SOME SPECIES OF SHARK HAVE TO SWIM THEIR ENTIRE LIVES, OTHERWISE THEY’LL DROWN
This is a fact which is something well known at my place of work where we have several species of shark. However, not all sharks have to keep swimming to keep from drowning.
“That there is some gooood oxygen…..”
Let’s take a look at how sharks and other fish breathe…
To breathe, sharks must remove oxygen from the water around them. The water enters the shark’s mouth (the shark’s nose is used exclusively for smell) and flows over the gills. Inside the gills are hundreds of feathery gill filaments. Each filament in turn has thousands of leaflike lamellae, or flaps, which contain bloodvessels. The blood absorbs the oxygen from the incoming water, and the excess water flows back out the shark’s body through gill slits. Sharks have five to seven pairs of gill slits, depending on the species.
With this method, sharks can extract about 80 percent of oxygen out of the meager 1 percent of oxygen that’s present in the water; to compare, humans have 21 percent of oxygen available in the air, but take in only about 25 percent. To maintain a steady flow, though, the shark constantly needs to be taking in water.
But does it have to constantly swim to take in this water? Scientists thought so because other fish seemed to have the equipment to actively pump the water through their mouth and over their gill slits, whereas sharks looked less developed. However, it turns out that not all sharks have to stay moving to breathe.
The oldest sharks, the modern sharks’ ancestors, didn’t have to constantly swim to breathe. Rather, they all pumped water through their mouth and over their gills. This method is known asbuccal pumping, named for the buccal, or cheek, muscles that pull the water into the mouth and over the gills. Many sharks retain this method today, such as nurse sharks, angel sharks and carpet sharks, also known as wobbegongs. Skates and rays, the shark’s cousins, also breathe this way. These species tend to spend most of their time lying on the bottom of the ocean floor.
In addition to an inactive lifestyle, there are some additional bodily differences that allow these sharks to breathe by buccal pumping. For example, many of the sharks that practice this method are dorsoventrally flattened (or squashed along the length of its back), like the angel shark. They have stronger muscles in the face. These sharks might also have a more prominent spiracle, which is a tube behind the eyes. When a shark is buried at the bottom of the ocean floor and can’t breathe through its mouth, the spiracle acts like a mouth by pulling in water. The water then exits through the gill slits.
As sharks evolved and became more active, however, this method of pumping became secondary. It was simply more energy efficient to take in water while swimming, in effect “ramming” the water into the mouth and letting it flow out through the gills slits. This method of breathing is known as ram ventilation. Most sharks can alternate between buccal pumping and ram ventilation, depending on what they’re doing. When they start swimming fast enough to force the water in more quickly than they could pump it, then they stop pumping. The sand tiger shark is an example of a shark that switches back and forth.
Some sharks, however, have completely lost the ability to breathe by buccal pumping, and these are the sharks that will indeed drown if they stop swimming and ramming water. These sharks are known asobligate ram breathers (or obligate ram ventilators); only about two dozen of the 400 identified shark species are required to maintain this forward swimming motion. These include the great white shark, the mako shark, the salmon shark and the whale shark.
So 24 hours a day, 365 days a year from birth to death, these sharks are constantly swimming to stay alive….