By Blaise Jones
Like most large marine organisms, sharks breathe through their gills. Gills are specialized organs that allow for chemical exchange from the surrounding environment and the animal’s circulatory system. Basically, they work like a checkpoint that allows certain chemicals that the animal needs, in this case oxygen, into the animal’s blood, while keeping others out.
Unlike bony fish however, sharks have to breathe the hard way. While bony fishes are able to pump water through their mouths and over their gills thanks to a mind-bogglingly complex variety of specialized bone structures and muscles in the skull, sharks have none of that and thus need to find other ways to breath.
Breathing Battering Ram
The vast majority of sharks are free-swimming, meaning that they are always on the move through the water column. These sharks rely on their own body movement in order to breathe. They constantly move forward with their mouths slightly agape, allowing water to flow through and over their gills. This is a process known as “ram ventilation.”
In fact, these sharks must keep moving for other reasons as well. Since sharks lack swim bladders that keep bony fish buoyant, if a shark were to stop swimming it would sink (5). Even more important than that is the fact that free-swimming sharks need to keep swimming in order to keep their blood flowing. Since free-swimming sharks tend to have small hearts, they rely upon the contracting of their swim muscles in order to keep their blood flowing .
However, free-swimming sharks do have a last ditch effort in case they can’t keep moving. By forcefully opening their mouths they can manually pump water over their gills. This gives these sharks a desperate, gasping appearance, which is entirely appropriate. Aside from the other factors, such as sinking and reduced blood flow, this process has the added issue of inefficiency. The process is exhausting and the sharks can only keep it up for so long. Eventually, they will run out of energy and drown if they can’t start swimming again.
However, out of all the shark species out there, only about 24 species must constantly be swimming in order to survive. Most sharks alternate between ram ventilation, and another, more ancient behavior.
Pump up the Jams
Ram ventilation is a relatively recent evolutionary occurrence in sharks. Their ancestors were more stationary and didn’t have to continue moving in order to survive. These sharks used a method called “buccal pumping.”
Buccal pumping involves opening the mouth with enough force to suck in water over their gills in large enough quantities to ensure enough oxygen is entering their system. These sharks had specialized cheek muscles that allowed them to suck in the needed amount of water with minimal effort. In fact, part of the reason these sharks were sedentary was because it lowered the amount of oxygen their bodies needed in order to function.
In modern times buccal pumping still exists, with the whitetip reef shark being a prime example of it. Other species, however, have evolved a modified version of buccal pumping using a very special physiological adaptation.
The mid-point between pure buccal pumping sharks and the more evolutionarily derived ram ventilators are sharks with spiracles. The spiracle is a modified gill slit had evolved extra structures that connect small opening, typically just behind the eye. This tiny orifice is most often found directly behind the eyes of the shark and can be mistaken for an ear. Sharks that have spiracles are able to efficiently pump water through them and over their gills with minimal effort, allowing them to remain motionless while exchanging oxygen with the environment. These sharks include nurse, wobbegong, and the angel sharks .
Special Spelunking Sharks
However, there are some special cases where even free-swimming, ram ventilating sharks can rest on the bottom. Certain underwater caves, such as those found near Isla Mujeres, have such a high amount of oxygen in them that free-swimming sharks can sit in them for hours with no apparent ill effects. These sharks came to be known as “sleeping sharks”, because when they were first discovered divers thought the sharks were asleep.
- “Sharks! The Mysterious Killers” by Downs Matthews
- “Marine Biology” by Jeffery S. Levinton
- “Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess
- Alan Moore, Director of the Coastal Marine Education and Research Academy