Whale sharks are the world’s largest living fish, measuring up to 18 metres long, but somehow they can suspend themselves in an upright position despite having a body density that is greater than that of seawater. A study of captive whale sharks suggests this may be due to air they take in as they feed at the water’s surface.
Scientists have previously observed whale sharks (Rhincodon typus) feeding vertically in the wild. Although they often use slow fin movements to hold themselves in place while feeding on plankton at the surface, the sharks occasionally remain completely still without sinking or tipping sideways, says Taketeru Tomita at the Okinawa Churashima Foundation Research Center in Japan.
He and his team found that the whale sharks in their aquarium didn’t have to move all the time to stay upright. Fascinated, they then realised that the animals would suck in air from the surface while taking in prey. When they stopped sucking in air, they would sink.
To understand how the air helps them float, the researchers calculated estimates for body volume, mass and density for two sharks. They then equipped one shark with underwater ultrasound equipment, which detected air in its gill cavities when it was feeding vertically, but not at other times.
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Using this data, they created a mathematical model to determine how much air the whale shark would need to stay vertically afloat and immobile, based on its dimensions.
Their calculations revealed that, in theory, around 0.2 cubic metres of air would provide the necessary buoyancy for each of their study sharks, says Tomita. Considering the size of the sharks’ mouths, the researchers say the animals could easily suck in that much air each time they reach the surface.
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Becoming buoyant might not be the main reason why whale sharks suck in air – although it could be a useful side effect, says Tomita. “We suspect that water suctioning at the surface is an efficient way to gather food floating on the surface layer of the water, but we do not completely understand this mechanism.”
He says the sharks could have other tricks for staying upright, like perhaps shooting jets of water downwards out of their gills. Further hydrodynamic studies, like computerised water flow simulations, may provide additional clues.
Journal reference: Zoology, DOI: 10.1016/j.zool.2021.125932