Water as a respiratory environment
Terrestrial organisms live in an oxygen-rich environment, but water contains considerably less oxygen than air – less than 1% by volume, as opposed to over 20% for air. Flowing or turbulent water may be well mixed, so oxygen may be somewhat evenly distributed. Still water, however, may have more oxygen at the surface due to diffusion from the air. Some fishes take advantage of this by coming toward the surface to breathe when oxygen is limited. For example, Sailfin Molly (Poeciliidae) use aquatic surface respiration(ASR) as well as an increase in ventilation frequency to cope with hypoxic conditions (Timmerman & Chapman 2004).The use of ASR diminishes, however, after a period of acclimation to the low oxygen conditions.
Gas solubility in liquids diminishes with increasing temperature. Warm water, therefore, contains less oxygen than cool water, making the challenges of meeting metabolic needs far greater for warm water fishes. Fresh water can hold about 25% more oxygen than sea water due to the diminished solubility of gases in water as the concentration of salts or other solutes increases. This salting out effect is true for all water solutions, including natural aquatic environments, blood plasma, cytoplasm, or a glass of carbonated beverage (just add some table salt and see what happens). The combined effects of temperature and salinity make oxygen availability especially low in warm, marine environments.
The relatively high density and viscosity of water means that more energy is required to simply move water acrossthe respiratory surfaces than is true of air. A fish may use as much as 10% or more of the oxygen that it gets from the water simply keeping the breathing muscles going(Jones & Schwarzfeld 1974), whereas for air-breathing animals the relative cost is much lower, around 1–2%.
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