FISH GOTTA SWIM…THEY ALSO GOTTA BREATHE. Or, more properly, they need to continuously restock their blood supply with oxygen from the surrounding water column to maintain the functions of living.
Key to how fish breathe is the constant streaming of water past thin, permeable membranes in their gills that enable the diffusion of oxygen from the water into the blood stream.
Fish maintain that flow of water by either of two methods – ram ventilation and buccal pumping, sometimes called active or normal ventilation.
HOW FISH BREATHE (I) – BUCCAL PUMPING
Buccal pumping (active ventilation) occurs when a fish pumps water through its mouth and over its gills. The term draws its name from the buccal cheek muscles with which they do this. You may sometimes hear the term “buccal cavity;” it’s simply a fancy term for the mouth, yours as well as fishes.’
Fishes that swim slowly, hold in place against the current, rest in place on the seabottom breathe this way this a lot. Skates, rays and nurse sharks, angel sharks and carpet sharks normally breathe this way.
HOW FISH BREATHE (II) – RAM VENTILATION
Ram ventilation is a simpler process in which a fish swims forward with its mouth open, taking in water that passes over the gills. The drawback to ram ventilation is that the fish has to swim continuously to be able to continue breathing.
A lot of sharks breathe this way, as do some large bony fish swimmers like tunas and billfishes.
Many fishes, including sharks like the sand tiger shark, can switch between buccal pumping and ram ventilation as the situation and their swimming speed dictates.
HOW FISH BREATHE (III) – OBLIGATE RAM VENTILATION
Some sharks and bony fishes, on the other hand, can’t accomplish buccal pumping at all and have to swim continuously to maintain the water/blood gas exchange. This is obligate ram ventilation. The bony fishes include the aforementioned tunas and billfishes. Obligate ram-ventilating sharks include great whites, makos, salmon sharks and whale sharks.
There’s an oft-stated “fact” that “sharks have to swim continuously to survive.” In reality, most don’t. Only some two dozen of the 400 or so known shark species are in the obligate ram ventilation category.
RAM VENTILATION EFFICIENCY
You might think that the need to swim continuously for ram ventilation would be quite inefficient in terms of a fish’s energy requirements, compared to sitting in one place and breathing by buccal pumping.
Research with remoras – bony fishes that can switch between the two fish breathing methods – found that maintaining the same volume of water/gill flow with buccal pumping consumed as much as 5.1 percent more energy for the fish than ram ventilation.
The gills in both cartilaginous and bony fishes are located on each side of the fish’s body, just behind the mouth. Water entering the mouth passes through the gills and exits through gill slits.
In cartilaginous fishes, the first pair of gill slits takes the form of spiracles, round openings just behind the eyes. This enables bottom-dwellers, like rays, to draw in water even when their mouths might be buried in the sediment.Sharks, rays and the like are characterized by gill slits opening directly on the surface, easily seen. It’s a vestige of their early appearance in the oceans, some 400 million years ago.
BONY FISH GILLS
With the later development of bony fishes, fish breathing achieved a more effective approach. With all their gills on each side contained in a single chamber, each chamber clears water though a single opening that’s protected by a cover called an operculum. When the fish opens its mouth, the operculum closes and the pharynx expands, drawing water in for passage through gill filaments for the gas exchange. Closing the mouth reverses the process, sending water out.
The filaments consist of lamellae, folds of thin plates that contain capillaries. The lamellae structure maximizes the surface for the maximum gas exchange. And, in fact, the blood in the capillaries and the water streaming through increase efficiency even more by flowing in opposite directions – countercurrent gas exchange. The exchange takes place because oxygen levels in the oxygen-depleted blood entering the gills are much lower than those in the incoming water.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Fish Respiration, Future Scientists, Texas A&M; Will a shark drown if it stops moving? Shark Breathing: Buccal Pumping and Ram Ventilation, Shark Sleeping, et.al., HowStuffWorks; Fish Physiology, Gill, Gill Slit, Remora, et. al., Wikipedia.