Aptly named icefishes thrive in waters just above
freezing with enormous hearts and blood that runs clear as vodka. Their blood
is transparent because they lack red blood cells and haemoglobin to transport
oxygen throughout the body. Icefishes’ loss of haemoglobin genes was less an
evolutionary adaptation than a happy accident, one that has allowed them to
absorb the oxygen-rich Antarctic waters through their skin.
The sighting occurred in February 2021 in the camera
room aboard a research ship, the Polarstern, which had come to the Weddell Sea
to study other things, not icefish. It was 3 am near Antarctica, meaning the
sun was out but most of the ship was asleep. To Boehringer’s surprise, the
camera kept transmitting pictures as it moved with the ship, revealing an
uninterrupted horizon of icefish nests every 20 seconds.
“It just didn’t stop,” Boehringer said. “They were
everywhere.”
Half an hour later, Autun Purser, a deep-sea biologist
at the same institute, joined Boehringer. On the camera feed, there remained
nothing but nests.
“We were like, is this ever going to end?” Purser
said. “How come no one has ever seen this before?”
The nests persisted for the entire four-hour dive,
with 16,160 recorded on camera. After two more dives by the camera, the
scientists estimated the colony of Neopagetopsis ionah icefish stretched across
92 square miles of the serene Antarctic sea, totalling 60 million active nests.
The researchers described the site — the largest fish breeding colony ever
discovered — in a paper published Thursday in the journal Current Biology.
“Holy cow,” said C-H Christina Cheng, an evolutionary
biologist at the University of Illinois-Urbana-Champaign, who was not involved
with the research. “This is really unprecedented,” she said. “It is crazy
dense. It is a major discovery.”
The paper provides “evidence of a complex and so far
undescribed benthic ecosystem in the Weddell Sea,” said Mario La Mesa, a
biologist at the Institute of Polar Sciences in Bologna, Italy, who was not
involved with the research.
“I would not be surprised to find other massive
colonies of breeding fishes elsewhere,” said La Mesa, who last year described
the same Antarctic icefish species’ nest-guarding behaviour from sites near the
newly discovered colony.
Each of the newly discovered nests held, on average,
1,735 large, yolky eggs — low fecundity for a fish. An unprotected clutch would
prove an easy snack for predators like starfish, polychaete worms and sea
spiders, Cheng said. So the males stand sentry to ensure their offspring are
not devoured, at least not before they have the chance to hatch, and may clean
the nests with their elongated lower jaw, according to Manuel Novillo, a researcher
at the Bernardino Rivadavia Museum of Natural Science in Argentina, who was not
involved with the research.
About three-quarters of the colony’s nests were
guarded by a single fish. The others had eggs but no fish, a fish carcass
furred white with bacteria or nothing at all. Near the edges of the colony,
many unused or abandoned nests cradled several icefish carcasses, many with
starfishes and octopuses feasting on their eyes and soft parts.
“If you die in the fish nest area, you rot there,”
Purser said. “But if you die at the edges, then it seems to be everyone grabs
you and starts eating you there.”
The researchers observed that the colony occupied an
unusually warm patch of deep water, with temperatures up to about 35 degrees
Fahrenheit — practically toasty compared to other Antarctic waters.
Although the discovery of the nests contributes to
scientists’ understanding of the icefish life cycle, it raises even more
questions. How often are the nests built, and are they reused? Do the fish die
after the eggs hatch? Or, perhaps the most obvious: “Why there?” Cheng asked.
The authors have no sure answers, only speculations.
Maybe the warm deep currents guide the fish to the grounds. Maybe there is a
bounty of zooplankton for the fry to devour. Or maybe it’s something else.
But there must be something special about the location
of the active colony. Around 31 miles west, the researchers found a patch of
seafloor similarly littered with nests: all empty. These nests were abandoned,
overtaken by sponges and corals — long-living creatures that take years to
grow, Purser said.
Waters above the icefishes’ expansive settlement also
host hungry, foraging Weddell seals. When the researchers collected satellite
tracking data from seals during the expedition and analysed it with historical
data, they found, unsurprisingly, that the seals dive primarily to the icefish
nests. “They’re having a nice dinner,” Purser said.
Before the end of the cruise, the researchers deployed
a camera that will photograph the site twice daily for two years, hopefully
revealing even more about the life cycle of the icefish. Novillo said he is
looking forward to seeing what the camera captures. “It might constitute the
first field observation of courtship behaviour and/or nest preparation,” he
wrote in an email.
New insights into how icefish reproduce and contribute
to polar food webs could help manage and conserve populations. The authors
argue the new paper provides enough evidence to protect the Weddell Sea under
the Convention on the Conservation of Antarctic Marine Living Resources.
“The seafloor is not just barren and boring,” Purser
said. “Such huge discoveries are still there to be made, even today in the 21st
century.”
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‘Major discovery’ beneath Antarctic seas: a giant icefish breeding colony
