Science & Technology

Ups and downs of big vertical movement

Every night, the dusk darkens, and hordes of sea creatures, from small zooplankton to giant sharks, rise from the deep sea and spend the night near the surface. They feed and mate in upper waters before retreating before dawn.

This mass movement, known as diel vertical migration, is often referred to as the largest synchronous movement on Earth. As the planet rotates around its axis and the sea patch moves toward or away from the sunshine, it happens in constant change around the world.

Georges Cuvier, a naturalist, has a plankton called Daphnia pulex. Disappears and reappears In a shallow freshwater lake on a daily cycle. Later, during World War II, the “deep sea scattering layer” was discovered. This is a zone of the sea that unexpectedly deflects the Navy’s sonar pings and mysteriously disappears every night like a phantom seabed.

Martin Johnson, a scientist at the Scripps Institution of Oceanography, suggested an explanation. The deep-sea scattering layer can be a marine animal that moves to the surface of the sea. In June 1945, he tested his ideas on an overnight excursion in the waters off Point Loma, California.The zooplankton, jellyfish, and various crustaceans he caught in a series of 14 hauls proved that the mobile layer was actually composed. Creatures that move in the evening..

Since then, scientists have discovered this regular commute in almost every body of water they have seen. “It is universal throughout the habitat, whether in the ocean, freshwater or brackish waters,” said Kanchana Bandara, a marine scientist at the Norwegian Arctic University. “It is universal across geographic locations, from the tropics to the polar regions, and across taxonomic groups, from small zooplankton and phytoplankton to large whales and sharks.”

However, despite its widespread use, puzzles remain. Studies show that changes in light cause evening trekking, so it is unclear when animals in the waters around the Earth’s poles with a moon with constant or complete sunlight will move. Researchers are not only working to understand this, but also finding out when various creatures travel and why some choose not to travel at all.

According to scientists, it is important to understand these nuances. Diel vertical migration is because it acts as a huge conveyor belt that carries carbon bitten by surface water into the deep sea. Otherwise, carbon may stay on the surface of the sea or return to the atmosphere. This is a costly habit. It is estimated that the total energy spent on commuting by zooplankton alone is equivalent to about a year’s worth of energy consumption in the United States.

“It’s an unimaginable amount of energy,” says Bandara.

Diel vertical migration due to moonlight

There is consensus among scientists about many creatures, including: Zooplankton Like Daphnia pulex, migration helps prevent them from eating. The deeper, darker waters provide shelter from the eyes of predators during the day. Visits to surfaces with more food are the safest under the night cover.

Scientists also agree that changing light intensity is a major environmental clue for migrants, says Heather Blacken Grissom, a marine biologist at Florida International University. As the light begins to diminish, it can cause a rise to the surface.

But that’s not all. Scientists have long assumed that under a ray-tracing model, daily movement would stop during the Arctic winter, when there is a moon without sunlight.

But in 2008, researchers did say that zooplankton Participate in evening migration In the Arctic Ocean off Svalbard during a long polar night. Recent studies have confirmed that this pattern is widespread and can be driven by moonlight. According to a 2016 report, a team of Norwegian and British scientists surveyed the waters around the Arctic for several months before and after the winter solstice, when the sun was always below the horizon. Using underwater acoustic sampling technology, the team discovered that small marine organisms had shifted their migration. Synchronize them with the moonlight Than that of the sun. And in addition to the daily cycle, there was a monthly signal: during the bright light of the full moon, the animals were regularly moving into the deep sea.

Scientists are also learning more about zooplankton’s highest susceptibility to changes in light. Teams working in the North Pacific used sonar-like acoustic sampling to detect the daily movements of creatures such as copepods, ostracods, salps, and krill.The recorded weather was consistently cloudy, gray and drizzle, but zooplankton was still possible. Detect changes in cloud cover thickness And adjusted their depth, the team reported in PNAS In August. A difference in brightness of only 10 to 20 percent was enough to encourage a 50-foot mini-movement. There was no small trek for small animals.

The constant sunlight of polar summers doesn’t seem to stop Zooplankton from their nightly pilgrimage.. For several years off the west coast of Antarctica, researchers have used special nets to collect samples at specific depths. Examining the content, I found that the creatures continued to move in the constant light of summer, but some people had shorter commuting times as the days went on.

The fact that small marine animals preserved their daily cycles in the absence of darkness suggests that other signals, either independently or in combination with light, probably cause their movement in either the internal circadian clock. Research co-author Patricia Tivodo, a plankton ecologist at the university, says of Rhode Island.Through genetic research and laboratory and field experiments, scientists have recently established such a thing. The clock guides the daily cycle of some immigrants, Including copepods Calanoida Finmarquicus And Antarctic krill Krill superba..

Studies suggest that evolution supported the development of an internal circadian cycle for diel vertical migration, as stakes are so high (moving or edible), as a backup for dependence on environmental cues. doing.

Predators can influence migration decisions

High stakes on daily travel also seem to shape the behavior of creatures on their way to work. Studies show that immigrants off the coast of Santa Catalina Island in California tend to gather in coherent groups and schools during their travels, which may reduce their risk of being eaten. Large, prominent animals, such as fish, move slower (about 80 minutes after sunset) than small, unobtrusive animals that begin moving 20 minutes before sunset.

The presence of Predators also urge some migrants to delay their trekking.. For example, when Risso’s dolphin, which eats squid, was in the area, researchers observed that the squid was waiting in the deep sea and postponed the journey by about 40 minutes.

And one day, some people seem to skip commuting altogether. Researchers suspect that traveling may not always be hungry enough to feel worth the risk. This idea, known as the “hunger / satiety hypothesis,” assumes that individuals in a population are motivated by their level of hunger.

A team, including Tracey Sutton, a marine ecologist at Nova Southeastern University, tested this theory using a trawl survey in the Gulf of Mexico after the Deepwater Horizon oil spill. An automated net system has collected specimens from sampling stations throughout the bay for seven years, both in the deep sea and in surface water. 588 of them were sent to the lab, and the team “could break their stomachs to see what they were eating,” he said. Deep sea food web In 2017 Annual Review of Marine Science..

Scientists have discovered that those who did not move still have food in their stomachs. This suggests that they chose not to go trekking because they were still bored from the night before. And mobile people were more likely to be hungry. But there were exceptions. One fish and two crustacean species do not follow that pattern, with individuals within the population “Choose” whether to migrate, Researchers reported in February Frontier of oceanology.. Species with mismatched migration patterns also migrate shallowly and may metabolize faster than other species. Variables can interact and it is difficult to draw universal conclusions.

Starvation, optogenetics, genetics, etc. — Scientists continue to investigate these and other factors that influence this wonderful commute, such as salt, temperature, and exposure to UV light. Studying these variables is the key to understanding which animals are eating when, who is eating who. Earth’s carbon cycle, Says Sutton, and how this massive commute helps isolate it over time.

“If you’re really tracking carbon, the transition is more or less all,” he says.

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Ups and downs of big vertical movement

https://www.scientificamerican.com/article/the-ups-and-downs-of-a-great-vertical-migration/ Ups and downs of big vertical movement

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