DO SHARKS HAVE TO SWIM CONSTANTLY IN ORDER TO BREATHE? The answer is yes – for the relatively small number of shark species that excite us the most, like great whites and hammerheads.
But it’s not the case with most of the 500-plus species of sharks in the oceans, like the familiar nurse shark and lesser-known species like bullhead, angel and carpet sharks.
WATER, OXYGEN & GILLS
Like all fish, sharks breathe by extracting oxygen from water that passes through their gills. Sharks have been around for more than 400 million years, and the earliest sharks drew water into their mouths and through their gills for the gas exchange by pumping cheek muscles called buccal muscles.
Buccal pumping has survived in many species of modern sharks, particularly in bottom-dwelling sharks like the nurses and carpets. Skates and most rays flourish via buccal pumping, as well (as do bony fishes).
These are animals that receive far less cable television attention, probably because they tend to be less active. As sharks that don’t have to swim continuously, they spend much time resting on the seafloor – not a good strategy for TV ratings.
They’re re aided by the presence of spiracles, openings on the tops of their heads that they can use to pump water over their gills even when still. Some sharks – prime example: the angel shark – that have a habit of not just lingering on the seafloor but hiding in the sand. For this, spiracles come in handy.
ALONG CAME RAM VENTILATION
Over time, more active species evolved, prizing speed, pelagic seafaring and aggressive pursuit of prey over sedentary lifestyles. For them, it took less energy to let water pass over their gills as they swam than to rely on buccal pumping. This is “ram ventilation,” as in “ramming” water into their gas exchange systems.
Most sharks, like tiger sharks, sand tiger sharks (a different species) and lemon sharks can alternate breathing techniques. When swimming fast enough, they can cease buccal pumping and rely on ram ventilation.
OBLIGATE RAM VENTILATION: WHEN SHARKS HAVE TO SWIM
But about two dozen species are in the category of sharks that can’t switch and do require constant forward motion to breathe – “obligate ram ventilation.”
They’re mostly pelagic – great whites, hammerheads, whale sharks, makos and the reef sharks common encountered during reef dives. For them, it’s swim or die.
EXCEPTIONAL CIRCUMSTANCES
Which isn’t to say that obligate ram ventilators can’t get their rest. For one thing, like many species of bony fish, some are able restore themselves by “resting” rather than actually sleeping. Studies have suggested that a shark’s swimming movements are coordinated by its spinal cord rather than its brain, an arrangement that would enable it to continue to swim while resting its brain.
And it’s also been suggested that there are circumstances in which ram-ventilating sharks don’t have to swim continuously. They may be able to lie in one spot on the seafloor and let a strong current do the job of sweeping water over their gills.
And, then, there’s the Caves of the Sleeping Sharks in Isla Mujeres, Mexico. There, reef sharks have been observed lying motionless in water with a very high oxygen content and low salinity that makes motionless breathing easier.
HUMAN THREATS
Sharks have challenges enough just living the world’s oceans but human activity has increased their peril. Pelagic sharks captured even inadvertently in fishing fleet nets face suffocation because of their inability to swim away.
And, the barbaric practice of shark finning – slicing off a captured shark’s fin and returning the living animal to the water – has the same result. Unable to swim, their end from suffocation is inevitable. All of us should contact our government and support legislation to end this atrocity.
PRINCIPAL SOURCES: “Will a shark drown if it stops moving?,” Molly Edmonds, HowStuffWorks; “Must Sharks Keep Swimming to Stay Alive?,” Joseph Castro, LiveScience; “Exploding Myths – Island of the Sharks,” Nova; “Sharks: FAQ’s,” Florida Museum of Natural History; “Shark Anatomy: From The Outside In,” SharkSider; “Shark,” Wikipedia.