Reptiles
Olive Ridley Sea Turtle
Lepidochelys olivacea · also called olive ridley, olive ridley sea turtle, Pacific ridley
The olive ridley is the smallest and most numerous sea turtle, and the one responsible for one of the strangest sights in nature: the arribada, when a single beach fills from the waterline to the dunes with nesting turtles, so many that they climb over one another and dig up each other’s eggs.
Overview
Olive ridleys range through the warm waters of the Pacific, Indian, and southern Atlantic Oceans. They are the most omnivorous sea turtle, eating jellyfish, crabs, shrimp, urchins, fish, and algae both in open water and off the bottom. The shell is a small, rounded, heart-shaped olive-grey, and adults weigh only 35–50 kilograms — around a fifth of a green turtle.
The arribada
Most sea turtles nest alone, at intervals. The olive ridley (and its relative the Kemp’s ridley) instead nests in an arribada — Spanish for “arrival” — a mass synchronised event in which tens of thousands to over half a million females storm one beach within a few nights, several times a year. There are only a handful of arribada beaches worldwide, in Costa Rica, Mexico, Nicaragua, and Odisha in India.
Why all at once
The arribada is a case of predator swamping. A lone nest would almost certainly be found and eaten; a million nests in a week overwhelm every crab, bird, mammal, and human that would otherwise take them, so a large number of hatchlings gets through. The cost is that later turtles dig up earlier nests — but the net output of surviving young is still higher than solitary nesting would give on these beaches.
Status
The olive ridley is Vulnerable and declining despite being the most abundant sea turtle. It drowns as bycatch in trawls, longlines, and gillnets, its eggs and adults are harvested, and its nesting beaches are lost and lit. Some populations, such as those of Suriname and Guyana, have already collapsed.
Olive Ridley Sea Turtle: common questions
What is an arribada?
An arribada (Spanish for "arrival") is a mass nesting event unique to the two ridley turtle species. Instead of nesting alone at intervals like other sea turtles, olive ridley females gather offshore and then come ashore together in a huge wave — at the largest beaches, tens of thousands or even hundreds of thousands of turtles over a period of three to seven nights, several times a year. The beach becomes carpeted with nesting turtles, often digging up each other's nests as they compete for space. Why the turtles synchronise so tightly is not fully understood, but lunar cycle, wind, and pheromones may all play a part.
Why do olive ridleys nest all at once instead of spreading out?
The leading explanation is predator satiation, or "swamping." A single olive ridley nest of about 100 eggs left alone on a beach would almost certainly be found and eaten by crabs, birds, raccoons, coyotes, dogs, or people. But when a million nests appear in one week, the same predators simply cannot consume more than a small fraction before the eggs hatch, so a large absolute number of hatchlings survives. The strategy has a cost — many nests are destroyed by later-nesting turtles digging in the same sand — but on balance it produces more surviving young than solitary nesting would on these particular beaches.
Is the olive ridley endangered?
It is the most abundant sea turtle in the world, but it is still listed as Vulnerable and is declining overall. Its problems are the standard ones for sea turtles, at scale — drowning as bycatch in shrimp trawls, longlines, and gillnets (turtle excluder devices help but are unevenly used), historical and continuing harvest of eggs and adults, loss and lighting of nesting beaches, and plastic and other marine debris. Some arribada populations have collapsed — the species is functionally gone from beaches in Suriname and Guyana where it once nested in huge numbers.
How is the olive ridley different from other sea turtles?
It is one of the two smallest sea turtles (with its close relative the Kemp's ridley), it has a distinctive rounded, heart-shaped, olive-grey shell with more scute pairs than other species, and it is the most omnivorous, eating a wide mix of animal and plant food from both the surface and the seabed. Behaviourally, the arribada is its signature — no other sea turtle except the Kemp's ridley nests this way. It also tends to nest more frequently but with fewer eggs per clutch than the larger species.