​Each biochemical pathway, each structure, each behavior has a potential cost and benefit. The cost may be the allocation of nutritional resources or the risk of the behavior.

​Cost-benefit Analysis

​The benefits may be survival, producing and raising offspring, attracting a mate, outrunning a predator, catching prey.

​Cheetahs have to determine if the risk of not catching prey is worth the attempt. Each attempt takes a huge amount of energy.

• Density and viscosity

• Hydrogen ions (pH)

• Salinity

• Oxygen acquisition

• Depth/light

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Aquatic habitat considerations

​Density and Viscosity

​Density - d=M/V, how much of a substance is present in a given volume.

Viscosity - fluid's resistance to flow

Within an animal, the combination of muscle, fat, bone, air filled organs change that organisms density. This will determine if an animal is bouyant or if it sinks.

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​Animals and Density

​Density strategies

• Lung capacity

• Fat/blubber

• reduced bone structures

• swim bladders

​Viscosity strategies

Streamlined bodies help ogranisms move in the water.

The soft umbrella shapes of sea jellies allow them to slowly sink.

​pH - acidity

​Aquatic habitats absorb many contaminants from terrestrial habitats and the air. One impact from this is increased acidity. While aquatic organisms can often tolerate lower pHs, this buildup of acid through acid rain and CO2 absorption. Smaller fish seem to be able to deal with more acidic conditions, so we may see fewer and fewer larger fish if ocean acidification continues. (related to metabolic rates of fish of different sizes)

​Salinity

​Salinity Strategies

​Fresh water fish must excrete large amounts of water to maintain their internal balance.

Salt water fish must excrete a large amount of salts/solutes.

​Petrels, a pelagic bird, excretes excess salt through specialized tubes on its beak.

​Oxygen Acquisition

​Water runs over the gills and the oxygen dissolved in the water is absorbed by the capillaries.

​Marine mammals must surface for oxygen

​Depth & Light

​At greater depths, organisms deal with the lack of light and high pressure. Many organisms do not invest in colorful displays, but do invest in bioluminescence.