The Phases of the Moon, Rainfall, and the Black Volcanic Sand of Ostional Beach, Costa Rica.There are few natural spectacles on Earth as powerful, mysterious, and humbling as the arribada of the Olive Ridley sea turtles at Ostional Beach, Costa Rica. It is one of those rare events where nature operates on such a massive scale that human presence fades into the background. Witnessing it is not simply a travel experience—it is a profound reminder of the ancient rhythms that still govern life on this planet.
If you have ever dreamed of seeing sea turtles nest in the wild, then Ostional, located on the central Pacific coast of Costa Rica on the Nicoya Peninsula, is one of the most extraordinary places to do so. Near the laid-back town of Nosara, this rugged and remote beach becomes the stage for one of the world’s largest and most important nesting events for marine turtles.
What Is an “Arribada”?
The word “arribada” comes from Spanish and means “arrival.” In Costa Rica, it refers specifically to the mass nesting events of the Olive Ridley sea turtle (Lepidochelys olivacea). Unlike most sea turtles that nest individually, Olive Ridleys arrive together, sometimes in astonishing numbers.
During an arribada, hundreds of thousands—and in rare cases millions—of turtles emerge from the ocean over a period of several days. They crawl onto the beach, often piling over one another, to dig nests and lay their eggs in the warm, dark sand.
The sheer scale of this phenomenon defies description. The beach seems to move, alive with flippers, shells, and determination. The sound of waves mixes with the scraping of sand as turtles dig their nests. The air feels charged with something ancient and powerful.
Ostional Beach: A Unique Setting
Ostional Beach is unlike the postcard-perfect white sand beaches many travelers imagine. Instead, it is composed of black volcanic sand, rich in minerals and heat-retaining properties. This sand plays a crucial role in the nesting success of the Olive Ridley turtles.
The beach stretches for approximately 15 kilometers, providing ample space for mass nesting. Offshore, the waters are nutrient-rich, and the coastline remains relatively undeveloped—key factors in why turtles have returned here for countless generations.
The Costa Rican government, recognizing the global importance of this site, established the Ostional Wildlife Refuge (Refugio Nacional de Vida Silvestre Ostional) to protect both the turtles and their habitat. It is one of the most significant marine conservation areas in the world.
Timing the Arribada: Moon Phases and Rainfall
The arribada is not random. It is guided by a complex combination of lunar cycles, rainfall patterns, ocean currents, and instinctual memory passed down through generations of turtles.
The largest arribadas typically occur during the last quarter of the moon, when nights are darker. Scientists believe that reduced moonlight may help protect nesting turtles and hatchlings from predators.
Equally important is rainfall. The heaviest arribadas take place during Costa Rica’s rainy season, from August through December, when the Nicoya Peninsula receives increased precipitation. Rain softens the sand, making it easier for turtles to dig nests, and helps regulate the temperature of the eggs buried beneath the surface.
When these elements align—the moon phase, rainfall, ocean conditions, and internal biological clocks—the turtles arrive.
A Record-Breaking Natural Event
The most astonishing arribada on record occurred in 1995, when an estimated 500,000 Olive Ridley turtles came ashore at Ostional within a short period. Over the course of the nesting event, they laid up to ten million eggs along the beach.
To imagine half a million turtles emerging from the sea is almost impossible until you see it with your own eyes. The beach, as far as the eye can see, becomes a living carpet of shells and movement.
Events of this magnitude underscore why Ostional is considered one of the most important Olive Ridley nesting sites on Earth.
The Nesting Ritual
Once ashore, each female turtle begins a carefully choreographed process:
- Finding a Nesting Spot
The turtle crawls above the high tide line, often navigating around other turtles, to find a suitable place. - Digging the Nest
Using her powerful hind flippers, she digs a deep, flask-shaped hole in the sand. - Laying the Eggs
The turtle lays approximately 80–120 eggs, each about the size of a ping-pong ball. - Covering and Camouflaging
Once the eggs are laid, she carefully covers the nest and disguises it, using her flippers to scatter sand. - Returning to the Sea
Exhausted but driven by instinct, she makes her way back to the ocean, leaving the eggs to incubate on their own.
The entire process can take several hours, often continuing throughout the night.
From Egg to Ocean: The Race for Survival
After approximately 45 to 55 days, the eggs hatch. Tiny hatchlings break free from their shells and dig upward through the sand, emerging mostly at night to reduce the risk of predation.
What follows is one of the most vulnerable moments of their lives. The hatchlings instinctively orient themselves toward the brightest horizon—ideally, the reflection of moonlight on the ocean—and begin their desperate race to the sea.
Birds, crabs, and other predators take their toll, but those that reach the water disappear into the vast Pacific Ocean, beginning a long and perilous journey.
A 15-Year Journey Home
Perhaps the most remarkable aspect of the Olive Ridley’s life cycle is its natal homing instinct. After leaving Ostional as hatchlings, these turtles spend approximately 15 years roaming the open ocean.
They travel thousands of miles, navigating by Earth’s magnetic fields, ocean currents, and celestial cues. And then—against all odds—they return to the exact beach where they were born.
They return to Ostional, to the same black volcanic sands, to continue the cycle that has existed for millennia.
Conservation and Community Involvement
Ostional is unique not only for its turtles but also for its community-based conservation model. Under strict government regulation, local residents are allowed to collect a limited number of eggs from the earliest nests of an arribada. These eggs would otherwise be destroyed by later-arriving turtles digging new nests in the same area.
This carefully managed system provides economic support to the community while promoting conservation and protection of the turtles and their habitat. It is a rare example of humans and wildlife coexisting in a mutually beneficial way.
Visiting Ostional Responsibly
If you plan to witness an arribada, it is essential to do so responsibly:
- Always go with an authorized local guide
- Never touch or interfere with nesting turtles
- Do not use flash photography
- Keep a respectful distance
- Follow all refuge regulations
These guidelines help ensure that future generations—both human and turtle—can continue to experience this miracle.
A Humbling Experience
Standing on Ostional Beach during an arribada, you feel small—and grateful. Grateful that such places still exist. Grateful that ancient instincts remain intact. Grateful that Costa Rica has chosen to protect this treasure for the world.
The Olive Ridley turtles remind us that nature does not belong to us—we belong to it.
To witness an arribada is to witness life, persistence, and hope written across black volcanic sand under a tropical moon.