It is
the first time such behavior has been recorded in marine animals. The discovery
is published this week in the journal Scientific Reports.
Brennan
Phillips, URI assistant professor of ocean engineering, was leading the operations
team of remotely-operated vehicle operators during the expedition to explore
the underwater
Robotic arm picks up Pacific skate egg from ocean floor (Ocean Exploration Trust) |
“We
were on a really deep dive in a hydrothermally-active rift valley, with walls
30 meters tall on either side, and the ROV was meandering back and forth
looking for vents,” Phillips explained. “We started noticing all these egg
cases, and we recorded their location and collected a few but then just kept
going.”
Later,
when the egg locations were analyzed and compared to hydrothermal vent
locations and recorded water temperatures, the scientists concluded that the
eggs were likely intentionally placed by the skates where the water was warmer
than average.
In
total, 157 mobile-phone-sized egg cases were observed, which DNA analysis revealed
to be from the Pacific white skate (Bathyraja spinosissima). About 58 percent
of the egg cases were found within 20 meters of a black smoker, the hottest
kind of hydrothermal vent, and 89 percent of the egg cases were laid in water
that was hotter than the background temperature of 2.76 degrees Centigrade.
“The
eggs weren’t right next to the active vents, because the water can get so hot –
hundreds of degrees – that it would kill them,” Phillips said. “We found most
of them in the lukewarm water not far from the vents and near some extinct
vents.
“The
kicker is that we showed our data to a bunch of shark experts, and they had
seen anecdotal evidence of shark and ray egg cases near hydrothermal vents, but
they never had the data to put the story together,” he added.
Several
land animals have been recorded laying their eggs in similar situations. A rare
bird native to Tonga, the Polynesian megapode, often nests in
volcanically-heated soils, for instance. Fossils of some sauropod dinosaurs of
the Cretaceous era are believed to have done so as well.
This
isn’t the first time Phillips has observed unexpected behaviors by shark family
members in heated water. In 2014, he investigated an underwater volcano near
the Solomon Islands in the western Pacific Ocean, where he watched several
species of shark swim right through the bubbling hot water in the volcano’s
crater.
“Our
cameras caught images of schools of sharks, smaller fish and even jellyfish
living in the hot acidic plume,” he said at the time. “This presumably toxic
environment supports a whole community of life, even though every once in a
while it blows up.”
Coupled
with his hydrothermal vent observation, Phillips believes that sharks, rays and
skates – which evolved about 500 million years ago – have adapted to the
unusual conditions.
“Seafloor
volcanism comes and goes, and it is often one of the causes of mass
extinctions,” he said. “It’s interesting to me that we’re seeing sharks and
skates thriving around volcanoes and vent sites, like they’re especially
resilient and have evolved to withstand the hot water environment.”
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