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The olive ridley sea turtle (Lepidochelys olivacea), also known as the Pacific ridley, is a species of sea turtle.

Description

The olive ridley is a small extant sea turtle, with an adult carapace length averaging 60 to 70 cm ¹. The heart-shaped carapace is characterized by four pairs of pore-bearing inframarginal scutes on the bridge, two pairs of prefrontals, up to nine lateral scutes per side^1,2 . Olive ridleys are unique in that they can have a variable and asymmetrical lateral scute count ranging from five to nine plates on each side, with six to eight being most commonly observed ¹.Each side of the carapace has 12–14 marginals. The carapace is flattened dorsally and highest anterior to the bridge. It has a medium–sized, broad head that appears triangular in planar view ^1,2. The head has concave sides, most obvious on the upper part of the short snout. It has paddle-like forelimbs, each having two anterior claws ¹. The upperparts are grayish green to olive in color, but sometimes appear reddish due to algae growing on the carapace ². The bridge and hingeless plastron of an adult varies from greenish white (younger) to a creamy yellow on older specimens ¹.

Hatchlings are dark gray with a pale yolk scar, but appear all black when wet ¹. Carapace length ranges from 37–50mm ². A thin white line borders the carapace, as well as the trailing edge of the fore and hind flippers².Both hatchlings and juveniles have serrated posterior marginals, 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 reached¹.

Olive ridleys rarely weigh over 50 kilograms. A study in Oaxaca, Mexico reported an adult sample ranging from 25 to 46 kilograms. 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 to 23.3 grams. Adults are somewhat sexually dimorphic. Mature males have a longer and thicker tail than females, which is used for copulation¹. The presence of an enlarged and hooked claw on the front flipper of males allows them to grasp the female carapace during copulation. Males have a longer, tapered carapace than females, which have a round, dome-like carapace ¹. Males also have a more concave plastron, believed to be another adaptation for mating. The plastra of males may also be softer than females ².

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 India 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, there have been records of the olive ridley 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 ² 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 ³. Historically, this species has been widely regarded as the most abundant sea turtle in the world ¹. According to Carr (1972), more than 1 million olive ridleys were commercially harvested off the coasts of Mexico in 1968 alone. Cliffton et al. (1982) had estimated the population of Pacific Mexico to be at least 10 million prior to the era of mass exploitation ¹. More recently, Spotilia (2004) estimated that the global population of annual nesting females has been reduced to approximately 2 million, and Abreu-Gabrois and Plotkin (2008) estimated that number to have been further reduced to 852 550. This indicated a dramatic decrease of 28–32% in the global population within only one generation (i.e. 20 years) ³.

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 ridley’s nest around Costa Rica, Mexico, Nicaragua, and the Northern Indian Ocean; the breeding colony in Mexico was listed as endangered in the U.S. on July 28, 1978.^[1]

Nesting grounds

Olive ridley turtle nesting on Escobilla, Oaxaca

Nesting

Olive ridley turtles are best known for their behavior of synchronized nesting in mass numbers, termed arribadas ². In the India Ocean, the majority of olive ridleys nest in two or three large bundles near Gahirmatha in the Orissa ⁴. In 1991, over 600,000 turtles nested along the coast of Orissa in one week ¹. 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 ⁴. 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. There are two active arribadas in Nicaragua; Chacocente and La Flor; and a small nesting ground in Pacific Panama. Historically, there were several arribadas in Mexico, yet only one remains at Playa Escobilla in Oaxaca⁴.

Although olive ridleys are famed for their arribadas, many of the nesting grounds can only support relatively small to moderate-sized aggregations (e.g. 1,000 nesting females). The overall contribution and importance of these nesting beaches to the population may be underestimated by the scientific community¹.

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.^[2] These contaminants have been shown to decline the productivity of the benthic community, which negatively affects these turtles that get their nutrition from these communities.^[2] The increasing demand to build marinas and docks near Baja California and southern California are also negatively impacting the olive ridleys in these areas, in which an increase in oil and gas will be released into these sensitive habitats.^[2] Another major threat to these turtles are power plants; specifically the San Diego Gas & Electric (SDG&E) plant in Carslbad, and the Southern California Edison Nuclear Generating Station in San Onofre.^[2] Both these plants have documented juvenile and sub-adult turtles becoming entrained and entrapped within the saltwater cooling intake systems.^[2]

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, 1843^1,5. Because Eschscholtz was the first to propose the specific epithet olivacea, though, he was credited with the valid name Lepidochelys olivacea Eschscholtz, 1829^1,5. 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 ¹.The etymology of the English vernacular name olive is somewhat easier to resolve, as its carapace is olive green in color⁶. 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) ¹.

Ecology and behavior

An olive ridley sea turtle laying eggs

Reproduction

Mating is often assumed to occur in the vicinity of nesting beaches, however, copulating pairs have been reported over 1 000 kilometers from the nearest beach¹. Research from Costa Rica revealed that the number of copulating pairs observed near the beach could not be responsible for the fertilization of the tens of thousands of gravid females. Therefore, it was believed that a significant amount of mating must have occurred elsewhere at other times of the year ¹.

Olive ridleys generally begin to aggregate near nesting beaches approximately 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 (i.e. arribadas) occurring between September and December. Nesting beaches can be characterized as relatively flat, mid-beach zone, and free of debris ⁵. Beach fidelity is common, but not absolute. Nesting events are usually nocturnal, however diurnal nesting has been reported especially during large arribadas¹. 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 ¹. Females can lay up to three clutches per season, but most will only lay one or two clutches ². The female will remain near shore for the inter-nesting period, which is approximately one month. Mean clutch size varies throughout its range and decreases with each nesting attempt⁴. 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) ². The incubation period is usually between 45–51 days under natural conditions, but may extend to 70 days in poor weather conditions. Eggs incubated at temperatures of 31–32 degrees Celsius will produce only females; eggs incubated at a temperature of 28 degrees or less will produce solely males; and incubation temperatures of 29–30 degrees will produce a mixed sex clutch². 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 kilometers of mainland shores in protected, relatively shallow marine waters (22-55m)². Olive ridleys will occasionally occur in open waters. The multiple habitats and geographical localities used by this species vary throughout its life cycle ⁵. More research is needed to acquire data on and use of pelagic habitats ¹.

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 ². The olive ridley is also known to feed on filamentous algae in areas devoid of other food sources ¹. Captive studies have indicated some level of cannibalistic behavior in this species¹.

Threats

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

Known predators of olive ridley nests include raccoons, coyotes, feral dogs and pigs, opossums, coatimundi, caimans, ghost crabs, and the sunbeam snake². 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 ¹. Adults have relatively few known predators, other than sharks and killer whales responsible for occasional attacks ¹. Females are often plagued by mosquitoes 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 hides².

Other major threats include mortality associated with boat collisions and incidental takes in fisheries. Trawling, gill nets, ghost nests, long line, and pot fishing, have significantly impacted olive ridley populations, as well as other species of marine turtles ^1,7. During 1993–2003, more than 100 000 olive ridley turtles were reported dead in Orissa, India from fishery-related practices ⁸. 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 grounds¹. Additionally, coastal development also threatens newly hatched turtles through the effects of light pollution.^[3] Hatchlings which use light cues to orient themselves to the sea are now misled into moving towards land, and die from dehydration, 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 that previously laid nests are inadvertently dug up and destroyed by other nesting females¹. 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 arribadas event successfully hatched¹. Although some of this loss had resulted from predation and high tides, the majority was attributed to conspecifics unintentionally destroying existing nests. The extent that arribadas contribute to the population status of olive ridleys has created debate among scientists. Many believe that the massive reproductive output of these nesting events is critical to maintaining populations, while others maintain that traditional arribada beaches fall far short of their reproductive potential and are most likely not sustaining population levels¹. In some localities, 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, and so the unique practice of allowing a sustainable (legal) egg harvest has been attempted in several localities ⁴.Numerous case studies have been conducted in regions of arribadas beaches to investigate and understand the socioeconomical, 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 viable⁴. Since egg collection became legal in 1987, local villagers have been able to harvest and sell approximately 3 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 success¹. Most participating households reported egg harvesting as their most important activity, and salaries 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 beaches⁴. The Ostional project retained more local profits than similar egg collection projects in Nicaragua⁴. However, one must consider the complexity of evaluating egg harvesting projects such as this, and realize that case studies can only provide a snapshot in time and also suffer from the site specificity of findings⁴. In most regions, illegal poaching of eggs is considered a major threat to olive ridley populations, and thus the practice of allowing legal egg harvests continues to attract criticism from conservationists and sea turtle biologists. Pamela Plotkin's Biology and Conservation of Ridley Sea Turtles, particularly the report from Lisa Campbell entitled Understanding Human Use of Olive Ridleys, provides further research on the Ostional Harvest (as well as other harvesting projects), while Scott Drucker's documentary, Between the Harvest, offers a glimpse into this world and the debate that surrounds it.

Conservation status

Two olive ridley (Lepidochelys olivacea) 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 CITES⁷.These listings were largely responsible for halting the large scale commercial exploitation and trade of olive ridley skins⁷. The Convention on Migratory Species (CMS) and the Inter-American Convention for the Protection and Conservation of Sea Turtles (IAC) have also provided olive ridleys with protection, and led 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 non-government organizations, which focused primarily on public outreach and education.Arribada management has also played a critical role in conserving olive ridleys ⁴.Lastly, enforcing the use of Turtle Excluder Devices(TEDs) in the shrimp trawling industry has also proved effective in some areas ⁷. Globally, the olive ridley continues to receive less conservation attention than its close relative, the Kemp ridley (L. kempii)⁸.

See also

• Chelonioidea – the sea turtle superfamily
• Caretta caretta – the loggerhead turtle
• Chelonia mydas – the green turtle
• Dermochelys coriacea – the leatherback turtle
• Eretmochelys imbricata – the hawksbill turtle
• Natator depressus – the flatback turtle

References

¹ National Marine Fisheries Service and U.S Fish and Wildlife Service.(1998). Recovery Plan for U.S Pacific Populations of the Olive Ridley Turtle (Lepidochelys olivacea). National Marine Fisheries Service, Silver Spring, MD.

² Barbour, Roger, Ernst, Carl, & Jeffrey Lovich. (1994). Turtles of the United States and Canada. Washington, DC: Smithsonian Institution Press.

³ Department of the Environment, Water, Heritage and the Arts (2010). Lepidochelys olivacea in Species Profile and Threats Database, Department of the Environment, Water, Heritage and the Arts, Canberra. Available from: http://www.environment.gov.au/sprat. Accessed 20 Apr 2010.

⁴ Plotkin, P. (2007). Biology and conservation of ridley sea turtles, (Ed.), Baltimore, Maryland: The Johns Hopkins University Press.

⁵ Lepidochelys olivacea Eschscholtz 1829". Encyclopedia of Life, available from "http://www.eol.org/pages/1056177". Accessed 07 Apr 2010

⁶ Ellis, Richard. (2003). The Empty Ocean. Washington, D.C : Island Press.

⁷ Abreu-Grobois, A. and Plotkin, P. 2008. Lepidochelys olivacea. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.1. <www.iucnredlist.org>. Downloaded on 7 April 2010.

⁸ Shanker, K et al.(2004). Phylogeography of olive ridley turtles (Lepidochelys olivacea) on the east coast of India: implications for conservation theory.Molecular Ecology,13, 1899–1909