Olive ridley sea turtle

Appearance

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 side1,2. 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 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, while adult males weighed significantly less averaging 33.00 kg. 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

The olive ridley turtle has a cirumtropical 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, more than 1 million olive ridleys were commercially harvested off the coasts of Mexico in 1968 alone. Cliffton et al. had estimated the population of Pacific Mexico to be at least 10 million prior to the era of mass exploitation 1. More recently, Spotilia estimated that the global population of annual nesting females has been reduced to approximately 2 million, and Abreu-Gabrois and Plotkin 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 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.

Status

The olive ridley is classified as Vulnerable according to the International Union for Conservation of Nature And Natural Resources, 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 and the Inter-American Convention for the Protection and Conservation of Sea Turtles 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 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 8.

Habitat

Most observations are typically within 15 kilometers of mainland shores in protected, relatively shallow marine waters 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.

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 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 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 was observed in Surinam, while nesting females from the eastern Pacific were found to have an average of 105 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.

Food

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 and fish eggs may be indicative of pelagic 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.

Predators

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. 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.

Defense

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. The overall contribution and importance of these nesting beaches to the population may be underestimated by the scientific community1.