Oceans

The number of known mountains in Earth’s oceans has roughly doubled. Global satellite observations have revealed nearly 20,000 previously unknown seamounts, researchers report in the April Earth and Space Science.

Just as mountains tower over Earth’s surface, seamounts also rise above the ocean floor. The tallest mountain on Earth, as measured from base to peak, is Mauna Kea, which is part of the Hawaiian-Emperor Seamount Chain.

These underwater edifices are often hot spots of marine biodiversity (SN: 10/7/16). That’s in part because their craggy walls — formed from volcanic activity — provide a plethora of habitats. Seamounts also promote upwelling of nutrient-rich water, which distributes beneficial compounds like nitrates and phosphates throughout the water column. They’re like “stirring rods in the ocean,” says David Sandwell, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego.

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More than 24,600 seamounts have been previously mapped. One common way of finding these hidden mountains is to ping the seafloor with sonar (SN: 4/16/21). But that’s an expensive, time-intensive process that requires a ship. Only about 20 percent of the ocean has been mapped that way, says Scripps earth scientist Julie Gevorgian. “There are a lot of gaps.”

So Gevorgian, Sandwell and their colleagues turned to satellite observations, which provide global coverage of the world’s oceans, to take a census of seamounts.

The team pored over satellite measurements of the height of the sea surface. The researchers looked for centimeter-scale bumps caused by the gravitational influence of a seamount. Because rock is denser than water, the presence of a seamount slightly changes the Earth’s gravitational field at that spot. “There’s an extra gravitational attraction,” Sandwell says, that causes water to pile up above the seamount.

Using that technique, the team spotted 19,325 previously unknown seamounts. The researchers compared some of their observations with sonar maps of the seafloor to confirm that the newly discovered seamounts were likely real. Most of the newly discovered underwater mountains are on the small side — between roughly 700 and 2,500 meters tall, the researchers estimate.

However, it’s possible that some could pose a risk to mariners. “There’s a point when they’re shallow enough that they’re within the depth range of submarines,” says David Clague, a marine geologist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., who was not involved in the research. In 2021, the USS Connecticut, a nuclear submarine, ran into an uncharted seamount in the South China Sea. The vessel is still undergoing repairs at a shipyard in Washington state. More

How do you know if the world’s largest living fish is expecting babies? Not by her bulging belly, it turns out.

Scientists thought that an enlarged area on the undersides of female whale sharks was a sign of pregnancy. But a technique used for the first time on free-swimming animals showed only skin and muscle. These humps might instead be a secondary sex characteristic on mature females, like breasts on humans, researchers report in the March 23 Endangered Species Research.

The ultrasound is part of a suite of new methods including underwater “jet packs” and blood tests that scientists hope could unlock secrets about this creature’s reproduction.

Whale sharks (Rhincodon typus) are classified as globally endangered by the International Union for Conservation of Nature. There are only an estimated 100,000 to 238,000 individuals left worldwide, which is more than a 50 percent decline in the last 75 years.

In part because whale sharks are relatively rare, their reproductive biology is mostly a mystery (SN: 8/1/22). Nearly everything biologists think they know is based on the examination of one pregnant female caught by a commercial fishing boat in 1995.

“Protecting organisms without knowing about their biology is like trying to catch a fly with our eyes closed,” says Rui Matsumoto, a fisheries biologist with the Okinawa Churashima Foundation in Japan. The organization researches subtropical animals and plants to maintain or improve natural resources in national parks.

To learn more about these gentle giants, Matsumoto and shark biologist Kiyomi Murakumo of Japan’s Okinawa Churaumi Aquarium had to figure out how to keep up with them. Like superheroes in a comic book, the biologists used underwater jet packs — propellers attached to their scuba tanks — to swim alongside the fish, which average 12 meters in length and move about five kilometers per hour.

Then the researchers had to maneuver a 17-kilogram briefcase containing a waterproof ultrasound wand on the undersides of 22 females swimming near the Galápagos Islands and draw blood with syringes from their fins. Until this study, the ultrasound wand had never been used outside of an aquarium on free-swimming wildlife.

Fisheries biologist Rui Matsumoto uses a propeller mounted on his scuba tank to keep pace with a female whale shark to take an ultrasound of her belly.S. Pierce

Performing these two tests on whale sharks is especially challenging, says study coauthor Simon Pierce, a whale shark ecologist with the Marine Megafauna Foundation, a nonprofit organization that uses research to drive marine conservation.  The fish “have some of the thickest skin of any animal — up to about 30 centimeters thick.”

Another challenge is the seawater itself, which can contaminate blood samples. The researchers developed a two-syringe system, where the first syringe creates a vacuum and allows the second syringe to draw only blood. 

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Back in the lab, the blood plasma from six of the females showed hormone levels similar to levels obtained from a captive immature female in an aquarium, indicating those wild females were not old enough to reproduce.

Ultrasound imagery showed egg follicles in two of the 22 female sharks, meaning those females were mature enough to reproduce but not pregnant. The biologists did not locate a pregnant whale shark.

Pioneering these noninvasive techniques on whale sharks has opened the door to possibly learning more about other endangered marine animals, too. Waterproof ultrasound wands mounted on a pole, Pierce says, are now being used on tiger sharks in places where the predators are drawn in by bait.

Rachel Graham agrees developing these underwater sampling techniques is an “astounding feat.” But the marine conservation scientist and founder of MarAlliance, a marine wildlife conservation nonprofit, doubts whether most free-ranging wild marine animals, particularly faster-swimming sharks or marine mammals, would tolerate similar tests.

“What makes whale sharks fairly unique … is that they move relatively slowly at times, have the ability to remain stationary, and they tolerate the presence of other animals — such as us — nearby,” says Graham, who has studied shark species around the world and was not involved in the new study.

Coupled with satellite tracking, the new methods, could eventually show us where whale sharks give birth, Pierce says. Little is known about whale shark pups, including whether they are born in shallow or deep water, and whether pups are born one-at-a-time or if mothers gather to give birth together. “Assuming they do have some sort of breeding or pelagic nursery area we can identify … then that obviously goes quite a long way towards conserving the population.” More

Oceans may be shrinking — Science News, March 10, 1973

The oceans of the world may be gradually shrinking, leaking slowly away into the Earth’s mantle…. Although the oceans are constantly being slowly augmented by water carried up from Earth’s interior by volcanic activity … some process such as sea-floor spreading seems to be letting the water seep away more rapidly than it is replaced.

Update

Scientists traced the ocean’s leak to subduction zones, areas where tectonic plates collide and the heavier of the two sinks into the mantle. It’s still unclear how much water has cycled between the deep ocean and mantle through the ages. A 2019 analysis suggests that sea levels have dropped by an average of up to 130 meters over the last 230 million years, in part due to Pangea’s breakup creating new subduction zones. Meanwhile, molten rock that bubbles up from the mantle as continents drift apart may “rain” water back into the ocean, scientists reported in 2022. But since Earth’s mantle can hold more water as it cools (SN: 6/13/14), the oceans’ mass might shrink by 20 percent every billion years. More

Setting sail into a plastic sea —Science News, February 10, 1973

Scientists on an oceanographic voyage in the Central North Pacific last August became startled about the number of manmade objects littering the ocean surface. [Far from civilization and shipping lanes], they recorded 53 manmade objects in 8.2 hours of viewing. More than half were plastic. They go on to compute that there are between 5 million and 35 million plastic bottles adrift in the North Pacific.

Update

The Great Pacific Garbage Patch is larger now than it was in 1973, containing an estimated 1.8 trillion pieces of plastic within an area twice the size of Texas (SN Online: 3/22/18). In recent years, marine biologists have started seeing evidence that garbage is disrupting ocean ecosystems. For instance, large pieces of trash have helped species cross into new territories (SN: 10/28/17, p. 32). But an even greater threat may lurk beneath the waves. Tiny bits of plastic concentrate hundreds of meters deep where they can be eaten by filter feeders and potentially make their way into the guts of larger predators (SN: 7/6/19 & 7/20/19, p. 5). More

After decades of population declines, the future is looking brighter for several tuna and billfish species, such as southern bluefin tuna, black marlins and swordfish, thanks to years of successful fisheries management and conservation actions. But some sharks that live in these fishes’ open water habitats are still in trouble, new research suggests.

These sharks, including oceanic whitetips and porbeagles, are often caught by accident within tuna and billfish fisheries. And a lack of dedicated management of these species has meant their chances of extinction continue to rise, researchers report in the Nov. 11 Science. 

The analysis evaluates the extinction risk of 18 species of large ocean fish over nearly seven decades. It provides “a view of the open ocean that we have not had before,” says Colin Simpfendorfer, a marine biologist at James Cook University in Australia who was not involved in this research.

“Most of this information was available for individual species, but the synthesis for all of the species provides a much broader picture of what is happening in this important ecosystem,” he says.

In recent years, major global biodiversity assessments have documented declines in species and ecosystems across the globe, says Maria José Juan-Jordá, a fisheries ecologist at the Spanish Institute of Oceanography in Madrid. But these patterns are poorly understood in the oceans.

To fill this gap, Juan-Jordá and her colleagues looked to the International Union for Conservation of Nature’s Red List, which evaluates changes in a species’s extinction risk. The Red List Index evaluates the risk of extinction of an entire group of species. The team specifically targeted tunas, billfishes and sharks — large predatory fishes that have influential roles in their open ocean ecosystems. 

Red List Index assessments occur every four to 10 years. In the new study, the researchers built on the Red List criteria to develop a way of tracking extinction risk continuously over time, rather than just within the IUCN intervals.

Juan-Jordá and her colleagues did this by compiling data on species’ average age at reproductive maturity, changes in population biomass and abundance from fish stock assessments for seven tuna species, like the vulnerable bigeye and endangered southern bluefin; six billfish species, like black marlin and sailfish; and five shark species. The team combined the data to calculate extinction risk trends for these 18 species from 1950 to 2019.

The team found that the extinction risk for tunas and billfishes increased throughout the last half of the 20th century, with the trend reversing for tunas starting in the 1990s and billfishes in the 2010s. These shifts are tied to known reductions in fishing deaths for these species that occurred at the same time.

The results are positive for tunas and billfishes, Simpfendorfer says. But three of the seven tunas and three of the six billfishes that the researchers looked at are still considered near threatened, vulnerable or endangered. “Now is not the time for complacency in managing these species,” Simpfendorfer says.

But shark species are floundering in these very same waters where tuna and billfish are fished, where the sharks are often caught as bycatch. 

Many open ocean sharks — like the silky shark (Carcharhinus falciformis) (pictured) — continue to decline, often accidentally caught by fishers seeking other large fish.Fabio Forget

“While we are increasingly sustainably managing the commercially important, valuable target species of tunas and billfishes,” says Juan-Jordá, “shark populations continue to decline, therefore, the risk of extinction has continued to increase.”

Some solutions going forward, says Juan-Jordá, include catch limits for some species and establishing sustainability goals within tuna and billfish fisheries beyond just the targeted species, addressing the issue of sharks that are incidentally caught. And it’s important to see if measures taken to reduce shark bycatch deaths are actually effective, she says. 

“There is a clear need for significant improvement in shark-focused management, and organizations responsible for their management need to act quickly before it is too late,” Simpfendorfer says.  More

One common chemical in sunscreen can have devastating effects on coral reefs. Now, scientists know why.

Sea anemones, which are closely related to corals, and mushroom coral can turn oxybenzone — a chemical that protects people against ultraviolet light — into a deadly toxin that’s activated by light. The good news is that algae living alongside the creatures can soak up the toxin and blunt its damage, researchers report in the May 6 Science.

But that also means that bleached coral reefs lacking algae may be more vulnerable to death. Heat-stressed corals and anemones can eject helpful algae that provide oxygen and remove waste products, which turns reefs white. Such bleaching is becoming more common as a result of climate change (SN: 4/7/20).

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The findings hint that sunscreen pollution and climate change combined could be a greater threat to coral reefs and other marine habitats than either would be separately, says Craig Downs. He is a forensic ecotoxicologist with the nonprofit Haereticus Environmental Laboratory in Amherst, Va., and was not involved with the study.

Previous work suggested that oxybenzone can kill young corals or prevent adult corals from recovering after tissue damage. As a result, some places, including Hawaii and Thailand, have banned oxybenzone-containing sunscreens.

In the new study, environmental chemist Djordje Vuckovic of Stanford University and colleagues found that glass anemones (Exaiptasia pallida) exposed to oxybenzone and UV light add sugars to the chemical. While such sugary add-ons would typically help organisms detoxify chemicals and clear them from the body, the oxybenzone-sugar compound instead becomes a toxin that’s activated by light.

Anemones exposed to either simulated sunlight or oxybenzone alone survived the length of the experiment, or 21 days, the team showed. But all anemones exposed to fake sunlight while submersed in water containing the chemical died within 17 days.

Algae can soak up oxybenzone and its toxic by-products, a study shows. Sea anemones lacking algae (white) died sooner than animals with algae (brown) when exposed to oxybenzone and UV light.Djordje Vuckovic and Christian Renicke

The anemones’ algal friends absorbed much of the oxybenzone and the toxin that the animals were exposed to in the lab. Anemones lacking algae died days sooner than anemones with algae.

In similar experiments, algae living inside mushroom coral (Discosoma sp.) also soaked up the toxin, a sign that algal relationships are a safeguard against its harmful effects. The coral’s algae seem to be particularly protective: Over eight days, no mushroom corals died after being exposed to oxybenzone and simulated sunlight.

It’s still unclear what amount of oxybenzone might be toxic to coral reefs in the wild. Another lingering question, Downs says, is whether other sunscreen components that are similar in structure to oxybenzone might have the same effects. Pinning that down could help researchers make better, reef-safe sunscreens.   More

The first global atlas of ocean light pollution shows that large swaths of the sea are squinting in the glare of humans’ artificial lights at night.

From urbanized coastlines along the Persian Gulf to offshore oil complexes in the North Sea, humans’ afterglow is powerful enough to penetrate deep into many coastal waters, potentially changing the behaviors of creatures that live there, researchers report December 13 in Elementa: Science of the Anthropocene. Regional and seasonal differences — such as phytoplankton blooms or sediment from rivers — also affect the depth to which light penetrates.

Artificial lights are known to affect land dwellers, such as by swelling or shrinking certain insect populations, or by making it harder for sparrows to fight off West Nile virus (SN: 3/30/21; SN: 8/31/21; SN: 1/19/18). But the bright lights of coastal cities, oil rigs and other offshore structures can also create a powerful glow in the sky over the sea.

To assess where this glow is strongest, marine biogeochemist Tim Smyth of Plymouth Marine Laboratory in England and colleagues combined a world atlas of artificial night sky brightness created in 2016 with ocean and atmosphere data (SN: 6/10/16). Those data include shipboard measurements of artificial light, satellite data collected monthly from 1998 to 2017 to estimate the prevalence of light-scattering phytoplankton and sediment, and computer simulations of how different wavelengths of light move through the water.

Not all species are equally sensitive to light, so to assess impact, the team focused on copepods, ubiquitous shrimplike creatures that are a key part of many ocean food webs. Like other tiny zooplankton, copepods use the sun or the winter moon as a cue to plunge en masse to the dark deep, seeking safety from surface predators (SN: 1/11/16; SN: 4/18/18).

Humans’ nighttime light has the most impact in the top meter of the water, the team found. Here, artificial light is intense enough to cause a biological response across nearly 2 million square kilometers of ocean, an area roughly that of Mexico. Twenty meters down, the total affected area shrinks by more than half to 840,000 square kilometers.

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Octopuses living in the deep sea off the coast of California are breeding far faster than expected.

The animals lay their eggs near geothermal springs, and the warmer water speeds up embryonic development, researchers report February 28 at the virtual 2022 Ocean Sciences Meeting. That reproductive sleight of hand means that the octopus moms brood for less than two years, instead of the estimated 12.

In 2018, scientists working off the coast of California discovered thousands of deep-sea octopuses (Muusoctopus robustus) congregated on a patch of seafloor about 3,200 meters below the surface. Many of the grapefruit-sized animals were females brooding clutches of eggs, leading researchers to dub the site the Octopus Garden.

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But with water temperatures hovering around a frigid 1.6° Celsius, growth in this garden was predicted to be leisurely. In octopuses, embryonic development tends to slow down at low temperatures, says marine ecologist Jim Barry of the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. “When you get really cold, down near zero, that’s when brood periods get really long.”

The record for the longest brood period of any animal, just over four years, is held by a different species of octopus living in warmer water (SN: 7/30/14). M. robustus, thriving in the chilly depths of the Octopus Garden, was therefore a serious contender to snatch that title, Barry says. “If you look at its predicted brood period at 1.6° C, it’s over 12 years.”

To verify what would be a record-setting stint of motherhood, Barry and his colleagues repeatedly visited the Octopus Garden from 2019 to 2021 using a remotely operated vehicle. The team trained cameras at the octopus eggs, which resemble white fingers, to monitor their rate of development. With one of the submersible’s robotic arms, the researchers also gently nudged dozens of octopuses aside and measured the water temperature in their nests.

The team found that relatively warm water — up to 10.5° C — bathed all the egg clutches. The female octopuses are preferentially laying their eggs in streams of geothermally heated water, the researchers realized. That discovery was a tip-off that these animals are not the long-haul moms people thought them to be, Barry says. “We’re virtually certain these animals are breeding far more rapidly than you’d expect.”

Deep-sea octopuses (Muusoctopus robustus) brood clutches of eggs, which look like white fingers.Ocean Exploration Trust, NOAA

Based on observations of the developing eggs, Barry and colleagues calculated that the moms brooded for only about 600 days, or about a year and a half. That is much faster than predicted, says Jeffrey Drazen, a deep-sea ecologist at the University of Hawaii at Manoa who was not involved in the research. “They’re cutting a huge amount of time off of their parental care period.”

There is also an evolutionary advantage to seeking out warmer water: Shorter brood periods mean that fewer eggs are likely to be gobbled up by predators. And these octopuses seem to know that, Barry says. “We believe they’re exploiting that thermal energy to improve reproductive success.”

Only a few other marine animals, such as icefish in Antarctica’s Weddell Sea (SN: 1/13/22), are known to seek out warmer conditions when breeding. But there are probably other species that do the same, Drazen says. The challenge is finding them and their breeding grounds in the vast expanse of the deep ocean. “I imagine that as we keep looking, we will keep finding really interesting sites that are important to certain species,” he says. More