The Effect of Atmospheric Gases on Shore Organisms
The atmospheric gases are oxygen, carbon dioxide, nitrogen and water vapor. In addition, there are the variously called rare, noble or inert gases and various pollutants. The first four are the most important for living things. This is a good place to deal with respiration in shore organisms. The need for oxygen for this vital process is nearly universal amongst them, although anaerobic respiration (fundamentally, the release of energy from food without oxygen) is carried out by some. The process of aerobic (with oxygen) respiration is dealt here, here, anaerobic (without oxygen) here. This page provide lots of links and there is much else on the WWW for those who search!. Both processes are fundamental to most Advanced Biology courses.
One fundamental difference between aerobic and anaerobic respiration is the yield of energy, anaerobic is only about 1/20 as good at this as aerobic, so oxygen is very desirable. A feature shown by many shore creatures is the ability to reduce metabolism when oxygen is in short supply. Shore organisms show a range of adaptations, including an ability to respire without oxygen and to air breathe when the tide is out (air contains 100,000 more oxygen than water!!).
A supply of oxygen is not quite vital to all living things because of the existence of anaerobic respiration, but it is very advantageous because of the MUCH greater efficiency of aerobic respiration mentioned above. Shore organisms get their oxygen from the water and/or the air. The situation with regard to water is dealt with here, to air, below.
Many shore animals can obtain oxygen from the air, the most extreme being the terrestrial crabs (Gecarcinidae, see here). Other crabs, more characteristic of the shore can breathe air, as can the intertidal 'woodlouse' or sea-slater Ligia oceanica.
Intertidal mollusca can also breathe air, all of the Littorinids, the top shells and Dog-whelks have been shown to have the ability. Perhaps the most surprising air-breathers of all are the fish with the eels, blennies and gobys being the most well known (refs here and here).
In all these, however, the balance has always to be struck between gaining atmospheric air and losing water. The non-shelled forms such as the annelids and fish tend to seek humid places, under rocks and so forth, the shelled forms have more resistance to desiccation because of the shell. However, in all of them some anaerobic respiration will be needed at the most unfavourable times during the tidal cycle.
Apart from its relative inefficiency, anaerobic respiration leads to the build up, in animals, of lactic acid (in plants the waste product is ethanol). This can subsequently be either broken down when oxygen returns to release some of its energy, or disposed of. A considerable amount of research on the barnacles shows that much of the lactic acid made is excreted, the 'oxygen debt' that they accumulate during periods of anaerobiosis being only very partially 'paid off'.
We can summarise the situation as follows, in all but the highly adapted upper shore species, on the recession of the tide, high levels of metabolism are reduced to a quiescent state. Aerobic respiration maintains this, until dessication becomes intolerable at which point the shell shuts and anaerobic respiration takes over.