Aquatic respiration refers to the process whereby an aquatic animal obtains oxygen from the surrounding water. The level of oxygen in water is low, and can be very low.
Very small organisms can obtain sufficient oxygen through the skin (e.g. flatworms) but larger organisms must have special structures, such as gills, and must have ways of increasing water flow over those structures.
contain four gill arches (bony fishes) on each side of the fish head
each gill arch has 2 rows 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. Sharks use this system. When they swim, water flows into the mouth and across the gills. Relying on this technique means that a shark can never stop swimming.
Fish use a type of countercurent 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.
Pulmonary respiration is usually supplemented by rapid throat pulsations (buccal pumping) that push air over the vascular lining of the buccopharyngeal (mouth and throat) region, and into the lungs, while others use their nares for ventilation.
Aquatic, globular type egg masses can obtain sufficient oxygen in cooler waters because the higher oxygen level is able to penetrate to the innermost eggs in the mass.
Aquatic eggs produce small amounts of urea, as the toxic ammonia is diluted through diffusion of water into the perivitelline fluid.
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