These deep-sea skates use hydrothermal vents as egg incubators

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.

A "black smoker", a type of hydrothermal vent, in the Galapagos Rift (left). Skate egg cases collected in the area (right). Images: Ocean Exploration Trust

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 Nautlius noticed 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."

A Pacific white skate (Bathyraja spinosissima) swims over lava seafloor about 1,800 meters below the surface at Axial Seamount, an underwater volcano off the coast of Oregon. These large skates can reach a whopping two metres in length! Image: MBARI

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. (For a bit of perspective, there were shark-like animals swimming in ancient oceans 280 million years before the first dinosaurs.)

The location of the Iguanas-Pinguinos site, where the eggs were discovered.

"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.

Imagery from the remotely operated vehicle (ROV) Hercules showing egg cases at the Iguanas-Pinguinos hydrothermal vent site at the Galapagos Marine Reserve. Image: Ocean Exploration Trust

Image: Ocean Exploration Trust

"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."

Keeping toasty can pay a role in the development of other animals, too – the sex of many reptiles, for example, is determined by their "hot or not" status. Still, only very few species have been known to harness the heat from geothermal activity to help their young go from embryo to hatchling. It's thought that some sauropod dinosaurs built nests in volcanically heated soils during the Cretaceous period, some 145 to 66 million years ago. And today, just one dinosaur descendant – the Polynesian megapode (an endangered bird native to Tonga) – employs this tactic.

More work is needed to find out just how many Pacific white skates prefer their eggs "soft boiled", or if this behaviour happens in other locations, too. However, layers of hatched cases also discovered around the Galapagos vents indicate that the species has been using the site for years. It just took a pinch of serendipity to reveal it.

Ebert has seen similar egg deposits off the coast of both California and Oregon. "I thought it was interesting, but I didn't make the connection," he says. "I didn't have the full scope – and these guys were able to get it with their survey. Most of the team members who stumbled upon this discovery are geologists, and it opens up a whole new world that we don't know about."

It's possible that skates along North America's Pacific coast also lay their eggs near underwater seamounts, but that's just speculation for now. An unexpected twist during the study, however, may have shown scientists where to look for answers.

When the team was digging into genetics data from the eggs collected in the Galapagos, they found that their genetic signature matched that of a skate caught during a survey in Vancouver, Canada back in 2008. That animal was thought to be a spiny-tailed skate (Bathyraja spinicauda), but Ebert and his colleagues realised it had been misidentified all those years ago.

The skates from that Canadian survey had been stored in a museum fish collection in Victoria, and photographs of the supposed "spiny-tailed skate" revealed it was actually a Pacific white. This revelation extends the known range for the species, and because British Columbia has some notable geothermal activity, any nearby rifts or seamounts could be a starting point to hunt for egg cases.

Image: Ocean Exploration Trust

A closer look at the haul from this decade-old survey also resulted in the identification of two more skates – the broad skate and the fine-spine skate – which have now been added to the roster of Canadian fauna.

"Two completely different studies, done ten years apart, together filled in blanks about these animals," says Ebert. "From so many angles, it is such a cool story!"

The team hopes to continue this multidisciplinary collaboration, and to extend it as deep-sea surveys continue springing up in the coming years. We don't know just how unique the use of geothermal "incubators" might be, but at least for now, the Galapagos skates get to keep their shiny title as the only known example in our oceans.

"This all happened on one dive," adds Phillips. "Just fifteen hours flying around underwater. Imagine what we might find if we were expressly looking for this."

Maps created during the project have helped see Iguanas-Pinguinos included in the Galapagos Marine Reserve, a recently established World Heritage Site – but many other vent fields around the world remain unprotected. And because valuable minerals abound near black smokers, sea floor-mining initiatives are often in conflict with conservation efforts. A mountain of skate eggs, the team explains, gives conservationists leverage: it demonstrates that the ocean's volcanic habitats are also crucial to the survival of some species, likely far more than we realise.

"Now experts [from different fields] know what to look for," adds Ebert. "We're going to try to look at some other areas. Hopefully we're going to start gathering more information. It's wide open now."

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Top header image: MBARI