But little did Darwin know that an even more impressive testament to life’s stunning versatility was unfolding 30 miles out to sea and 5,500 feet below the waves.
There in the utter darkness and crushing pressures of the deep ocean, a rare stingray-like creature called a Pacific white skate today lays its eggs among the hot plumes that gush from hydrothermal vents. This seems daring — the underwater equivalent of a bird building its nest at the mouth of a volcano — but it may be a stunningly sophisticated maneuver, scientists say. Eggs incubate faster when they’re warm, increasing the likelihood that the offspring will survive to perpetuate their parents' DNA.
“I think it’s phenomenal,” said Dave Ebert, program director of the Pacific Shark Research Center at Moss Landing Marine Labs in California and co-author of a new study about the discovery. As far as he is aware, it is the first time any marine animal has been seen exploiting the hydrothermal vent environment for this purpose.
“This is a very complex behavior pattern,” Ebert said, “and it gets into why there are so many different species” of skate.
Skates, sometimes called “flat sharks,” are a diverse family within the class of fish called Elasmobranchs, which also includes sharks and rays. Like their cousins, skates are ancient (some date to before the age of dinosaurs), boneless and predatory. Their kite-shaped bodies have been seen soaring over the bottoms of every ocean in the world.
The Pacific white skate is the deepest-dwelling species in this group. Ranging between half a mile to nearly two miles below the surface, they are an enigma to scientists — beyond the reach of all but the sturdiest submersibles.
In 2015, Salinas and his colleagues surveyed the hydrothermal-vent field known as Iguanas-Pinguinos, north of the Galapagos, aiming to establish a baseline of biodiversity for this literal hot spot. Two tectonic plates are drifting away from each other there as new ocean crust emerges. Superhot fluid darkened by dissolved minerals gushes from fissures in the sea floor, creating the billowing phenomena known as “black smokers.”
On a warm June morning, the researchers dropped their remote-operated underwater vehicle, or ROV, into the azure waters of the Pacific. For 90 minutes it sank deeper and deeper, the ocean around it darkening to cobalt, then navy, then black. A long fiber-optic cable tethered the craft to the ship where scientists watched a live feed of the descent.
No sooner had the ROV reached the sea floor than they spotted a cluster of rectangular pouches clustered near the base of one of the black-smoker chimneys.
“It was instant jackpot,” Salinas recalled.
The egg cases were about the size of iPhones and the color of banana peels, with tails at the corners that give them their common name — “mermaid’s purse.” More than half were spotted within 65 feet of a chimney, and nearly 90 percent were in places where the water temperature was higher than its average of 37 degrees Fahrenheit.
Using the ROV's robotic arm, the team retrieved a handful of cases to bring to the surface.
Team members can’t say for sure why the skates are laying their eggs there, although faster incubation seems to be the most likely explanation. Amid the deprivation of the deep sea, Pacific white skates manage to eke out an existence by living long and slow. They swim sluggishly, eat infrequently and can live for decades. Their embryos take a correspondingly long time to develop — up to four years. But research suggests that heating the surrounding water just a fraction of a degree can shorten that period by several months.
Above the seas, other creatures brave a volcano on behalf of their offspring. An endangered bird called the Tongan megapode buries its eggs in volcanic ash to keep them warm. And researchers in Argentina have found fossilized nesting sites suggesting that long-necked dinosaurs took advantage of geothermal heat for the same purpose.
“From an evolutionary point of view, it makes sense,” Salinas said. Every day an egg spends incubating is one during which it is utterly defenseless against a changing environment and hungry predators. A savvy parent of any species would do well to try to limit that time span.
To Ebert, the discovery was a profound reminder of the truth of Darwin’s scribbled observation about the Galapagos finches.
From a single common ancestor that lived hundreds of millions of years ago, skates have evolved to exploit almost every ocean environment. They dwell in the deep sea, on sandy slopes, among kelp beds just below the surface. They breathe through spiracles — small holes on their backs — rather than their mouths, allowing them to almost bury themselves in silt and sand. Some are more than six feet long; others are the size of a dinner plate. Some have special sensory organs that allow them to detect the weak electric fields produced by moving prey.
“It’s so cool to see that this skate has such a unique niche,” Ebert said.
But both scientists are concerned about what the future holds. Even as humans begin to understand the weird wonders of the ocean bottom, we are also changing it. Overfishing has broken food chains and pushed trawlers to drop their nets deeper and deeper as surface populations decline.
“We are extracting from the deep sea before we even know what is there,” Salinas said.
The researchers’ report of their discovery was published Thursday in the journal Scientific Reports. They continue to work on their analysis of the biodiversity survey at Iguanas-Pinguinos, with Salinas estimating that they identified about 30 new species. There probably are hundreds, if not thousands more, to be found. The ocean floor is the largest habitat on Earth, but the surfaces of the moon and Mars are better known.
“When we are realizing these unique behaviors, it highlights the need for going down there and exploring more,” Salinas said. “We have a whole alien world in our own back yard.”