The problems many animals face with depth compensation are associated with breathing and buoyancy. At greater depth, the pressure exerted on a body increases. Dramatic pressure changes can cause discomfort and death for animals that have air spaces within their bodies because gases compress as pressure increases (at depth) and expand as pressure decreases (at altitudes).

When diving or flying, we become acutely aware of the air spaces in our bodies -- especially our sinuses and Eustachian tubes. The feeling you get when your ears "pop" depends on your distance from sea level and the partial pressure of gases (air) at various altitudes and depths.

Air breathers, such as humans, get their oxygen from gas vapor (air), then exchange that gas across moist tissues lining the lungs where oxygen gas must dissolve into liquid before being used throughout the body. Water breathers, such as lobsters and fish, get their oxygen pre-dissolved in water. The dissolved oxygen is taken up directly by tiny blood vessels located in the gills.

For animals with gills, gas exchange (breathing) does not present a significant problem as pressure increases with depth because the breathing apparatus does not include open air spaces.

However, an animal with lungs has air sacs that get smaller and larger with respective increases and decreases in depth. Air breathing animals need time to compensate for changes in pressure before coming up to the surface from depths deeper than 10 meters (33-1/3 feet or depth at one atmosphere of pressure). Because lobsters and fishes do not use air-filled breathing structures, gas exchange does not present a problem for them when being brought to the surface from depths.

Buoyancy, like breathing, can be dependent upon air spaces. Our lungs make us positively buoyant so we float. We can achieve neutral buoyancy (weightlessness) by carrying heavy objects such as lead weights used by scuba divers.

Many fishes have air or swim bladders (gas-filled sacs) within their bodies to maintain neutral buoyancy at various depths. These structures enable fishes to control their location at depth within the water column.

When fishes with swim bladders are brought to the surface from depths greater than 10 meters, they will often swell up. The swelling is due to the expansion of gases with decreasing pressure. This is not a problem for harvested fish because the flesh will taste just as good, but it can be a problem for fish conservation because juvenile fish and nonharvested species may not recover from the trauma.

Lobsters have no problem coping with changes in depth because they simply do not have air spaces within their bodies. They use gills to breathe and, since they have no need to maintain themselves at certain depths within the water column, they are negatively buoyant and sink alive to the sea floor.