The majority of inhabitants of the seashore are essentially aquatic organisms which have evolved directly from fully marine forms. If left uncovered to the air, they lose water by evaporation and will eventually die from dehydration. To make the transition from sea to shore successfully, forms which live in exposed positions on the surface of the shore must have some means of retarding the rate of water loss sufficiently to permit survival during the periods when they are left uncovered by the receding tide. The danger of drying is most severe where exposure to air also involves exposure to sun or wind.
Water loss can be fatal in several ways. Death may be due to disturbances of metabolism resulting from the increasing concentration of internal fluids. In many cases the immediate hazard is asphyxia. Some organisms require a continuous current of water over the gills for adequate gaseous exchange. Others can survive for a time in air, but all must preserve at least a film of water over their respiratory surfaces. In a fish out of water the weight of the gill lamellae unsupported by water causes them to collapse against each other and adhere by surface tension. So little area of gill is then left exposed for respiratory exchange that the fish asphyxiates despite the high oxygen content of the air.
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