Aquatic respiration

Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water.

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Earth's natural bodies of water have a low oxygen concentration--much lower than the level of oxygen in air at the earth's surface. Smaller organisms can obtain sufficient oxygen through the skin (e.g. flatworms), but larger organisms require special structures to collect enough oxygen to sustain life. This oxygen comes from molecules of oxygen gas (O₂) dissolved in the water. The oxygen atom present in the water molecule (H₂O) is not suitable for respiration.

Fish have developed gills for respiration which have:

• large surface area
• high blood flow
• small/short diffusion distances
• contain 4 gill arches (Bony fishes), two gill arches (Cartilaginous fish) or 7 gill baskets (Lampreys) on each side of the fish's head
• each gill arch has 2 rows (hemibranchs) of gill filaments
• each gill filament has many lamellae

The operculum in fish is a long bony cover for the gill that can be used for pushing water. Some fishes pump water using the operculum. Without an operculum, other methods are required, such as ventilation. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.

Bony fish use a type of countercurrent flow to maximize the intake of oxygen that diffuse through the gill. Countercurrent flow is when deoxygenated blood moves through gill in one direction while oxygenated water moves in the gill in the opposite direction. This mechanism maintains the concentration gradient and increasing the efficiency of the respiration process as well. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that a bony fishes have.

The genesis of the aquatic respiratory rhythm

One of the main caracteristic of aquatic respiration is that, just like terrestrial respiration, it is rhytmic. In fact, aquatic respiration consists in an evergoing alternance between an inspiratory phase and an expiratory phase. During expiration, gill muscles are contracted, which expulses water from the gill baskets. During inspiration, muscles are relaxed, and the elasticity of the gill baskets that have been contracted during the expiratory phase stretches them, causing fresh water to enter. The duration of one respiratory cycle can vary for different species and during different moments of the day from 0.5 seconds to 4 seconds.

Scientists have investigated what part of the body is responsible for maintaining the respiratory rhythm. They found that neurons located in the brainstem of fishes are responsible for the genesis of the respiratory rhythm (Rovainen, 1985). The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which caused debates about the homology of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this rhythm are still not completely understood (see Involuntary control of respiration).

Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body. Similarly to what is observed in mammals, fishes breath faster and heavier when they do physical exercice. The mechanisms by which these changes occur have been strongly debated over more than 100 years between scientists (Waldrop, Iwamoto and Haouzi, 2006). The authors can be classified in 2 schools :

1. those that think that the major part of the respiratory changes are pre-programmed in the brain, which would implie that neurons from locomotion centers of the brain connect to respiratory centers in anticipation to movements.

2. those that think that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. This would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.

Most scientists now agree that both mechanisms are probably present and complementary.