What do fish do at night? 588 marine species reveal something surprising

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Every evening after dusk gives way to dark hordes of sea creatures – from tiny zooplankton to giant sharks – rise from the depths to spend the night close to the surface. They revel in the upper waters, feed and mate, before retiring before dawn.

Known as the daily vertical migration, this mass movement is often touted as the largest synchronous migration on Earth. As the planet rotates on its axis and areas of the ocean turn towards or away from sunlight, this is happening in a continuous flow around the world.

Migration was first documented in the early 1800s when naturalist Georges Cuvier noted that plankton called daphnia – water fleas – were disappear and reappear in a daily cycle in a shallow freshwater lake.

Then, during World War II, came the discovery of the “deep scattering layer”: an area in the oceans that unexpectedly deflected the Navy’s sonar pulses and mysteriously vanished every night, like a seabed. Phantom.

Scripps Institution of Oceanography scientist Martin Johnson offered an explanation: the deep diffusion layer could be marine animals migrating to the surface. In June 1945, he tested the idea on an overnight excursion in the waters off Point Loma, California. The zooplankton, jellyfish and various crustaceans he captured in a series of 14 hauls established that the moving layer was indeed made up of enterprising living beings. an evening migration.

Since then, scientists have spotted this steady course in almost every water they’ve examined.

“It’s universal in all habitats,” whether marine, freshwater or brackish, says Kanchana Bandara, marine scientist at the Arctic University of Norway. “It is universal in all geographic locations, from the tropics to the poles, and it is universal in all taxonomic groups, from small zooplankton or phytoplankton to large whales and sharks.”

But despite its omnipresence, puzzles remain. Research suggests that changes in light trigger the evening hike, so it’s unclear how animals in the waters around the Earth’s poles – where there are months when sunlight is constant or completely absent – know when it’s time to migrate.

How do fish know when it’s time to migrate?Cavan Images / Cavan / Getty Images

Researchers are working to understand this, as well as to determine when various creatures make their journeys – and why some choose not to travel at all.

These nuances are important to understand, scientists say, as daily vertical migration serves as a giant conveyor belt transporting carbon that is nibbled from surface waters to the depths – carbon that might otherwise remain on the ocean surface. or return to the atmosphere.

It’s an expensive habit: Estimates suggest that over a year, the collective energy expended on daily trips by zooplankton alone is equivalent to about a year of energy consumption in the United States.

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

Daily migration in action

There is a consensus among scientists that for many creatures, including zooplankton like daphnids, migration helps them avoid being eaten. Deeper, darker waters provide refuge from the eyes of predators during the day. Visits to the surface, where food is most abundant, are done safely under cover of night.

Scientists also agree that changing light intensity is the main environmental signal for migrants, says Heather Bracken-Grissom, marine biologist at Florida International University. When the light begins to fade, it can trigger the ascent to the surface.

But that’s not the whole story. Scientists had long assumed, as part of the light-tracking model, that daily migrations would cease during arctic winters, when there are months without daylight.

But in 2008, researchers reported that zooplankton was, indeed, participate in an evening migration in the arctic waters of Svalbard during the long polar night. More recent research has established that this pattern is widespread – and can be caused by moonlight.

In 2016, a team of scientists from Norway and Great Britain studied the waters around the Arctic in the months before and after the winter solstice, when the Sun is still below the horizon. Using hydroacoustic sampling techniques, the team found that the tiny sea creatures had moved their migrations, synchronize them with the light of the moon rather than that of the sun.

And in addition to the daily cycle, there was a monthly signal: The animals regularly moved to deeper waters during the bright light of the full moon.

Pisces use the Moon as a signal.Photo by Marianna Armata / Moment / Getty Images

Scientists are also learning more about the supreme sensitivity of zooplankton to changes in light. Working in the northern Pacific Ocean, a team used sonar-like acoustic sampling to detect the daily movement of creatures, including copepods, ostracods, salps and krill.

The recorded weather was consistently overcast, gray and drizzly, but the zooplankton could still detect variations in the thickness of the cloud cover and adjust their depth, the team reported in PNAS in August. A difference in brightness of just 10 to 20 percent was enough to cause 50-foot mini-migrations – no short haul for the tiny animals.

The constant daylight of a polar summer also doesn’t seem to stop zooplankton from their night pilgrimage. For several years in the waters off the west coast of Antarctica, researchers used specialized nets that collected samples at specific depths.

Examining the content, the team found that the critters continued to migrate throughout the constant summer light, although for some the trips were shorter when the days were longer.

The fact that the tiny marine animals retained their daily cycle even without darkness suggests that another signal is triggering their migration, either independently or in combination with light – possibly an internal circadian clock, according to the co-author. study Patricia Thibodeau, plankton ecologist at the University. from Rhode Island.

Through genetic studies and experiments in the laboratory and in the field, scientists recently established that such a clock does not guide daily cycles certain migrants, including the copepod Calanus finmarchicus and Antarctic krill Superb euphausie.

Research suggests that since the stakes are so high – to migrate or to be eaten – evolution has fostered the development of an internal circadian cycle for daily migration, as a safeguard to reliance on environmental cues.

How predators influence migration

The important issues associated with daily migration also seem to shape the behavior of creatures as they move. Research has shown that migrants off Santa Catalina Island in California tend to congregate in cohesive groups or schools when moving, which can reduce the risk of being eaten.

Larger, more visible animals such as fish migrate later – around 80 minutes after sunset – than smaller, less visible animals, which begin their migration up to 20 minutes before sunset.

The presence of predators also encourage some migrants delay their trip. When the squid-eating Risso’s dolphins, for example, were in the area, researchers observed that the squids waited in deeper water, delaying their trip by about 40 minutes.

And some people on some days seem to skip the ride altogether. Researchers suspect they may not always be hungry enough to think the trip is worth it. This idea, known as the “hunger / satiety hypothesis,” postulates that individuals in a population are motivated by their own level of hunger.

A team including Nova Southeastern University marine ecologist Tracey Sutton put this theory to the test, taking advantage of trawl surveys in the Gulf of Mexico following the Deepwater Horizon oil spill. Over a period of seven years, automated net systems collected specimens at sampling stations across the Gulf, in deep and surface waters.

Of these, 588 creatures were then sent to labs, so the team could “open their stomachs and see what they’re eating,” says Sutton, who co-wrote an overview of deep ocean food webs. in the 2017 Annual Journal of Marine Sciences.

Scientists found that those who had not migrated still had food in their stomachs, suggesting that they had chosen not to make the trip because they were still full from the night before. And migrating individuals were more likely to have an empty stomach.

But exceptions remained: one fish and two species of crustacean did not follow this pattern, suggesting that individuals in a population “Choose” if or not to migrate, the researchers reported in February in Frontiers in marine sciences.

Fish species whose migration patterns did not align have also had shallower migrations and may have faster metabolism than other species – variables that can interact, Sutton says, making it difficult to draw conclusions. universal.

Hunger, light, genetics and more – scientists continue to probe for these and other factors that affect this great journey, including salinity, temperature, and UV exposure. Studying these variables, which animals move with when and who eats whom, is key to understanding the Earth’s carbon cycle, says Sutton, and how this massive transport helps sequester it over time.

Migration, he says, “is more or less everything, if you really follow carbon.”

This article originally appeared in Known magazine, an independent journalistic company of Annual Reviews. Subscribe to bulletin.

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