Picture 1: The oceans cover more than 70% of our planet. Photo credit: Wiki Commons
Sharks must breathe oxygen to survive. However, the logistics of obtaining oxygen is different than that of terrestrial animals. Sharks must extract oxygen from the surrounding water. For sharks to breathe, fresh seawater must flow across the respiratory organ known as a gill. Sharks have a pair of gills that are located internally behind the jaw, on both sides of the body. These organs are composed of numerous filaments of thin, highly vascular tissue. Each filament contains thousands of folds known as lamellae. Lamellae are important for gas exchange because they have a high density of capillaries. These capillaries contain blood with a lower oxygen concentration than the passing water. This allows the oxygen to diffuse into the blood stream of the shark. Furthermore, lamellae increase the amount of tissue that is in contact with the ambient water, strengthening the efficiency of gas exchange.
Picture 2: The great white shark is one of approximately two-dozen species of sharks that have evolved into obligate ram breathers. Other noticeable species include the shortfin mako shark, salmon shark, blue shark, whale shark, and porbeagle shark. Photo credit: Wiki Commons
Water must enter through the mouth of the shark in order to come into contact with the gills. Ram pumping is a passive process by which the shark opens its mouth while swimming. The swimming force drives water into the mouth, allowing seawater to flow over the gills. Ram breathing is the main pumping mechanism for pelagic shark species. These sharks swim for the duration of their lives. Consequently, some have evolved into obligate ram breathers. In other words, if they stop swimming, water will not pass through their mouth or flow over their gills. Ultimately, the shark will drown. In result, obligate ram breathers, will remain swimming even when they are resting.
This may seem counterintuitive to think that the shark must swim while resting. However, research has shown that the spinal cord, and not the brain, may coordinate swimming. This would allow swimming to occur even when brain activity is slowed. Furthermore, the body shape of pelagic sharks is next to perfectly streamlined. In addition, sharks are only slightly negatively buoyant, and thus the effects of gravity are minimal. Therefore, the energy to maintain slow cruising speeds is next to none. As a result, swimming while sleeping is not strenuous for these sharks.
Picture 3: The spotted wobbegong shark is an example of a benthic species of shark that uses buccal ventilation to breathe when stationary. Photo credit: Wiki Commons
Not all sharks will drown when they stop moving. Lying in a stationary position is a common behavior to many shark species, especially those of the benthos or bottom dwelling sharks. This can be advantageous for cryptic predation and antipredator behaviors, or for sharks that do not swim while they sleep. When stationary, the ram breathers will switch to a mechanism known as buccal pumping. Buccal pumping is an active process where the cheek muscles rapidly expand, drawing water into the mouth of the shark.
Picture 4 and 5: Examine the difference of head shape between the benthic leopard shark (left) and the pelagic shortfin mako shark (right). Photo credit: Wiki Commons
Buccal pumping is the ancestral breathing mechanism of sharks. Through time, pelagic species evolved a more streamlined body shape, resulting in less energy required for swimming. During this evolutionary process, the muscles of pelagic sharks became greatly reduced, or in some cases, lost all together (i.e. obligate ram breathers). As a result, it is common that benthic sharks have a larger, more rounded head than pelagic sharks.
Picture 6: Spiracles are another passageway for water to enter the respiratory system of benthic or bottom dwelling sharks when the mouth is buried in the sediment. Photo credit: Wiki Commons
It is not uncommon for shark species to have features known as spiracles. Spiracles are external openings located behind the eyes. Spiracles act in a similar manner to buccal pumping. The opening expands at a high rate, drawing water down a tube and across the gills. Spiracles are very common on benthic species because they allow the shark to breathe when the mouth is buried in sediment. They have essentially been reduced, ineffective or even lost in species that are constrained to pelagic waters, such as the obligate ram ventilators.
Picture 7: This oceanic blacktip shark is an example of a shark with 5-gill slits. Most sharks have 5 gill slits, while some have 6 or 7. Photo Credit: Scott Seamone
Once the water flows past the gills, it must exit the body. All shark species have between 5 to 7 gills slits located behind their head. Gill slits are external openings that allow water to escape from the shark after the oxygen has been removed.
As you read this, take a deep breath. You will notice the process of inhaling and exhaling air occurs through the same passageways. This is known as bi-directional respiration. Sharks are similar to us because they also must breathe oxygen to survive. However, sharks differ from us because they depend on a unidirectional respiratory system where water enters the mouth, flows over gills (where the oxygen diffuses into the blood stream), and then exits the body via gill slits in a single direction. Furthermore, sharks can only pump water through the respiratory system via the mouth. The nostrils of a shark have evolved into passageways used solely for smell. Sharks have evolved two main mechanisms to pump water through their body: (1) buccal pumping and (2) ram pumping. The style of pumping generally coincides with the sharks location in the water column, and showcases one of the many diversifying features found among these species.
Written by: Scott Seamone, ShARCC Intern