Catherine Brahic New Scientist 28 Jan 11;
THE egg is exactly the same shape, size and colour as a ping-pong ball, but soft and covered in a thin, glistening fluid. Just seconds ago it was dumped into a hole in the sand by a female olive ridley sea turtle.
It's midnight on La Escobilla beach on Mexico's Pacific coast, where two biologists and I squat near the solitary turtle. She's one of the stragglers; last week, some 50,000 females swam in to lay their eggs, a phenomenon known as an arribada. The beach plays host to several arribadas each year. In 2007, 1.4 million nests were dug in this 15-kilometre stretch of sand, making it a contender for the largest sea-turtle nesting site in the world.
That's remarkable when you consider that just 20 years ago, the number hovered around 140,000. In those days, the slaughterhouse in nearby San Augustinillo was licensed to kill between 1000 and 3000 turtles a day. The beach, which now attracts a mix of Mexican and European tourists, was drenched in blood and littered with left-over turtle parts. I'm told the bay teemed with sharks, and the stench - so remarkable that it has its own name, chuquilla - could be smelled in the next town, 2.5 kilometres down the coast.
Then, in 1990, a national ban came into force, and La Escobilla was brought under state protection. The locals are allowed onto the beach during the day, says Erika Peralta Buendia, the biologist who runs the beachside monitoring station, but at night you need a permit. The lure of the supposedly aphrodisiac eggs is still strong.
But something far more insidious than humans threatens the colony. Hide beetles (Omorgus suberosus), each just 1 centimetre long, are eating their way through the stock of eggs and hatchlings. The damage is such that Ernesto Albavera of the Mexican Turtle Centre in nearby Mazunte estimates that just 2 per cent of the eggs that are laid here on average each year actually hatch. He believes he is witnessing the natural death of an arribada beach.
Arribadas are one of nature's most spectacular phenomena. Just two of the seven known species of sea turtles nest in synchrony - the olive ridley and the Kemp's ridley - and they do so on only a handful of beaches around the world (see map). No one knows why they do it, but what is becoming clear is that arribada beaches have life cycles of their own, driven by what the mass nesting does to the ecosystem.
On average, about 100 million eggs are laid in the sand each year by the turtles. They have no maternal instinct, and disappear into the night after they have laid their clutch. Latecomers kick up eggs that were laid just days earlier, creating a feast for crabs, birds, racoons - and the local population of hide beetles.
Albavera's studies have identified a sharp decline in fertility of the beach, and he thinks the beetles are responsible. But while he calls it "a big problem" he also thinks it is part of a natural cycle. "The arribada phenomenon is temporary," he says. "The turtles will move somewhere else."
Work by Ken Lohmann at the University of North Carolina at Chapel Hill points to what could be driving this cycle. His studies have shown that turtle hatchlings imprint on their beach of birth and use its geomagnetic signature to return later in life to lay their own eggs. If the number of eggs hatching at La Escobilla plummets, the number of females that arrive each year to lay should follow suit a few years later.
How arribada beaches are "born" is a greater mystery. The small town of Ixtapilla, 800 kilometres north of here, appears to be at the beginning of a new cycle. In 1997, it unexpectedly played host to its first arribada, and has done so regularly ever since. Albavera's limited funding allows him to send only the occasional student to Ixtapilla to pass on advice to the locals, who are keen to protect the turtles.
Back in La Escobilla, a tiny black turtle crawls over my foot on its way to the ocean. The most recent arribada has just drawn to a close, but the first eggs are starting to hatch from the previous event, some 45 days earlier. The baby may one day return to lay its own eggs, but the odds of survival are slim for any hatchling.
Whatever its future, it has already played a small part in the grand cycle that Albavera and his colleagues are trying their best to preserve.
