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Basking Sharks Look Ferocious, But Prefer Plankton to People

Basking shark
The basking shark (Cetorhinus maximus), which can reach up to 40 feet in length, is the second-largest living shark, after the whale shark. It's also one of only three plankton-eating shark species, along with the megamouth shark and the whale shark. George Karbus Photography/Getty Images

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The original "Jaws" hit theaters on June 20, 1975. During that blockbuster's 40th anniversary weekend in 2015, something fishy happened. A lone diver and two separate groups of fishermen all reported seeing a great white shark off the coast of Maine. But when a photograph was produced, experts realized the creature had been misidentified. This wasn't some seal-stalking great white. Instead, it was a basking shark — an even larger species with habits far removed from its better-known cousin's.

Capable of growing 40 feet (12 meters) long, the basking shark can weigh up to 5 tons (4.5 metric tons). Of all the fish alive today, it's the second biggest — outsized only by the mighty whale shark. It gets its name from its tendency to "bask" in the sun near the surface of the water.

Colossal animals have colossal appetites, but they don't always go after big game. Basking sharks are filter-feeders, gathering zooplankton and other tiny animals (like shrimp) in bulk as they roam the seas with mouths wide open.

Straining for Plankton, not Humans

Sharks, rays, skates and their relatives make up the elasmobranchs, a subclass of fish that's over 1,000 species strong. Filter-feeding is a rare strategy within their ranks; only 13 elasmobranch species are known to consume prey in this manner.

Whale sharks and basking sharks both crack the list, but they use different techniques. The former actively sucks water — and swimming prey — into its maw. Basking sharks don't bother.

At its widest point, a full-grown basking shark's mouth is about 3 feet (1 meter) across. When hunger strikes, the boat-sized fish open their jaws and push themselves forward. During this process, seawater passes through the gills at a rate of 1,500 tons (1,360 metric tons) per hour. Thousands of bristles on the gill slits, each measuring about 3.9 to 4.7 inches (10 to 12 centimeters) long, sift the waters and capture hapless victims.

Every 60 minutes, a 22-foot (7-meter) basking shark burns up an estimated 663 calories. Getting that kind of nutrition out of itty-bitty zooplankton may seem like a fool's errand. Yet huge sea creatures do it every day.

"Many of the largest animals on Earth filter feed like this — whale sharks and basking sharks, manta rays and the baleen whales," says David Shiffman, a marine conservation biologist who studies sharks, in an email.

"If plankton is abundant, which it often is when humans aren't messing things up, just swimming around with your mouth open in any random direction is a lot more energy efficient than having to chase and subdue prey — and there's a lot less risk of your prey escaping and you starving to death."

Well-traveled Fish

The Industrial Revolution was hard on these sharks. Fishermen in the North Atlantic (and other places) harvested the fish en masse for their sizable livers, the oils from which fueled lanterns, lubricated machines and lent themselves to cosmetics.

Today, the basking shark is considered endangered by the International Union for Conservation of Nature (IUCN). Actions have been taken to keep this species afloat, including a 2007 measure which barred commercial fishing vessels from hunting or trans-shipping the peaceful sharks in European Union waters.

Your odds of seeing one may vary by the season. Interested parties in Scotland and New England can observe the fish from springtime to autumn. Experts once thought basking sharks overwintered at the bottom of the sea, hibernating in underwater canyons and whatnot.

Tagged specimens have proven otherwise. At least some of the fish are migratory, following the zooplankton to lower latitudes or deeper waters far offshore when Old Man Winter comes a-knocking.

Lilian Lieber is well-acquainted with their habits. A post-doctoral fellow at Queen’s University Belfast, she’s led multiple studies on the species. “Basking sharks are highly mobile,” Lieber tells us in an email. Other scientists have observed tagged fish descending more than 3,281 feet (1000 meters) below the waves — and they’re fully capable of crossing the Atlantic.

“However, unlike some baleen whales, basking sharks don’t seem to have strict migration routes,” adds Lieber. “Their movements are highly driven by zooplankton dynamics … and their seasonal migrations are more variable.”

New research indicates basking sharks prefer the company of kin. Lieber was the lead author of a genetic tagging report published February 3, 2020, in the journal Nature. She and her colleagues gathered DNA samples from hundreds of individuals in the North Atlantic.

Specifically, they took mucus swabs. “Basking sharks have a relatively thick layer of dark mucus covering their skin and a sample of this mucus (‘slime’) contains their genetic signature,” Lieber says. “We used relatively cheap tools (mostly DIY) to sample basking sharks. For instance, we attached some sterile cotton clothes at the end of an extendable pole.”

Aboard small boats, the researchers would drift beside each fish and carefully “[swab] the pole in the region of their large dorsal fin.” The data paints an interesting picture: Apparently, sharks who forage together tend to have more genetic similarities than we might expect. Future conservation efforts may benefit from this discovery.

It's All Relative

Since we're talking about genetics, let's put the big fish in its taxonomic place. Basking sharks are part of the order lamniformes, a subgroup of sharks characterized by their spine-free dorsal fins and the lack of protective membranes over their eyeballs. Also, every known lamniform has five pairs of gill slits.

"This order contains some of the most famous species like the great white, and some of my favorite weird and wonderful species like the basking shark, the goblin shark, the megamouth shark (which has the coolest scientific name in the ocean, Megachasma pelagios, or 'giant mouth of the deep') and threshers," Shiffman tells us. "The world's fastest shark, the shortfin mako, is also in this order."

Still, none of those other fish have the great white's marquee value. The aforementioned Maine story wasn't an anomaly: Experts say boaters and beachgoers are constantly mistaking basking sharks for their smaller cousins. And that can provoke unrest.

"People should perhaps ask a scientist to confirm that something is a great white before posting breathless fearmongering headlines about scary sharks right off the beach, because a lot of times the photo is obviously of a basking shark," cautions Shiffman.

Basking sharks might not want to take a bite out of you, but — for reasons scientists don't yet understand — they sometimes breach out of the water. Three people were actually killed near Scotland in 1937 when their boat came too close to a leaping basker.

So remember, whether they eat plankton, seals or seaweed, all sharks deserve respect.

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