Shark cousin uses heat from ocean vents to incubate its eggs

Birds might sit on their eggs to keep them warm until they hatch, but deep-sea fish called skates have found a less boring and time-consuming way to incubate their eggs — they lay them near hot hydrothermal vents on the sea floor.

Skates are relatives of sharks that have flattened bodies like stingrays and typically live more than a kilometre below the surface of the ocean. They lay eggs that can take three or four years to hatch in the cold waters of the deep ocean, although the eggs could theoretically hatch more quickly in warmer environments.

Scientists exploring the volcanically active sea floor off the Galapagos Islands with a robotic sub or ROV (remotely operated vehicle) stumbled across a "nursery" in 2015 where Pacific white skates had laid 157 yellow egg cases, each about the size of a smartphone and shaped like a pillow with horns at all four corners.

When they analyzed the data, they discovered that over 89 per cent of the eggs had been laid in places where the water was warmer than average, the researchers reported in a paper published today in the journal Scientific Reports.

About 68 per cent of the egg cases were laid within 20 metres of a black smoker chimney, a hydrothermal vent in the ocean floor that spews hot water along with clouds of dark-coloured particles.

The researchers, led by Pelayo Salinas-de-Leon, senior marine ecologist at the Charles Darwin Foundation in the Galapagos Islands, suggest that the skates are choosing this place to lay their eggs in order to use the warmer temperatures to make them develop and hatch faster. That's because they're cold-blooded animals, and the rate at which all bodily processes happen depends on the surrounding temperature.

Previous research has found that in the lab, the incubation period for some sharks can be reduced from two years to one just by increasing the temperature half a degree, said David Ebert, one of the co-authors of the new study.

Vulnerable stage

Ebert, program director of the Pacific Shark Research Center at Moss Landing Marine Laboratories in California, added that Pacific white skates are already the size of a dinner plate when they hatch and can grow to be up to two metres in diameter.

"The most vulnerable time in this skate's life history is in going to be in the egg case," said Ebert. The developing embryo can be eaten by predators like worms and snails, and the faster it can hatch, the shorter this period of vulnerability.

Some land animals have already been known to use geothermal vents to heat their eggs, including sauropod dinosaurs during the Cretaceous and modern birds called megapodes.

Salinas-de-Leon said he and his research team was "very excited to document this behaviour for the first time in the marine environment."

Accidental discovery

The team hadn't originally been planning to study skate egg cases. They were on a three-week expedition organized by the Ocean Exploration Trust, a non-profit group dedicated to exploring the oceans with help of its 64-metre Nautilus research vessel and its Argus and Hercules ROVs, which can dive to depths of 6,000 metres.

Their goal was to map and explore the biodiversity around the Galapagos Islands — the area where the world's first hydrothermal vents were discovered in 1977 — in order to figure out how much the Galapagos Marine Park needed to be expanded and protected from human activity like mining and fishing.

Just by chance, Salinas-de-Leon recalled, the Hercules ROV landed right next to a black smoker chimney at the start of a 24-hour dive. Its cameras beamed back images of egg cases that the team initially thought were shark egg cases.

The team decided to collect two samples with the Hercules's manipulator arm. DNA from those samples were later used to identify the species. They matched a Pacific white skate caught off Vancouver Island in B.C. during a Fisheries and Oceans Canada expedition and catalogued at the Royal B.C. Museum.

When the team reviewed the footage from the dive, they noticed that the eggs were concentrated near the black smokers and wondered if those might be areas of warmer water.

Brennan Phillips was a PhD student on the expedition who piloted the ROV, and is now an assistant professor of ocean engineering at the University of Rhode Island. He checked the temperatures automatically recorded by the ROV above each of the egg cases. When he saw the results, he said, "Well, I was really excited."

While most of the readings were less than 0.1 C higher than the normal water temperature of 2.76 C, they were taken more than a metre above the egg cases, where it was cooler. Salinas-de-Leon estimates that the egg cases themselves may have been closer to 1 degree above the surrounding temperature.

'Parental care'

Chris Mull is a postdoctoral researcher at Simon Fraser University in Burnaby, B.C., who studies sharks and their relatives, including skates, but was not involved in the study. He says many shark species that give birth to live young will move to warmer waters when they're pregnant, presumably to speed up gestation.

But this is the first time he's seen an egg-laying species seek out warmer waters to incubate their eggs.

"You could almost think of it as a form of parental care," he said, adding that skates weren't previous known for caring for their eggs or their young in any way.

Salinas-de-Leon and his colleagues ended their report with some good news. In March 2016, after the researchers showed the high level of shark biodiversity in the area, the Ecuadorian government created a 40,000 square kilometre marine sanctuary around Darwin and Wolf Islands in the Galapagos that includes the Pacific white skate nursery.

The study was funded by the Helmsley Charitable Trust and the Save our Seas Foundation.