their reproductive grounds more regularly than female turtles (Limpus, 1993; Hays et al., 2010). Nesting is typically seasonal, lasting between 2 to 6 months of the year. Nesting behavior is most often solitary, with individuals nesting independently of each other; however, Kemp’s ridley and olive ridley turtles also nest in dense aggregations called arribadas (Spanish for arrival). During an arribada thousands of individual females nest on a single beach over a period of 3 to 10 consecutive days (Figure 4).

Life History and Distribution Nests generally hatch en mass as the pipping movements of each hatchling prompt adjacent turtles to break free from their eggs. Vocalizations between eggs may also be important considering that some other reptiles, such as freshwater turtles and crocodilians, synchronize their hatching through vocalizations (Vergne and Mathevon, 2008; Ferrera et al., 2013). Regardless, no published studies have currently recorded evidence of vocalizing in sea turtle hatchlings. After hatching, the hatchlings dig upwards to the surface of the sand, a journey that can take 2 to 3 days. The temperature of the surface sand influences when the hatchlings emerge from the nest. During the daytime, hatchlings become immobile if they encounter hot sand; however, when temperatures cool at night the hatchlings reanimate (Figure 5). On the surface, the hatchlings rapidly crawl towards the brightest horizon. On dark, unlit beaches this is usually the ocean, illuminated by the reflection of light from the moon and stars. In contrast, on developed beaches, artificial lights that shine out onto the beach can disorientate the hatchlings by competing with natural seafinding cues and hinder the ability of the hatchlings to find the ocean (Tuxbury and Salmon, 2005). The hatchlings that make it to the water swim almost continuously for their first day, in a period called the ‘frenzy’ (Wyneken and Salmon, 1992). Over the next week, swimming activity gradually decreases as the hatchlings progress further out to sea (Salmon and Wyneken, 1987). Eventually the hatchlings will become entrained in open-ocean gyres and they will spend many years drifting within these currents, associating with planktonic communities (Arthur et al., 2008). Open-ocean environments contain fewer predators than coastal areas and are thus considered beneficial development locations for juvenile sea turtles. The distribution of sea turtle nesting beaches may even be strongly linked to locations where the offshore currents readily advect hatchlings away from the beach and out into open water (Putman et al., 2010; Shillinger et al., 2012). Once juvenile turtles grow to about the size of a dinner plate, they typically migrate to tropical or subtropical estuaries, reefs, or other inshore habitats (Gonza´lez Carman et al., 2011; Berube et al., 2012). Juvenile turtles remain in these areas until adulthood, at which point they begin their reproductive migrations. Sea turtles tend to nest on the beaches on which they were born (Bowen et al., 2004; Lohmann et al., 2008). It has been demonstrated that sea turtles are able to navigate using the earth’s magnetic field (Lohmann et al., 2012). This ability together with the use of olfactory cues and chemical imprinting from natal beaches may account for their natal homing ability. It is hypothesized that crystals of biogenic magnetite provide the capacity for sea turtles to sense the Earth’s magnetic field (Irwin and Lohmann, 2005), but this is yet to be confirmed.

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Figure 5 Mass emergence of loggerhead turtle hatchlings head to the water. Photo taken by Nathan J. Robinson.

Adult sea turtles can be found in all the oceans of the world except for the polar oceans as they are too cold. Leatherback and olive ridley turtles are largely pelagic and their foraging movements can encompass entire ocean basins. In contrast, most green, hawksbill, and flatback turtles reside in near-shore habitats and occupy relatively small home-ranges. Loggerhead turtles appear to be able to combine both coastal and pelagic life-history behaviors and often demonstrate multiple migratory strategies within a single nesting population.

Survival Rates and Threats Sea turtles have evolved a life history strategy that includes high mortality as hatchlings, but is offset by high reproductive potential and longevity. This strategy has worked for millions of year, but many sea turtle populations are now in decline due to increasing mortality from anthropogenic sources (Bjorndal et al., 1993; Spotila et al., 2000; Witherington et al., 2009). Nevertheless, there have also been some remarkable recoveries (Dutton et al., 2004; Balazs and Chaloupka, 2004). All sea turtles are currently listed in Appendix I of the Convention on International Trade in Endangered Species of Flora or Fauna (CITES Convention). All species except the flatback turtle are also listed as threatened or endangered the International Union for Conservation of Nature (IUCN) Red List. During the early life-stages, survival rates are at their lowest and only an estimated one in a thousand hatchlings will survive until sexual maturity. Many sea turtle nests are flooded by high tides, washed away by storms, over-heated, harvested by humans, or predated by both native and introduced species such as dogs and raccoons. The hatchlings that survive and successfully leave their nest must then find the ocean. Many hatchlings are predated as they crawl to the water by crabs, birds, dogs, raccoons and varanid lizards. Other individuals become disoriented by artificial lights on the beach and die of dehydration before they reach the ocean. The hatchings that do make it to the ocean are at risk of predation by fish and birds, especially in coastal waters. As hatchlings progress towards the open ocean is it thought that predation rates decrease. Predation rates will further decrease as the turtle begins to grow and the protective shell becomes tougher. Adult sea turtles have few natural predators. On the beach, nesting adults are at risk from dogs, crocodilians and large cats. In the water, adult turtles are only preyed upon by large sharks and killer whales. In many parts of the world, however, humans continue to harvest adult sea turtles for their meat and shells (Humber et al., 2011). Furthermore, large commercial fisheries, such as pelagic long-liners and shrimp trawlers, catch numerous sea turtles as by-catch (Wallace et al., 2010). While the capture of sea turtles is mostly incidental, many individuals are injured or die as a result of swallowing hooks or becoming entangled in nets.

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Recently there is increasing concern about how sea turtles will be impacted by climate change. Rising sea-level will reduce the amount of available nesting area for many populations (Fuentes et al., 2010). In addition, rising temperatures can lead to increasingly female biased sex-ratios, due to temperature-dependent sex determination (Janzen, 1994), and reductions in hatching success (Santidria´n Tomillo et al., 2012; Saba et al., 2012).

Cheloniidae Genus Chelonia: Chelonia mydas (Linnaeus, 1758) the Green Turtle The genus Chelonia contains only one extant species: the green turtle (Figure 6). The green turtles that nest on the shores of the East Pacific Ocean, however, have a unique shell-shape and color. This has led some researchers to separate the green turtle into two separate species or races: the green turtle (Chelonia mydas mydas) and the East Pacific green turtle or black turtle (Chelonia mydas agassizii Bocourt, 1868) (Figure 7). Nevertheless, molecular studies have indicated that both green and black turtles are, at least genetically, the same species (Karl and Bowen, 1999). Green turtles actually have brownish colored carapaces with yellowish plastrons. The name green turtle comes from the large, greenish fat deposit found under the carapace that is the main ingredient in turtle soup. The black turtle is distinguished by the dark black color of both the carapace and plastron in the adult form. The posterior portion of the carapace is also more concave in black turtles and more convex in green turtles. Both green and black turtles have one pair of prefrontal scutes with four pairs of costal scutes. Adult green turtles are between 80 and 120 cm in straight-line carapace length (SCL) and weigh between 65 and 200 kg. Black turtles tend to be smaller at 65 to 90 cm SCL and weighing between 50 and 150 kg.

Figure 6 Adult green turtle. Photo taken by Nathan J. Robinson.

Figure 7 Adult black turtle. Photo taken by Gabriela S. Blanco.

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Both green and black turtles are typically herbivorous. Green turtles often feed on sea grass and marine plants of the genera Zostera, Thallassina, Enhaus, Posidonia, and Halodule, whereas black turtles rely heavily on red algae. However, recent studies have indicated that this species may also supplement their diet with gelatinous prey items (Amorocho and Reina, 2007; Arthur et al., 2007). Green turtles are found in shallow tropical waters and are seasonal visitors to more temperate locations. Significant breeding rookeries for green turtles are found along both Atlantic and Pacific Coasts of Central America, Hawaii, the islands of Western Africa, Ascension Island, the islands of Malaysia, and the islands of northern Australia. A small and highly endangered population of green turtles is also found in the Eastern Mediterranean. Green turtles are the only species of turtle that comes ashore for reasons other than to nest. At locations in Hawaii and the Galapagos, it is not uncommon for male and female green turtles to emerge from the water and remain on the beach for multiple hours during the day. The reason these turtles do this is unknown but it may be to escape predators, raise body temperate through basking, or destroy ectoparasites.

Genus Lepidochelys: Lepidochelys kempii (Garman, 1880) the Kemp’s Ridley Turtle, Lepidochelys olivacea (Eschsholtz, 1829) the Olive Ridley Turtle The genus Lepidochelys is comprised of two species, the Kemp’s ridley (Figure 8) and the olive ridley (Figure 9) turtles. Interestingly, the Kemp’s ridley is the rarest sea turtle whereas the olive ridley is the most abundant. These two species were thought to have diverged with the closure of the Isthmus of Panama about 14 million years ago, a closure that also resulted in the isolation of the Kemp’s ridley turtle within the Gulf of Mexico (Bowen et al., 1991).

Figure 8 Adult Kemp’s ridley turtle. Photo taken by Julianne Koval.

Figure 9 Adult olive ridley turtle. Photo taken by Tera C. Dornfeld.

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Both Kemp’s and olive ridleys have drab-olive colored carapaces and yellowish-white plastrons. The carapaces are highly domed and are generally as wide as they are long. Anatomically ridleys are the smallest sea turtles and the adults of both species grow to between 50 and 80 cm in SCL and weigh between 30 and 50 kg. Kemp’s ridley turtles are carnivores, feeding primarily on swimming crustaceans and mollusks. Olive ridley turtles are omnivorous and have a more varied diet that includes salps (Mettcalfina spp.), algae, jellyfish, fish, benthic invertebrates, mollusks, crustaceans, and bryozoans. Ridleys are the only turtles to nest both solitarily and as part of arribada assemblages. The largest arribadas occur at Orissa, India and at this site over 100 000 olive ridley turtles have been recorded emerging in a single night. The density of nesting means that many turtles dig up eggs that were laid on previous nights. The large quantities of rotting eggs also provide fertile conditions for bacterial growth. The bacteria reduce oxygen concentrations in the sand, elevate temperatures, and infect and kill many eggs. Consequently, the hatching success of arribada nests is often only between 1% and 10% (Valverde et al., 2010). Kemp’s ridley turtles are found in shallow waters in the Gulf of Mexico, although juveniles will also frequent estuaries along the east coast of the USA during the summer months. Only two significant breeding rookeries for Kemp’s ridley turtles exist, Rancho Nuevo, Mexico and South Padre Island, USA, with arribadas only occurring on Rancho Nuevo. Olive ridley turtles are found in warm pelagic waters in the Atlantic, eastern Pacific, and northern Indian Oceans. Significant olive ridley rookeries are found on the Pacific coast of Central America, western Africa, eastern India, and the tropical Atlantic coasts of South America. In the 1950s, the scientific community first discovered the Kemp’s ridley arribadas occurring on Rancho Nuevo. Only shortly afterwards, it was also discovered that this population was in rapid decline. These declines were attributed to wide-scale harvesting of eggs and the large number of adult Kemp’s ridleys killed by shrimping nets in the Gulf of Mexico. Before it was too late, the Mexican government protected the species by law and the USA prohibited the importation of shrimp harvested in a manner that would negatively affect the turtles. The Kemp’s ridley is now recovering although it is still far below historic population sizes.

Genus Eretmochelys: Eretmochelys imbricata (Linnaeus, 1766), the Hawksbill Turtle The hawksbill turtle gets its name from its narrow, elongated jaw that resembles the beak of a raptor (Figure 10). The carapace of a hawksbill turtle is mottled black, brown, and yellow, while the plastron is bright yellow to beige. Adult hawksbill turtles have highly variable shell lengths ranging from 50 to 90 cm in SCL and weigh between 40 and 80 kg as adults. The sharp beak of the hawksbill is perfectly adapted to a diet of sponges. Hawksbill turtles are one of only a few spongivorous animals and electron micrographs of their gastrointestinal tract have shown microvilli with millions of silica spicules imbedded in the tissue. Tunicates, sea anemone, bryozoans, mollusks, and marine plants are also important components of a hawksbill’s diet (Witzell, 1983). Hawksbill turtles are generally found in tropical, shallow-water coral reefs. Significant breeding rookeries for hawksbill turtles are found in the Caribbean, equatorial Brazil, Southeast Asia, and the western Indian Ocean. Small populations of hawksbills have also been recently discovered in the Eastern Pacific Ocean. Interestingly, these Eastern Pacific Ocean hawksbills are more commonly found associated with mangroves than coral reefs (Gaos et al., 2011). Hawksbill turtles have been subject to intense harvesting worldwide for the beauty of their carapace. Historically eyeglass frames and hair combs were made from their shell scutes as ‘tortoiseshell’. Hawksbill trinkets can still be found for sale in many parts of the world. Taxidermied adults or shells are also prized in Asian cultures as symbols of longevity.

Figure 10 Adult hawksbill turtle. Photo taken by Nathan J. Robinson.

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Genus Caretta: Caretta caretta (Linnaeus, 1759): the Loggerhead Turtle Loggerhead turtles (Figure 11) have a dark-brown to maroon carapace and yellow plastrons. They are named loggerhead because they have the largest head and jaw, relative to body size, of all sea turtles. Adult loggerhead turtles are between 70 and 110 cm in SCL and weigh between 60 and 200 kg. Loggerheads are the most omnivorous of all the sea turtles. Their large head and powerful beak enables them to eat many hardshelled invertebrates such as mollusks and crabs. Their diet can also includes tubeworms, sea pens, fish, algae, whip corals, sea anemones, barnacles and shrimp. Their habitats appear to be quite diverse, reflecting their varied diets, and they will shift between deeper continental shelf areas and shallow river estuaries and lagoons. Significant breeding rookeries for loggerhead turtles are found on the east and west coasts of the USA, eastern and western Africa, eastern Mediterranean, east Asia, and Australia. Loggerhead turtles nest in the most temperate locations of all sea turtles. Loggerhead turtles conduct one of the longest migrations in the animal kingdom. Loggerhead turtles hatched on the beaches of Japan follow the North Pacific Gyre across the Pacific Ocean to juvenile foraging grounds along the Mexican coastline. After reaching sexual maturity, these turtles then migrate over 10 000 km back across the Pacific Ocean to back to their nesting beaches in Japan (Nichols et al., 2000).

Genus Natator: Natator depressus (Garman, 1880) the Flatback Turtle Flatback turtles (Figure 12) have a low-domed carapace with an upturned rim at the posterior of the animal. The carapace is a dull olive-grey and the plastron is a pale-cream. The shell scutes of flatback turtles are oily and relatively thin. Adult flatback turtles are between 80 and 100 cm in CCL and weigh between 60 and 100 kg. Flatbacks are omnivorous and feed on a large variety of mainly soft-bodied prey including sea grasses, mollusks, jellyfish, crabs, shrimp, fish, soft corals, and sea cucumbers.

Figure 11 Adult loggerhead turtle. Photo taken by Nathan J. Robinson.

Figure 12 Adult flatback turtle. Photo taken by Liz Sim.

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Figure 13 Adult leatherback turtle. Photo taken by Frank V. Paladino.

The flatback turtle has a very limited distribution and is only found in the shallow, protected coastal waters off Northern Australia and Gulf of Carpentaria. Significant breeding rookeries are similarly found along the northern coasts of Australia. The flatback turtle is the only sea turtle that does not spend its juvenile years in the open ocean (Walker and Parmenter, 1990). Due to the remoteness of their nesting grounds, a thorough population assessment for this species has not yet been possible; however, many of the nesting beaches for this species are located near large oil extraction or refinement facilities. The impact that such industry might be having on the nesting flatback populations is currently of significant interest.

Dermochelyidae Genus Dermochelys: Dermochelys coriacea (Vandelli, 1761) the Leatherback Turtle Leatherback turtles (Figure 13) are the largest and most visually distinct of the extant sea turtle species. They do not have the characteristic ‘hard-shell’ of the Cheloniidae and instead their shell consists of cartilaginous osteoderms beneath a layer of leathery skin. Seven cartilaginous ridges extend along the anterior-posterior axis of the carapace. These facilitate laminar flow of water over the body of the turtle and thus decreasing drag. Leatherbacks’ coloration is a distinctive black or dark blue with white spots and a white plastron. As adults they range from 130 to 190 cm in CCL and can weigh between 200 and 1000 kg. Leatherbacks are obligate carnivores and only feed on soft-bodied, gelatinous invertebrates such as cnidarians, ctenophores, and salps. Their mouths even have two saber tooth-like projections on the upper beak that function to pop the air bladders of floating cnidarians and colonial medusas. Leatherback turtles primarily occupy pelagic environments. Their foraging movements are often focused toward prey aggregations that are typically located along dynamic oceanographic features, such as fronts and up-welling zones, that lead to large food aggregation (Shillinger et al., 2008). The migratory patterns of leatherback turtles can be likened to prolonged sojourns in vast feeding areas that can encompass entire ocean basins. Significant breeding rookeries are found on the Pacific and Atlantic sides of Central America, Papua, eastern and western Africa, Sri Lanka, and the Guyanas. Adult leatherback turtles have the largest range of any reptile. Remarkably they have been found in water temperatures of 7 to 10  C while maintaining a core body temperature above 20  C (Frair et al., 1972). Enabling leatherback turtles to maintain elevated body temperatures is an array of adaptations termed gigantothermy (Paladino et al., 1990). These include their large rotund body-shape, a thick layer of fat, and counter-current heat exchange in their circulatory system. Leatherback turtles also make use of a high activity rate to generate muscle-derived heat.

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Further Reading Bustard R (1973) Sea turtles: Natural history and conservation. New York: Taplinger Publishing. Carr A (1986) The sea turtle: So excellent a fishe. Austin, TX: University of Texas Press. Carr A (1991) Handbook of turtles. Ithaca, NY: Comstock Publishing Associates of Cornell University Press. Lutz PL and Musick J (1997) The biology of sea turtlesI:Boca Raton, FL: CRC Press. Lutz PL, Musick J, and Wyneken J (2002) The Biology of Sea Turtles, Vol II. Boca Raton, FL: CRC Press. Seminoff JA and Wallace BP (2012) Sea Turtles of the eastern Pacific: Advances in research and conservation. Tucson, AC: University of Arizona Press. Spotila JR (2004) Sea turtles: A complete guide to their biology, behavior, and conservation. Balitmore, MD: The John Hopkins University Press. Wyneken, J. (2001). The Anatomy of Sea Turtles. U.S. Department of Commerce NOAA Technical Memorandum NMFS-SEFSC-470, pp 1–172 Wyneken J, Lohmann KJ, and Musick JA (2013) The biology of sea turtles, III:Boca Raton, FL: CRC Press.

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