The great white shark has the most fearsome reputation of all sharks. But it might not be the biggest of the predator sharks. That honor might go to the Pacific sleeper shark. The biggest one ever seen appeared to be about 23 feet long—longer than the biggest great white.

The Pacific sleeper is found mainly in cold waters around the rim of the northern Pacific Ocean. But some have been seen in warmer waters close to the equator.

The shark got its name because it was thought to spend most of its time near the bottom, waiting for prey to swim by—a “sleepy” sort of behavior. But at least one study found otherwise. The sharks were found to move up and down through the water column, from the bottom to near the surface. And some covered as much as three or four miles a day.

Pacific sleepers will eat just about anything. They prefer fish that dwell on the bottom, along with giant octopus. But their stomach contents also show other types of fish, snails, sea lions, and other prey. They might have hunted down some of them, and gobbled the already dead remains of others.

The shark hasn’t been studied that much. The largest one ever caught was about 14 feet long and weighed half a ton. But video cameras caught one that was estimated at 23 feet.

Pacific sleepers probably grow slowly and have a low reproduction rate. So they could be threatened by overfishing, mostly as bycatch—draining the population of what might be the largest of all predator sharks.

Currents at the bottom of the ocean can be just as fickle as wind currents at the surface. They can turn, speed up or slow down, and even reverse course. And they can change in just days or even hours.

That’s the conclusion of the most detailed study of sea-floor currents to date. Researchers anchored 34 instrument packages across a thousand-square-mile region off the coast of Mozambique, at the southeastern corner of Africa. The instruments monitored the currents for four years.

The study took place on the continental slope, at depths of up to a mile and a half. The slope is steep, and sharp canyons notch into it. Sediments tumble down the slope and through the canyons.

At the bottom of the slope, the currents generally flow from south to north. And in the canyons, they generally flow downhill. Speeds range from about a half to one-and-a-half miles per hour.

But researchers found a lot of variation. The speed changes, and so does the direction. Currents can even reverse direction—even in the canyons, where they sometimes flow uphill. Some of the changes are related to the tides or to passing storms or eddies. And others are related to the seasons, so they play out over days or weeks.

The researchers say a better understanding of sea-floor currents can tell them more about where ocean sediments come from. That can help them better understand changes in climate, the sources of pollution, and more—swirling along at the bottom of the sea.

In the spring of 1956, a doctor in the Japanese village of Minamata reported an outbreak of a troubling new disease. It was seen mainly among children, and it affected the central nervous system. The disease quickly spread, with hundreds of cases reported, then thousands. It took years for scientists to work out the cause: poisoning from industrial pollution in Minamata Bay—the first known case of a disease caused by polluted seawater.

A chemical factory was pumping huge amounts of wastewater into the bay. The water was laced with mercury. Some of it was methylmercury—an especially nasty form.

Microscopic organisms gobbled the stuff up, then were eaten by larger organisms. The amount of mercury built up to higher and higher levels with each link in the food chain. So the fish and shellfish eaten by people were filled with it. That triggered Minamata disease. Symptoms included numbness, problems with vision and hearing, trouble walking, and tremors. The disease killed hundreds, and may have afflicted millions. And its effects are still being felt.

The company dredged the bay to remove contaminated sediments. And the nations of the world crafted a treaty to reduce the amount of methylmercury in the environment. It calls for less mercury in products and manufacturing, fewer emissions of it from coal-fired power plants, and better storage and disposal.

Even so, mercury and other chemicals still cause problems as they work their way up the marine food chain.

The parrotfish is like a house cleaner who does a great job of keeping things tidy, but sometimes breaks a glass. You want to keep them around, but you just wish they’d be a little less destructive.

For the parrotfish, the “houses” are coral reefs. They clean tiny organisms off the coral, keeping the coral healthy. But they also chip off pieces of the coral. If they chip away too much, they can damage the coral.

Parrotfish have strong teeth. They grind up the coral they chip off, then poop it out as grains that can wash up on the beach as white sand.

The scraping can scar the hard coral—the “skeleton” created by the living organisms inside, known as polyps. In many cases, the coral heals as new polyps move in. But in others, the coral can collapse. And if too many corals are destroyed, an entire reef can suffer.

Researchers spent a decade studying the coral-parrotfish relationship in four regions of the Caribbean Sea. They looked at individual corals, complete reefs, and wider areas that encompass many reefs. They also studied the parrotfish populations.

They found that more parrotfish generally meant more damage to the corals—but not always. Parrotfish prefer some species of corals over others. So in regions where those species weren’t as common—or where there was less variety of coral species—the damage was less severe.

The results may help managers control the parrotfish catch—perhaps improving the health of coral reefs across the Caribbean.

The exhaust produced by ocean-going ships can contribute to our warming climate. Most ships burn fossil fuels, so they spew out atmosphere-warming compounds. But some of their contribution to global warming may be a result of lower emissions—not of carbon, but of sulfur.

One of the compounds produced by burning fossil fuels is sulfur dioxide. Sunlight can cause it to interact with other compounds. That can yield droplets of acid rain, plus tiny grains of sulfur. Water can condense around those grains, forming clouds. The sulfur can stay in the air for days, so it can contribute to clouds for a long time.

The sulfur-based clouds are bright, so they reflect a lot of sunlight into space. That helps keep down the surface temperature.

In 2020, the International Maritime Organization passed some new regulations. It required shipping to cut sulfur emissions by 80 percent—reducing acid rain and cutting air pollution around ports.

A recent study looked at the possible impact that’s had on global warming. Researchers analyzed more than a million satellite images of ocean clouds. They compared those to maps of global temperature increases. And they used computer models to study what it all means.

The work found a big drop in ship-created clouds. And the drop correlated with areas of greater warming. The researchers concluded that the loss of clouds could have added about a tenth of a degree Fahrenheit to global temperatures—and could add more in the years ahead.

The many creatures that dig into the sediments at the bottom of the ocean are ecosystem engineers. Their burrowing, foraging, and even pooping change the ocean landscape—not just close by, but miles away.

Sediments have been described as the oceans’ compost heaps. They contain bits of rock and dirt washed out to sea by rivers. They also contain bits of organic material—everything from dead skin cells to the wastes of all the fish and other animals in the water above. And they’re loaded with bacteria and algae.

Many organisms spend much or all of their lives near the bottom—from shallow coastal waters to the deepest ocean trenches. That includes worms, fish, crustaceans, and others.

These critters dig burrows to protect them from predators or provide a safe haven for mating. They sculpt patterns in the soft sand or mud to attract mates. They poke through the sediments to scare up food. Some even scoop up the sediments, filter out tasty morsels, then poop out everything that’s not edible.

All of that activity changes things. It moves sediments from one spot to another. It scatters bacteria. It lifts eggs into the water. It brings nutrients to microscopic organisms.

The immediate effects are on a small scale—over a few inches or feet. But they add up. Most of the sea floor is covered with sediments, and as long as there is oxygen, there are animals burrowing and moving the sediments around. So the effects of all these ecosystem engineers add up.

About 12 million tons of plastics enter the oceans every year—the equivalent of a full garbage truck every minute. The total includes millions of grocery bags. But restrictions on the bags appear to be having a positive effect. Several studies have found big reductions in the number of bags found on beaches.

Plastic bags are a huge problem for ocean life. Animals can get tangled up in them. Birds and turtles mistake them for jellyfish and eat them. And fish eat bits of plastic if the bags fall apart. So reducing the number of bags in the oceans can save the lives of many creatures.

One study looked at the beaches in the United Kingdom. Governments there began cutting back on the bags more than a decade ago. Some of them banned the bags, while others required stores to charge for them. Since the restrictions went into effect, the number of bags picked up on the beaches has gone down by 80 percent.

There have also been big reductions in the United States. A dozen states have banned the bags, along with a couple of hundred cities and counties. Others require consumers to pay for the bags. A study by Ocean Conservancy found that volunteers picked up 29 percent fewer bags in 2022 and ’23 compared to the years before Covid-19. The numbers went way up during the pandemic as bag rules were suspended.

Millions of bags are still washing into the oceans. So birds, turtles, and other life still face a threat from this common form of trash.

Depending on which side of the country you live on, you probably either hate or love sea urchins. Off the coast of California, there are too many of the spiny creatures. They’re destroying kelp beds, harming the entire ecosystem.

But off the coast of Florida—and throughout the Caribbean Sea—there aren’t enough urchins. And without them, coral reefs are dying off.

Long-spined sea urchins used to be common in the Caribbean. They have black spines that can be up to a foot long. And their “teeth” are rocky plates that allow them to scrape algae from corals and other hard surfaces.

In 1983, a disease raced across the Caribbean. Within two weeks, it had killed 97 percent of the urchins. And in 2022, a parasite hit the still-recovering urchin population, wiping out most of the urchins on most reefs.

Without the urchins, the algae population has exploded. Algae can cover the corals, blocking the sunlight the living corals need to survive. The algae also coat the surfaces that young corals latch themselves to, preventing them from establishing new colonies.

Combined with climate change, ocean pollution, and other problems, that’s cut the amount of corals across the Caribbean by about 80 percent since the 1970s.

Today, scientists are raising urchins in the lab, then dropping them on reefs. It’s too early to tell how that’s working out. But researchers are hopeful that the efforts will begin to restore balance to Caribbean reefs.