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Deep-sea skates use hydrothermal vents as egg incubators

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For expecting parents, choosing just the right crib can be a bit of a nightmare, but generally speaking, setting up your progeny near a a bubbling plume of iron sulphide and superheated water is one option to avoid. Unless, it turns out, you're a deep-sea skate. New research suggests some of these rarely encountered fish use hydrothermal vents – fissures in the Earth's surface that spew forth hot water and steam – like incubators, reported Earth Touch News.

Like so many great discoveries, this one happened completely by chance. During a 2015 survey off the coast of the Galapagos Islands, a collaborative team aboard the Ocean Exploration Trust research vessel Nautliusnoticed a peculiar pattern: the region's towering, sulphide-rich hydrothermal vents (known as 'black smokers') were frequently visited by ghostly skates, close shark and ray relatives. The seafloor around these structures, meanwhile, was littered with ochre-coloured eggs.

Perplexed by the unusual sighting, the team reached out to Dr Dave Ebert, programme director at the Pacific Shark Research Center, who was able to identify the alabaster animals seen near the vents as Pacific white skates (Bathyraja spinosissima). Later genetic tests conducted on four of the eggs confirmed they harboured the same species behind their silky, collagen walls.

"For me, it was like being a kid on Christmas morning. I felt like yelling, 'Wahoo! This is awesome!'" says Ebert. "These fish live one, even two, thousand metres deep. We know almost nothing about them."

Like their close kin, skates take their time growing, but deep-dwelling members of this group (order Rajiformes) take "slow" to a whole new level: in some cases, it can take up to four years for a baby skate to emerge from its egg case as a fully formed smiling ravioli.

Add a little heat to the mix, however, and that time can be cut in half.

"We think these Pacific white skates are using the vents to speed up development," explains Ebert. "We know from past studies that if you take these egg cases and you warm them up, even by half a degree, or a degree Celsius, it will rapidly decrease the incubation period. So instead of being three to four years, it might be one to two."

With the help of geologists, marine biologists and geneticists from four countries, this heating-up hypothesis was published today in a study led by Charles Darwin Research Station senior scientist Dr Pelayo Salinas de León. If the hunch proves correct, it will mark the first time this strategy has been documented in any marine animal.

"This is a really cool finding!" says Dr Thomas Farrugia, a skate and ray researcher at the University of Alaska Fairbanks who was not involved with the study. "In one sense, it's not too surprising: skate egg cases are often found in high densities in specific areas, such as canyons or kelp beds. You would assume that they are laid in those areas for some purpose. But it is really intriguing that they might be using the thermal advantages of hydrothermal vents!"

Even for co-author Dr Brennan Phillips, a deep-sea biologist who has been piloting remotely operated vehicles (ROVs) for 14 years, the egg cases came as a surprise. Phillips studies the relationships between sharks and underwater volcanoes (he refers to this overlap as "sharkcanoes!").
Hydrothermal vents are often associated with volcanic activity beneath the sea floor, so the discovery piqued his interest right away.

"When I was piloting on this trip, and I saw the skate eggs, I was really pleased," says Phillips. "The egg cases added to the story of why these animals are there."
Sharks and their kin have been around for over 500 million years.

"And in that time, there have been periods of extreme vulcanism on this earth – periods where the volcanoes have become really active," says Phillips. "When that happens, most things die. So it's really interesting to me that we have a lot of sharks, skates and rays around still, that seem to not only be resilient to these harsh [volcanic] environments, but also to be using them to their advantage."

Hot water spewing from these underwater chimneys can reach an unimaginable 300°C (572°F), and the Iguanas-Pinguinos site, where the eggs were discovered, is known for especially vigorous venting.

Most of the 157 eggs observed by the team were nestled within 20 metres (66ft) of the openings. Even at this range, the surrounding water can be nearly a degree above the 2.76°C (37°F) average (that's positively balmy by deep-sea standards). What's more, the instruments used to measure the ambient temperatures weren't positioned quite as deep as the eggs themselves, so the water around the clutches could have been even warmer. "It might even be another half degree warmer," says Ebert.

There's evidence to suggest that skate development across the board is almost entirely driven by water temperature.
The Alaska skate, found in the frigid Bering Sea, takes about three years to develop. Skates found in tropical waters or coastal shallows, on the other hand, hatch faster. This means the incubation period required to produce a healthy baby skate probably has far less to do with biology than it does with habitat.
And that would mean denizens of the deep are at a serious disadvantage: the deeper you go, the colder it gets, and the longer a skate's babies take to join the watery world. It seems Pacific white skates have learned to cheat that system.

"It is ingenious, but we probably have to check our assumption that the fish 'know' to do this specifically," notes Farrugia. It's very possible that the behaviour is simply the result of evolution over time: skates that deposited their eggs near vents in the Galapagos Rift had shorter incubation periods, and were therefore able to produce offspring with greater success.

"On the other hand, I wouldn't rule out that they actually do seek out areas with warmer temperatures," he says. "Many elasmobranch species give birth or lay eggs in specific 'nursery' areas which present advantages for their young – either warmer temperatures, fewer predators or more food."

Ebert suspects that the latter scenario could be true. "But I doubt they came from far away," he adds. "Rather, I think these skates are generally in the area of these seamounts and rifts, but find the vents to specifically lay their eggs."

Hundreds of animal and microbe species have been documented near the planet's hydrothermal vents, even though these areas are among the most extreme habitats for life, with low oxygen content and monumental pressures. What's more, when hot, acidic seawater rises through volcanic rock, it releases all kinds of toxic minerals.

So, could the noxious chemicals emitted from Iguanas-Pinguinos have any negative effects on skate embryos? It's unlikely. For starters, these fish are quite hardy, and egg cases are closed off to the outside environment most of the time. When a baby skate is nearing the end of its stay, a small hole opens in the case to allow more oxygen in.

"In fact, the skate embryo will actually beat its tail to create some water flow and replenish the oxygen," explains Farrugia. "As long as sufficient oxygen comes in, they may be able to tolerate some of the other gases. But since the adults would have to be able to survive while they are depositing the egg cases, I'm guessing they are depositing them in areas that don't have a lot of the noxious gases while still having warmer temperatures."


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