THE LOGGERDEAD SEA TURTLES

The loggerhead sea turtle (Caretta caretta), or loggerhead, is an oceanic turtle distributed throughout the world. It is a marine reptile, belonging to the family Cheloniidae. The average loggerhead measures around 90 cm (35 in) long when fully grown, although larger specimens of up to 280 cm (110 in) have been discovered. The adult loggerhead sea turtle weighs approximately 135 kg (298 lb), with the largest specimens weighing in at more than 450 kg (1,000 lb). The skin ranges from yellow to brown in color, and the shell is typically reddish-brown. No external differences in gender are seen until the turtle becomes an adult, the most obvious difference being the adult males have thicker tails and shorter plastrons than the females.
The loggerhead sea turtle is found in the Atlantic, Pacific, and Indian Oceans, as well as the Mediterranean Sea. It spends most of its life in saltwater and estuarine habitats, with females briefly coming ashore to lay eggs. The loggerhead sea turtle has a low reproductive rate; females lay an average of four egg clutches and then become quiescent, producing no eggs for two to three years. The loggerhead reaches sexual maturity within 17–33 years and has a lifespan of 47–67 years.^[6]
The loggerhead sea turtle is omnivorous, feeding mainly on bottom-dwelling invertebrates. Its large and powerful jaws serve as an effective tool for dismantling its prey. Young loggerheads are exploited by numerous predators; the eggs are especially vulnerable to terrestrial organisms. Once the turtles reach adulthood, their formidable size limits predation to large marine animals, such as sharks.
Loggerheads are considered an endangered species and are protected by the International Union for the Conservation of Nature. Untended fishing gear is responsible for many loggerhead deaths. Turtles may also suffocate if they are trapped in fishing trawls. Turtle excluder devices have been implemented in efforts to reduce mortality by providing an escape route for the turtles. Loss of suitable nesting beaches and the introduction of exotic predators have also taken a toll on loggerhead populations. Efforts to restore their numbers will require international cooperation, since the turtles roam vast areas of ocean and critical nesting beaches are scattered across several countries.

 

 

 

 

Description

The carapace of this loggerhead is reddish-brown; five vertebral scutes run down the turtle's midline bordered by five pairs of costal scutes.

The loggerhead sea turtle is the world's largest hard-shelled turtle.^[7] Adults have an average weight range of 80 to 200 kg (180 to 440 lb) and a length range of 70 to 95 cm (28 to 37 in).^[7] The maximum reported weight is 545 kg (1,202 lb) and the maximum carapace length is 213 cm (84 in).^[7] The head and carapace (upper shell) range from a yellow-orange to a reddish-brown, while the plastron (underside) is typically pale yellow.^[8] The turtle's neck and sides are brown on the tops and yellow on the sides and bottom.^[9]
The turtle's shell is divided into two sections: carapace and plastron. The carapace is further divided into large plates, or scutes.^[8] Typically, 11 or 12 pairs of marginal scutes rim the carapace.^[10] Five vertebral scutes run down the carapace's midline, while five pairs of costal scutes border them.^[11] The nuchal scute is located at the base of the head.^[11] The carapace connects to the plastron by three pairs of inframarginal scutes forming the bridge of the shell.^[11] The plastron features paired gular, humeral, pectoral, abdominal, femoral, and anal scutes.^[10] The shell serves as external armor, although loggerhead sea turtles cannot retract their heads or flippers into their shells.^[12]
Sexual dimorphism of the loggerhead sea turtle is only apparent in adults. Adult males have longer tails and claws than females. The males' plastrons are shorter than the females', presumably to accommodate the males' larger tails. The carapaces of males are wider and less domed than the females', and males typically have wider heads than females.^[13] The sex of juveniles and subadults cannot be determined through external anatomy, but can be observed through dissection, laparoscopy (an operation performed on the abdomen), histological examination (cell anatomy), and radioimmunological assays (immune study dealing with radiolabeling).^[13]
Lachrymal glands located behind each eye allow the loggerhead to maintain osmotic balance by eliminating the excess salt obtained from ingesting ocean water. On land, the excretion of excess salt gives the false impression that the turtle is crying.^[14]

Distribution

Range of the loggerhead sea turtle

The loggerhead sea turtle has a cosmopolitan distribution, nesting over the broadest geographical range of any sea turtle. It inhabits the Atlantic, Indian, and Pacific Oceans and the Mediterranean Sea.^[15]
In the Atlantic Ocean, the greatest concentration of loggerheads is along the southeastern coast of North America and in the Gulf of Mexico. Very few loggerheads are found along the European and African coastlines.^[16] Florida is the most popular nesting site, with more than 67,000 nests built per year. Nesting extends as far north as Virginia, as far south as Brazil, and as far east as the Cape Verde Islands. The Cape Verde Islands are the only significant nesting site on the eastern side of the Atlantic. Loggerheads found in the Atlantic Ocean feed from Canada to Brazil.^[15]
In the Indian Ocean, loggerheads feed along the coastlines of Africa, the Arabian Peninsula, and in the Arabian Sea.^[17] Along the African coastline, loggerheads nest from Mozambique's Bazaruto Archipelago to South Africa's St Lucia estuary.^[18] The largest Indian Ocean nesting site is Oman, on the Arabian Peninsula, which hosts around 15,000 nests, giving it the second largest nesting population of loggerheads in the world. Western Australia is another notable nesting area, with 1,000–2,000 nests per year.^[17]
Pacific loggerheads live in temperate to tropical regions.^[18] They forage in the East China Sea, the southwestern Pacific, and along the Baja California Peninsula. Eastern Australia and Japan are the major nesting areas, with the Great Barrier Reef deemed an important nesting area.^[19] Pacific loggerheads occasionally nest in Vanuatu and Tokelau. Yakushima Island is the most important site, with three nesting grounds visited by 40% of all nearby loggerheads.^[17] After nesting, females often find homes in the East China Sea, while the Kuroshio Current Extension's Bifurcation region provides important juvenile foraging areas.^[18] Eastern Pacific populations are concentrated off the coast of Baja California, where upwelling provides rich feeding grounds for juvenile turtles and subadults. Nesting sites along the eastern Pacific Basin are rare. mtDNA sequence polymorphism analysis and tracking studies suggest 95% of the population along the coast of the Americas hatch on the Japanese Islands in the western Pacific.^[20] The turtles are transported by the prevailing currents across the full length of the northern Pacific, one of the longest migration routes of any marine animal.^[20] The return journey to the natal beaches in Japan has been long suspected, although the trip would cross unproductive clear water with few feeding opportunities.^[21] Evidence of a return journey came from an adult female loggerhead named Adelita, which in 1996, equipped with a satellite tracking device, made the 14500-km (9000-mi) trip from Mexico across the Pacific. Adelita was the first animal of any kind ever tracked across an ocean basin.^[22]
The Mediterranean Sea is a nursery for juveniles, as well as a common place for adults in the spring and summer months.^[16][23] Almost 45% of the Mediterranean juvenile population has migrated from the Atlantic.^[16] Loggerheads feed in the Alboran Sea and the Adriatic Sea.^[16] Greece is the most popular nesting site along the Mediterranean, with more than 3,000 nests per year.^[17] Because of this, Greek authorities do not allow planes to take off or land at night in Zakynthos due to the nesting turtles.^[24] In addition to the Greek coast, the coastlines of Cyprus and Turkey are also common nesting sites.^[17]
One record of this turtle was made in Ireland washed ashore on Ballyhealy Beach in Co. Wexford.^[25]

Habitat

 

 Loggerhead sea turtles spend most of their lives in the open ocean and in shallow coastal waters. They rarely come ashore, except for the females' brief visits to construct nests and deposit eggs. Hatchling loggerhead turtles live in floating mats of Sargassum algae.^[26] Adults and juveniles live along the continental shelf, as well as in shallow coastal estuaries.^[27] In the northwestern Atlantic Ocean, age plays a factor in habitat preference. Juveniles are more frequently found in shallow estuarine habitats with limited ocean access compared to non-nesting adults.^[28] Loggerheads occupy waters with surface temperatures ranging from 13.3-28.0°C (56-82°F) during non-nesting season. Temperatures from 27-28°C are most suitable for nesting females.^[29]
Juvenile loggerheads share the Sargassum habitat with a variety of other organisms. The mats of Sargassum contain as many as 100 different species of animals on which the juveniles feed. Some of the prey, such as ants, flies, aphids, leafhoppers, and beetles, are carried by the wind to these areas. Endemic prey of the mats include barnacles, small crab larvae, fish eggs, and hydrozoan colonies.^[26] Marine mammals and commercial fish, such as tuna, dolphin fish, and amberjacks, also inhabit the mats.^[30]

Ecology and behavior

A resting loggerhead sea turtle

Loggerhead sea turtles observed in captivity and in the wild are most active during the day. In captivity, the loggerheads' daily activities are divided between swimming and resting on the bottom. While resting, they spread their forelimbs to about midstroke swimming position. They remain motionless with eyes open or half-shut and are easily alerted during this state. At night, captives sleep in the same position with their eyes tightly shut, and are slow to react.^[29] Loggerheads spend up to 85% of their day submerged, with males being the more active divers than females. The average duration of dives is 15–30 min, but they can stay submerged for up to four hours.^[31] Juvenile loggerheads and adults differ in their swimming methods. A juveniles keeps its forelimbs pressed to the side of its carapace, and propels itself by kicking with its hind limbs. As the juvenile matures, its swimming method is progressively replaced with the adult's alternating-limb method. They depend entirely on this method of swimming by one year old.^[32]
Water temperature affects the sea turtle's metabolic rate.^[29] Lethargy is induced at temperatures between 13 and 15°C (55 and 59°F). The loggerhead takes on a floating, cold-stunned posture when temperatures drop to around 10°C.^[29] However, younger loggerheads are more resistant to cold and do not become stunned until temperatures drop below 9°C. The loggerheads' migration helps to prevent instances of cold-stunning.^[33] Higher water temperatures cause an increase in metabolism and heart rate. A loggerhead's body temperature increases in warmer waters more quickly than it decreases in colder water; their critical thermal maximum is currently unknown.^[33]
Female-female aggression, which is fairly rare in other marine vertebrates, is common among loggerheads. Ritualized aggression escalates from passive threat displays to combat. This conflict primarily occurs over access to feeding grounds. Escalation typically follows four steps.^[34] First, initial contact is stimulated by visual or tactile cues. Second, confrontation occurs, beginning with passive confrontations characterized by wide head-tail circling. They begin aggressive confrontation when one turtle ceases to circle and directly faces the other. Third, sparring occurs with turtles snapping at each other’s jaws. The final stage, separation, is either mutual, with both turtles swimming away in opposite directions, or involves chasing one out of the immediate vicinity.^[34] Escalation is determined by several factors, including hormone levels, energy expenditure, expected outcome, and importance of location. At all stages, an upright tail shows willingness to escalate, while a curled tail shows willingness to submit. Because higher aggression is metabolically costly and potentially debilitating, contact is much more likely to escalate when the conflict is over access to good foraging grounds.^[34] Further aggression has also been reported in captive loggerheads. The turtles are seemingly territorial, and will fight with other loggerheads and sea turtles of different species.^[31]

Feeding

An adult Aurelia jellyfish which loggerheads eat during migration through the open sea

The loggerhead sea turtle is omnivorous, feeding mainly on bottom-dwelling invertebrates, such as gastropods, bivalves, and decapods. It has a greater list of known prey than any other sea turtle. Other food items include sponges, corals, sea pens, polychaete worms, sea anemones, cephalopods, barnacles, brachiopods, isopods, insects, bryozoans, sea urchins, sand dollars, sea cucumbers, starfish, fish (eggs, juveniles, and adults), hatchling turtles (including members of its own species), algae, and vascular plants.^[35] During migration through the open sea, loggerheads eat jellyfish, floating molluscs, floating egg clusters, squid, and flying fish.^[9]
Loggerheads crush prey with their large and powerful jaws.^[9][36] Projecting scale points on the anterior margin of the forelimbs allow manipulation of the food. These points can be used as "pseudo-claws" to tear large pieces of food in the loggerhead's mouth. The loggerhead will turn its neck sideways to consume the torn food on the scale points.^[36] Inward-pointing, mucus-covered papillae found in the fore region of the loggerhead's esophagus filter out foreign bodies, such as fish hooks. The next region of the esophagus is not papillated, with numerous mucosal folds. The digestion rate in loggerheads is temperature-dependent; it increases as temperature increases.^[36]

Predators

A horned ghost crab (Ocypode ceratophthalma) preying on a loggerhead hatchling in Gnaraloo, Western Australia. Ghost crabs are one of the chief causes of egg and hatchling mortality in sea turtles.^[37][38][39]

The red fox is a predator of loggerhead nests in Australia.

Loggerheads have numerous predators, especially early in their lives. Egg and nestling predators include ghost crabs, oligochaete worms, beetles, fly larvae, ants, parasitoid wasp larvae, flesh flies, snakes, gulls, corvids, opossums, bears, rats, armadillos, mustelids, skunks, canids, procyonids, cats, pigs, and humans. During their migration from their nests to the sea, hatchlings are preyed on by dipteran larvae, crabs, toads, lizards, snakes, seabirds such as frigatebirds, and other assorted birds and mammals. In the ocean, predators of the loggerhead juveniles include fish, such as parrotfish and moray eels, and portunid crabs. Adults are more rarely attacked due to their large size, but may be preyed on by large sharks, seals, and killer whales. Nesting females are attacked by flesh flies, feral dogs, and humans. Salt marsh mosquitos can also pester nesting females.^[36][40]
In Australia, the introduction of the red fox (Vulpes vulpes) by British settlers in the 19th century led to significant reductions in loggerhead sea turtle populations. In one coastal section in eastern Australia during the 1970s, predation of turtle eggs destroyed up to 95% of all clutches laid.^[41] Aggressive efforts to destroy foxes in the 1980s and 1990s has reduced this impact; however, it is estimated that it will be the year 2020 before populations will experience complete recovery from such dramatic losses.^[42]
Along the southeastern coast of the United States, the raccoon (Procyon lotor) is the most destructive predator of nesting sites. Mortality rates of nearly 100% of all clutches laid in a season have been recorded on some Florida beaches.^[41] This is attributed to an increase in raccoon populations, which have flourished in urban environments. Aggressive efforts to protect nesting sites by covering them with wire mesh has significantly reduced the impact of raccoon predation on loggerhead sea turtle eggs.^[42] On Bald Head Island in North Carolina, wire mesh screens are used on every confirmed nest to prevent excavation by resident red foxes.^[43] A new concern with the steel cage technique is interference with the normal development of the nestlings' magnetic sense due to the use of ferrous wire,^[44] which may disrupt the turtles' ability to navigate properly. Efforts are underway to find a nonmagnetic material that will prevent predators gnawing through the barrier.
Up to 40% of nesting females around the world have wounds believed to come from shark attacks.^[40]

Disease and parasites

Infectious bacteria such as Pseudomonas and Salmonella attack loggerhead hatchlings and eggs. Fungi such as Penicillium infect loggerhead sea turtle nests and cloacae.^[40]
Fibropapillomatosis disease caused by a form of the herpes-type virus threatens loggerheads with internal and external tumors. These tumors disrupt essential behaviors and, if on the eyes, cause permanent blindness.^[45] Trematodes of the family Spirorchiidae inhabit tissues throughout the body of the loggerhead, including vital organs, such as the heart and the brain.^[46] Trematode infection can be highly debilitating. For example, inflammatory trematode lesions can cause endocarditis and neurological disease.^[46] A nematode, Angiostoma carettae, also infects loggerheads,^[47] causing histologic lesions in the respiratory tract.^[47]
More than 100 species of animals from 13 phyla, as well as 37 kinds of algae, live on loggerheads' backs.^[48] These parasitic organisms, which increase drag, offer no known benefit to the turtle, although the dulling effect of organisms on shell color may improve camouflage.^[48]

Life history

Early life

A hatchling loggerhead sea turtle on its journey to the ocean

Hatchlings range in color from light brown to almost black, lacking the adult's distinct yellows and reds.^[8] Upon hatching, they measure about 4.6 cm (1.8 in) and weigh about 20 g (0.7 oz).^[9] The eggs are typically laid on the beach in an area above the high-tide line. The eggs are laid near the water so the hatchlings can return to the sea.^[49] The loggerhead's sex is dictated by the temperature of the underground nest. Incubation temperatures generally range from 26-32°C (79-90°F). Sea turtle eggs kept at a constant incubating temperature of 32°C become females. Eggs incubating at 28°C become males. An incubation temperature of 30°C results in an equal ratio of male to female hatchlings.^[50] Hatchlings from eggs in the middle of the clutch tend to be the largest, grow the fastest, and be the most active during the first few days of sea life.^[41]
After incubating for around 80 days, hatchlings dig through the sand to the surface,usually occurs at night, when darkness increases the chance of escaping predation and damage from extreme sand surface temperatures is reduced.^[49] Hatchlings enter the ocean by navigating toward the brighter horizon created by the reflection of the moon and starlight off the water's surface.^[51]
Hatchlings can lose up to 20% of their body mass due to evaporation of water as they journey from nest to ocean.^[52] They initially use the undertow to push them five to 10 m away from the shore.^[52] Once in the ocean, they swim for about 20 hours, bringing them far offshore.^[8] An iron compound, magnetite, in their brains allows the turtles to perceive the Earth's magnetic field,^[53] for navigation. Many hatchlings use Sargassum in the open ocean as protection until they reach 45 cm (18 in).^[8] Hatchling loggerheads live in this pelagic environment until they reach juvenile age, and then they migrate to nearshore waters.^[8]

Maturation

A mature loggerhead sea turtle

When ocean waters cool, loggerheads must migrate to warmer areas or hibernate to some degree. In the coldest months, they submerge for up to seven hours at a time, emerging for only seven minutes to breathe. Although outdone by freshwater turtles, these are among the longest recorded dives for any air-breathing marine vertebrate.^[54] During their seasonal migration, juvenile loggerheads have the ability to use both magnetic and visual cues.^[55] When both aids are available, they are used in conjunction; if one aid is not available, the other suffices.^[55] The turtles swim at about 1.6 km/h (0.9 kn; 0.4 m/s) during migration.^[56]
Like all marine turtles, the loggerhead prepares for reproduction in its foraging area. This takes place several years before the loggerhead migrates to a mating area.^[57] Female loggerheads first reproduce at ages 28–33 in Southeastern United States and Australia, and at ages 17–30 in South Africa. Age at first reproduction in the Mediterranean, Oman, Japan, and Brazil are unknown.^[58] Nesting loggerheads have a straight carapace length of 70–109 cm (28–43 in). Because of the large range, carapace length is not a reliable indicator of sexual maturity.^[59] Their estimated maximum lifespan is 47–67 years in the wild.^[35]

Reproduction

Loggerhead turtle track on a beach

A loggerhead sea turtle laying eggs

Female loggerheads first reproduce between the ages of 17 and 33,^[58] and their mating period may last more than six weeks.^[57] They court their mates, but these behaviors have not been thoroughly examined.^[60] Male forms of courtship behavior include nuzzling, biting, and head and flipper movements.^[60] Studies suggest females produce cloacal pheromones to indicate reproductive ability.^[60] Before mating, the male approaches a female and attempts to mount her, while she resists. Next, the male and female begin to circle each other. If the male has competitors, the female may let the males struggle with each other. The winner then mounts the female; the male's curved claws usually damage the shoulders of the female's shell during this process. Other courting males bite the male while he is attempting to copulate, damaging his flippers and tail, possibly exposing bones. Such damage can cause the male to dismount and may require weeks to heal.^[60] While nesting, females produce an average of 3.9 egg clutches, and then become quiescent, producing no eggs for two to three years.^[57][61] Unlike other sea turtles, courtship and mating usually do not take place near the nesting beach, but rather along migration routes between feeding and breeding grounds.^[60] Recent evidence indicates ovulation in loggerheads is mating-induced.^[62] Through the act of mating, the female ovulates eggs which are fertilized by the male. This is unique, as mating-induced ovulation is rare outside of mammals.^[62] In the Northern Hemisphere, loggerheads mate from late March to early June. The nesting season is short, between May and August in the Northern Hemisphere and between October and March in the Southern Hemisphere.^[59]
Loggerheads may display multiple paternity.^[63] Multiple paternity is possible due to sperm storage. The female can store sperm from multiple males in her oviducts until ovulation.^[64] A single clutch may have as many as five fathers, each contributing sperm to a portion of the clutch.^[63] Multiple paternity and female size are positively correlated.^[63] Two hypotheses explain this correlation. One posits that males favor large females because of their perceived higher fecundity (ability to reproduce).^[63] The other states, because larger females are able to swim more quickly to mating grounds, they have longer mating periods.^[63]
All sea turtles have similar basic nesting behaviors. Females return to lay eggs at intervals of 12–17 days during the nesting season, on or near the beach where they hatched.^[60][61] They exit the water, climb the beach, and scrape away the surface sand to form a body pit. With their hind limbs, they excavate an egg chamber in which the eggs are deposited. The females then cover the egg chamber and body pit with sand, and finally return to the sea.^[65] This process takes one to two hours, and occurs in open sand areas or on top of sand dunes.^[61] The nesting area must be selected carefully because it affects characteristics such as fitness, emergence ratio, and vulnerability to nest predators.^[49] Loggerheads have an average clutch size of 112.4 eggs.^[66]

Taxonomy

Carolus Linnaeus gave the loggerhead its first binomial name, Testudo caretta, in 1758.^[4][10] Thirty-five other names emerged over the following two centuries, with the combination Caretta caretta first introduced in 1902 by Leonhard Stejneger.^[5] The English common name "loggerhead" refers to the animal's large head.^[9][67] The loggerhead sea turtle belongs to the family Cheloniidae, which includes all sea turtles except the leatherback sea turtle.^[68] The subspecific classification of the loggerhead sea turtle is debated, but most authors consider it a single polymorphic species.^[69] Molecular genetics has confirmed hybridization of the loggerhead sea turtle with the Kemp's ridley sea turtle, hawksbill sea turtle, and green sea turtles. The extent of natural hybridization is not yet determined; however, second-generation hybrids have been reported, suggesting some hybrids are fertile.^[70]

Evolutionary history

Although evidence is lacking,^[71] modern sea turtles probably descended from a single common ancestor during the Cretaceous period. Like all other sea turtles except the leatherback, loggerheads are members of the ancient family Cheloniidae, and appeared about 40 million years ago.^[1] Of the six species of living Cheloniidae, loggerheads are more closely related to the Kemp's ridley sea turtle, olive ridley sea turtle, and the hawksbill turtle than they are to the flatback turtle and the green turtle.
Around three million years ago, during the Pliocene epoch, Central America emerged from the sea, effectively cutting off currents between the Atlantic and Indo-Pacific Oceans. The rerouting of ocean currents led to climatic changes as the Earth entered a glacial cycle. Cold water upwelling around the Cape of Good Hope and reduction in water temperature at Cape Horn formed coldwater barriers to migrating turtles. The result was a complete isolation of the Atlantic and Pacific populations of loggerheads.^[72] During the most recent ice age, the beaches of southeastern North America were too cold for sea turtle eggs. As the Earth began to warm, loggerheads moved farther north, colonizing the northern beaches. Because of this, turtles nesting between North Carolina and northern Florida represent a different genetic population from those in southern Florida.^[72]
The distinct populations of loggerheads have unique characteristics and genetic differences. For example, Mediterranean loggerheads are smaller, on average, than Atlantic Ocean loggerheads.^[17] North Atlantic and Mediterranean loggerhead sea turtles are descendants of colonizing loggerheads from Tongaland, South Africa. South African loggerhead genes are still present in these populations today.^[72]

Conservation

Many human activities have negative effects on loggerhead sea turtle populations. The prolonged time required for loggerheads to reach sexual maturity and the high mortality rates of eggs and young turtles from natural phenomena compound the problems of population reduction as a consequence of human activities.^[73]

Threats

Loggerhead sea turtle nest roped off as part of the Sea Turtle Protection Project on Hilton Head Island

Loggerhead sea turtles were once intensively hunted for their meat and eggs; consumption has decreased, however, due to worldwide legislation. Despite this, turtle meat and eggs are still consumed in countries where regulations are not strictly enforced.^[74] In Mexico, turtle eggs are a common meal; locals claim the egg is an aphrodisiac.^[75] Eating turtle eggs or meat can cause serious illness due to harmful bacteria, such as Pseudomonas aeruginosa and Serratia marcescens, and high levels of toxic metals that build up through bioaccumulation.^[74][76]
The US West Coast is a critical migratory corridor for the Pacific loggerheads, in which these turtles swim across the Pacific to California’s coast from breeding grounds in Japan. Important foraging habitats for juveniles in the central North Pacific have been revealed through telemetry studies.^[77] Along with these foraging habitats, high levels of bycatch from industrial-scale fisheries have been found to overlap; with drift gillnets in the past and longline fisheries presently.^[77] Many juvenile loggerheads aggregate off the coast of Baja California Sur, Mexico, where small coastal fisheries increase these turtles' mortality risk; fishers have reported catching dozens of loggerheads with bottom-set gear per day per boat.^[77] The most common commercial fishery that accidentally takes loggerheads are bottom trawls used for shrimp vessels in the Gulf of California.^[78] In 2000, between 2,600 and 6,000 loggerheads were estimated to have been killed by pelagic longlining in the Pacific.^[79]
Fishing gear is the biggest threat to loggerheads in the open ocean. They often become entangled in longlines or gillnets. According to the 2009 status review of loggerheads by the Fisheries Service, drowning from entanglement in longline and gillnet fishing gear is the turtles’ primary threat in the North Pacific.^[78] They also become stuck in traps, pots, trawls, and dredges.^[9] Caught in this unattended equipment, loggerheads risk serious injury or drowning. Turtle excluder devices for nets and other traps reduce the number being accidentally caught.
Nearly 24,000 metric tons of plastic are dumped into the ocean each year. Turtles ingest a wide array of this floating debris, including bags, sheets, pellets, balloons and abandoned fishing line.^[80] Loggerheads may mistake the floating plastic for jellyfish, a common food item. The ingested plastic causes numerous health concerns, including intestinal blockage, reduced nutrient absorption and malnutrition, suffocation, ulcerations, or starvation. Ingested plastics release toxic compounds, including polychlorinated biphenyls, which may accumulate in internal tissues. Such toxins may lead to a thinning of eggshells, tissue damage, or deviation from natural behaviors.^[81]
Artificial lighting discourages nesting and interferes with the hatchlings' ability to navigate to the water's edge. Females prefer nesting on beaches free of artificial lighting. On developed beaches, nests are often clustered around tall buildings, perhaps because they block out the man-made light sources.^[49] Loggerhead hatchlings are drawn toward the brighter area over the water which is the consequence of the reflection of moon and star light. Confused by the brighter artificial light, they navigate inland, away from the protective waters, which exposes them to dehydration and predation as the sun rises.^[51] Artificial lighting causes tens of thousands of hatchling deaths per year.^[82]
Destruction and encroachment of habitat by humans is another threat to loggerhead sea turtles. Optimum nesting beaches are open-sand beaches above the high-tide line. However, beach development deprives them of suitable nesting areas, forcing them to nest closer to the surf.^[61] Urbanization often leads to the siltation of sandy beaches, decreasing their viability.^[61] Construction of docks and marinas can destroy near-shore habitats. Boat traffic and dredging degrades habitat and can also injure or kill turtles when boats collide with turtles at or near the surface.^[45]
Annual variations in climatic temperatures can affect sex ratios, since loggerheads have temperature-dependent sex determination. High sand temperatures may skew gender ratios in favor of females. Nesting sites exposed to unseasonably warm temperatures over a three-year period produced 87–99% females.^[83] This raises concern over the connection between rapid global temperature changes and the possibility of population extinction.^[84] A more localized effect on gender skewing comes from the construction of tall buildings, which reduce sun exposure, lowering the average sand temperature, which results in a shift in gender ratios to favor the emergence of male turtles.^[61]

Conservation efforts

Loggerhead sea turtle escapes from fishing net through a turtle excluder device

Since the loggerhead occupies such a broad range, successful conservation requires efforts from multiple countries.^[9]
Loggerhead sea turtles are classified as endangered by the International Union for the Conservation of Nature and are listed under Appendix I of the Convention on International Trade in Endangered Species, making international trade illegal.^[9] In the United States, the Fish and Wildlife Service and National Marine Fisheries Service classify them as a threatened species under the Endangered Species Act.^[9] Loggerheads are listed as endangered under both Australia's Environment Protection and Biodiversity Conservation Act 1999 and Queensland's Nature Conservation Act 1992. The Convention on Migratory Species works for the conservation of loggerhead sea turtles on the Atlantic coast of Africa, as well as in the Indian Ocean and southeast Asia.^[85][86] Throughout Japan, the Sea Turtle Association of Japan aids in the conservation of loggerhead sea turtles.^[87] Greece's ARCHELON works for their conservation.^[88] The Marine Research Foundation works for loggerhead conservation in Oman.^[89] Annex 2 of the Specially Protected Areas and Wildlife Protocol of the Cartagena Convention, which deals with pollution that could harm marine ecosystems, also protects them.^[9][90] Conservation organizations worldwide have worked with the shrimp trawling industry to develop turtle exclusion devices (TEDs) to exclude even the largest turtles. TEDs are mandatory for all shrimp trawlers.^[9]

A loggerhead sea turtle, Evelyn, is equipped with a satellite tag and a SeaTag-GEO solar-powered geomagnetic tag

In many places during the nesting season, workers and volunteers search the coastline for nests,^[91] and researchers may also go out during the evening to look for nesting females for tagging studies and gather barnacles and tissues samples. Volunteers may, if necessary, relocate the nests for protection from threats, such as high spring tides and predators, and monitor the nests daily for disturbances. After the eggs hatch, volunteers uncover and tally hatched eggs, undeveloped eggs, and dead hatchlings. Any remaining live hatchlings are released or taken to research facilities. Typically, those that lack the vitality to hatch and climb to the surface die.^[92] Hatchlings use the journey from nest to ocean to build strength for the coming swim. Helping them to reach the ocean bypasses this strength-building exercise and lowers their chances of survival.^[93]

Symbols

The loggerhead sea turtle is the official state reptile of South Carolina and also the state saltwater reptile of Florida.[9

THE OLIVE RIDLEY SEA TURTLE

The olive ridley sea turtle (Lepidochelys olivacea), also known as the Pacific ridley sea turtle, is a medium-sized species of sea turtle found in warm and tropical waters, primarily in the Pacific and Indian Oceans.

 

 Description

The olive ridley is a small sea turtle, with an adult carapace length averaging 60 to 70 cm.^[3] The heart-shaped carapace is characterized by four pairs of pore-bearing inframarginal scutes on the bridge, two pairs of prefrontals, and up to 9 lateral scutes per side. Olive ridleys are unique in that they can have variable and asymmetrical lateral scute 6 to 8 counts ranging from five to 9 plates on each side, with six to eight being most commonly observed.^[3] Each side of the carapace has 12–14 marginal scutes. The carapace is flattened dorsally and highest anterior to the bridge. It has a medium–sized, broad head that appears triangular from above. The head's concave sides are most obvious on the upper part of the short snout. It has paddle-like forelimbs, each having two anterior claws. The upperparts are grayish green to olive in color, but sometimes appear reddish due to algae growing on the carapace. The bridge and hingeless plastron of an adult varies from greenish white in younger individuals to a creamy yellow in older specimens.^[3][4]
Hatchlings are dark gray with a pale yolk scar, but appear all black when wet.^[3] Carapace length ranges from 37 to 50 mm. A thin, white line borders the carapace, as well as the trailing edge of the fore and hind flippers.^[4] Both hatchlings and juveniles have serrated posterior marginal scutes, which become smooth with age. Juveniles also have three dorsal keels; the central longitudinal keel gives younger turtles a serrated profile, which remains until sexual maturity is reached.^[3]
Olive ridleys rarely weigh over 50 kg (110 lb). A study in Oaxaca, Mexico, reported a sample of adults ranged from 25 to 46 kg; adult females weighed an average of 35.45 kg (n=58), while adult males weighed significantly less, averaging 33.00 kg (n=17). Hatchlings usually weigh between 12.0 and 23.3 g. Adults are somewhat sexually dimorphic. Mature males have longer and thicker tails, which are used for copulation,^[3] and the presence of enlarged and hooked claws on the males' front flippers allow them to grasp the female carapace during copulation. Males also have longer, more tapered carapaces than females, which have round, dome-like carapaces.^[3] Males also have more concave plastrons, believed to be another adaptation for mating. The plastrons of males may also be softer than females.^[4]

Distribution

Lepidochelys olivacea distribution map: red circles are major nesting grounds; yellow circles are minor nesting beaches.

The olive ridley turtle has a circumtropical distribution, living in tropical and warm waters of the Pacific and Indian Oceans from India, Arabia, Japan, and Micronesia south to southern Africa, Australia, and New Zealand. In the Atlantic Ocean, it has been observed off the western coast of Africa and the coasts of northern Brazil, Suriname, Guyana, French Guiana, and Venezuela. Additionally, the olive ridley has been recorded in the Caribbean Sea as far north as Puerto Rico. It is also found in the eastern Pacific Ocean from the Galapagos Islands and Chile north to the Gulf of California, and along the Pacific coast to at least Oregon Migratory movements have been studied less intensely in olive ridleys than other species of marine turtles, but they are believed to use the coastal waters of over 80 countries.^[5] Historically, this species has been widely regarded as the most abundant sea turtle in the world.^[3] More than one million olive ridleys were commercially harvested off the coasts of Mexico in 1968 alone.^[6]
The population of Pacific Mexico was estimated to be at least 10 million prior to the era of mass exploitation. More recently, the global population of annual nesting females has been reduced to about two million by 2004,^[7] and was further reduced to 852,550 by 2008.^[1][8] This indicated a dramatic decrease of 28 to 32% in the global population within only one generation (i.e., 20 years).^[5]
The olive ridley sea turtles are considered the most abundant, yet globally they have declined by more than 30% from historic levels. These turtles are considered endangered because of their few remaining nesting sites in the world. The eastern Pacific turtles have been found to range from Baja California, Mexico, to Chile. Pacific olive ridleys nest around Costa Rica, Mexico, Nicaragua, and the northern Indian Ocean; the breeding colony in Mexico was listed as endangered in the US on July 28, 1978.^[9]

Nesting grounds

Olive ridley hatchling

Nesting

Olive ridley turtles are best known for their behavior of synchronized nesting in mass numbers, termed arribadas.^[4] In the Indian Ocean, the majority of olive ridleys nest in two or three large groups near Gahirmatha in Odisha. In 1991, over 600,000 turtles nested along the coast of Odisha in one week. Nesting occurs elsewhere along the Coromandel Coast and Sri Lanka, but in scattered locations. However, olive ridleys are considered a rarity in most areas of the Indian Ocean.^[8]
They are also rare in the western and central Pacific, with known arribadas occurring only within the tropical eastern Pacific, in Central America and Mexico. In Costa Rica, they occur at Nancite and Ostional beaches. Two active arribadas are in Nicaragua, Chacocente and La Flor, with a small nesting ground in Pacific Panama. Historically, several arribadas were in Mexico, yet only one remains at Playa Escobilla in Oaxaca.^[8]
Although olive ridleys are famed for their arribadas, many of the nesting grounds can only support relatively small to moderate-sized aggregations (about 1,000 nesting females). The overall contribution and importance of these nesting beaches to the population may be underestimated by the scientific community.^[3]

Foraging grounds

Some of the olive ridley's foraging grounds near southern California are contaminated due to sewage, agricultural runoff, pesticides, solvents, and industrial discharges. These contaminants have been shown to decline the productivity of the benthic community, which negatively affects these turtles, which feed from these communities.^[3] The increasing demand to build marinas and docks near Baja California and southern California are also negatively affecting the olive ridleys in these areas, where an increase in oil and gas will be released into these sensitive habitats. Another threat to these turtles are power plants, which have documented juvenile and subadult turtles becoming entrained and entrapped within the saltwater cooling intake systems.^[3]

Taxonomy

The olive ridley was first described as Testudo mydas minor, Suckhow, 1798. It was later renamed Chelonian olivacea, Eschscholtz, 1829, and eventually Lepidochelys olivacea Fitzinger, 1843.^[10] Because Eschscholtz was the first to propose the specific epithet olivacea, though, he was credited with the valid name Lepidochelys olivacea Eschscholtz, 1829.
The genus name is derived from the Greek words lepidos, meaning scale, and chelys, which translates to turtle. This could possibly be a reference to the supernumerary costal scute counts characteristic of this genus.^[3] The etymology of the English vernacular name olive is somewhat easier to resolve, as its carapace is olive green in color.^[11] However, the origin of ridley is still somewhat unclear. Lepidochelys is the only genus of sea turtles containing more than one extant species: L. olivacea and the closely related L. kempii (Kemp's ridley). ^[12]

Ecology and behavior

An olive ridley sea turtle laying eggs

Reproduction

Mating is often assumed to occur in the vicinity of nesting beaches, but copulating pairs have been reported over 1,000 km from the nearest beach. Research from Costa Rica revealed the number of copulating pairs observed near the beach could not be responsible for the fertilization of the tens of thousands of gravid females, so a significant amount of mating is believed to have occurred elsewhere at other times of the year.^[3]
The Gahirmatha Beach in Kendrapara district of Odisha (India), which is now a part of the Bhitarkanika Wildlife Sanctuary, is the largest breeding ground for these turtles. The Gahirmatha Marine Wildlife Sanctuary, which bounds the Bhitarkanika Wildlife Sanctuary to the east, was created in September 1997, and encompasses Gahirmatha Beach and an adjacent portion of the Bay of Bengal. Bhitarkanika Mangroves were designated a Ramsar Wetland of International Importance in 2002. It is the world's largest known rookery of Olive Ridley sea turtles. Apart from Gahirmatha rookery, two other mass nesting beaches have been located which are on the mouth of rivers Rushikulya and Devi. The spectacular site of mass congregation of Olive Ridley sea turtles for mating and nesting enthralls both the scientists and the nature lovers throughout the world.
Olive Ridley sea turtles migrate in huge numbers from the beginning of November, every year, for mating and nesting along the coast of Orissa. Gahirmatha coast has the annual nesting figure between one hundred to five hundred thousand, each year. there has been decline in the population of these turtles in the recent past due to mass mortality. Olive Ridley sea turtle has found place in Schedule - I of Indian Wildlife (Protection) Act, 1972 (amended 1991). All the species of sea turtles in the coastal water of Orissa are listed as "endangered" as per IUCN Red Data Book. The sea turtles are protected under the 'Migratory Species Convention' and CITES (Convention of International Trade on Wildlife Flora and Fauna). India is a signatory nation to all these conventions. The 'Homing' characteristics of the Ridley sea turtles make them more prone to mass casualty. The voyage to the natal nesting beaches is the dooming factor for the sea turtles. Since Gahirmatha coast serves as the natal nesting beach for millions of turtles, it has immense importance on turtle conservation.^[13]
Olive ridleys generally begin to aggregate near nesting beaches about two months before nesting season, although this may vary throughout its range. In the eastern Pacific, nesting occurs throughout the year, with peak nesting events (arribadas) occurring between September and December. Nesting beaches can be characterized as relatively flat, midbeach zone, and free of debris.^[10] Beach fidelity is common, but not absolute. Nesting events are usually nocturnal, but diurnal nesting has been reported, especially during large arribadas.^[3] Exact age of sexual maturity is unknown, but this can be somewhat inferred from data on minimum breeding size. For example, the average carapace length of nesting females (n = 251) at Playa Nancite, Costa Rica was determined to be 63.3 cm, with the smallest recorded at 54.0 cm.^[3] Females can lay up to three clutches per season, but most will only lay one or two clutches.^[4] The female will remain near shore for the internesting period, which is about one month. Mean clutch size varies throughout its range and decreases with each nesting attempt.^[8]
A mean clutch size of 116 (30–168 eggs) was observed in Surinam, while nesting females from the eastern Pacific were found to have an average of 105 (74–126 eggs).^[4] The incubation period is usually between 45 and 51 days under natural conditions, but may extend to 70 days in poor weather conditions. Eggs incubated at temperatures of 31 to 32°C will produce only females; eggs incubated at 28°C or less will produce solely males; and incubation temperatures of 29 to 30°C will produce a mixed sex clutch.^[4] Hatching success can vary by beach and year, due to changing environmental conditions and rates of nest predation.

Habitat

Most observations are typically within 15 km of mainland shores in protected, relatively shallow marine waters (22–55 m deep).^[4] Olive ridleys are occasionally found in open waters. The multiple habitats and geographical localities used by this species vary throughout its life cycle.^[10] More research is needed to acquire data on and use of pelagic habitats.^[3]

Feeding

The olive ridley is predominantly carnivorous, especially in immature stages of the life cycle. Animal prey consists of protochordates or invertebrates, which can be caught in shallow marine waters or estuarine habitats. Common prey items include jellyfish, tunicates, sea urchins, bryozoans, bivalves, snails, shrimp, crabs, rock lobsters, and sipunculid worms. Additionally, consumption of jellyfish and both adult fish (e.g. Sphoeroides) and fish eggs may be indicative of pelagic (open ocean) feeding.^[4] The olive ridley is also known to feed on filamentous algae in areas devoid of other food sources. Captive studies have indicated some level of cannibalistic behavior in this species.^[3]

Threats

Olive Ridley entangled in a ghost net within the Maldives

Taken in a drifting net in the Maldives.

Floating in the Arabian Sea, possibly killed by a boat propeller

Hatchlings in Chennai

Known predators of olive ridley eggs include raccoons, coyotes, feral dogs and pigs, opossums, coatimundi, caimans, ghost crabs, and the sunbeam snake.^[4] Hatchlings are preyed upon as they travel across the beach to the water by vultures, frigate birds, crabs, raccoons, coyotes, iguanas, and snakes. In the water, hatchling predators most likely include oceanic fishes, sharks, and crocodiles. Adults have relatively few known predators, other than sharks, and killer whales are responsible for occasional attacks.^[3] Females are often plagued by mosquitos during nesting. Humans are still listed as the leading threat to L. olivacea, responsible for unsustainable egg collection, slaughtering nesting females on the beach, and direct harvesting adults at sea for commercial sale of both the meat and hides.^[4]
Other major threats include mortality associated with boat collisions, and incidental takes in fisheries. Trawling, gill nets, ghost nests, longline fishing, and pot fishing, have significantly affected olive ridley populations, as well as other species of marine turtles.^[3][1] Between 1993 and 2003, more than 100,000 olive ridley turtles were reported dead in Odisha, India from fishery-related practices.^[14] In addition, entanglement and ingestion of marine debris is listed as a major threat for this species. Coastal development, natural disasters, climate change, and other sources of beach erosion have also been cited as potential threats to nesting grounds.^[3] Additionally, coastal development also threatens newly hatched turtles through the effects of light pollution.^[15] Hatchlings which use light cues to orient themselves to the sea are now misled into moving towards land, and die from dehydration or exhaustion, or are killed on roads.
However, the greatest single cause of olive ridley egg loss results from arribadas, in which the density of nesting females is so high, previously laid nests are inadvertently dug up and destroyed by other nesting females.^[3] In some cases, nests become cross-contaminated by bacteria or pathogens of rotting nests. For example, in Playa Nancite, Costa Rica, only 0.2% of the 11.5 million eggs produced in a single arribada successfully hatched. Although some of this loss resulted from predation and high tides, the majority was attributed to conspecifics unintentionally destroying existing nests. The extent to which arribadas contribute to the population status of olive ridleys has created debate among scientists. Many believe the massive reproductive output of these nesting events is critical to maintaining populations, while others maintain the traditional arribada beaches fall far short of their reproductive potential and are most likely not sustaining population levels.^[3] In some areas, this debate eventually led to legalizing egg collection.

Economic importance

Historically, the olive ridley has been exploited for food, bait, oil, leather, and fertilizer. The meat is not considered a delicacy; the egg, however, is esteemed everywhere. Egg collection is illegal in most of the countries where olive ridleys nest, but these laws are rarely enforced. Harvesting eggs has the potential to contribute to local economies, so the unique practice of allowing a sustainable (legal) egg harvest has been attempted in several localities.^[8] Numerous case studies have been conducted in regions of arribadas beaches to investigate and understand the socioeconomic, cultural, and political issues of egg collection. Of these, the legal egg harvest at Ostional, Costa Rica, has been viewed by many as both biologically sustainable and economically viable. Since egg collection became legal in 1987, local villagers have been able to harvest and sell around three million eggs annually. They are permitted to collect eggs during the first 36 hours the nesting period, as many of these eggs would be destroyed by later nesting females. Over 27 million eggs are left unharvested, and villagers have played a large role in protecting these nests from predators, thereby increasing hatching success.^[3]
Most participating households reported egg harvesting as their most important activity, and profits earned were superior to other forms of available employment, other than tourism. The price of Ostional eggs was intentionally kept low to discourage illegal collection of eggs from other beaches. The Ostional project retained more local profits than similar egg collection projects in Nicaragua,^[8] but evaluating egg harvesting projects such as this suffers from the short timeline and site specificity of findings. In most regions, illegal poaching of eggs is considered a major threat to olive ridley populations, thus the practice of allowing legal egg harvests continues to attract criticism from conservationists and sea turtle biologists. Plotkin's Biology and Conservation of Ridley Sea Turtles, particularly the chapter by Lisa Campbell titled "Understanding Human Use of Olive Ridleys", provides further research on the Ostional harvest (as well as other harvesting projects). Scott Drucker's documentary, Between the Harvest, offers a glimpse into this world and the debate surrounding it.

Conservation status

Two olive ridley hatchlings moving into the ocean after being released from a conservation site in Puerto Vallarta, Mexico.

The olive ridley is classified as Vulnerable according to the International Union for Conservation of Nature and Natural Resources (IUCN), and is listed in Appendix I of CITES. These listings were largely responsible for halting the large scale commercial exploitation and trade of olive ridley skins.^[1] The Convention on Migratory Species and the Inter-American Convention for the Protection and Conservation of Sea Turtles have also provided olive ridleys with protection, leading to increased conservation and management for this marine turtle. National listings for this species range from Endangered to Threatened, yet enforcing these sanctions on a global scale has been unsuccessful for the most part. Conservation successes for the olive ridley have relied on well-coordinated national programs in combination with local communities and nongovernment organizations, which focused primarily on public outreach and education.Arribada management has also played a critical role in conserving olive ridleys.^[8] Lastly, enforcing the use of turtle excluder devices in the shrimp trawling industry has also proved effective in some areas.^[1] Globally, the olive ridley continues to receive less conservation attention than its close relative, the Kemp's ridley (L. kempii).
Several projects worldwide seek to preserve the olive ridley sea turtle population. For example, in Nuevo Vallarta, Mexico, when the turtles come to the beach to lay their eggs, some of them are relocated to a hatchery, where they have a much better chance to survive. If the eggs were left on the beach, they would face many threats such as getting washed away with the tide or getting poached. Once the eggs hatch, the baby turtles are carried to the beach and released.
Another major project, in India involved in preserving the olive ridley sea turtle population was carried out in Chennai, where the Chennai wildlife team collected close to 10,000 Olive Ridley turtle eggs along the Marina coast, of which 8,834 hatchlings were successfully released into the sea in a phased manner.^[16]