During the low-tide period, the inhabitants of the rocky shore are subject to the conditions of temperature, etc., existing in the surrounding air, except in so far as the organisms can isolate themselves from the aerial conditions by possessing thick shells, living in pools, burrowing, and other methods. Such factors as temperature, salinity, and insolation, which limit or modify the well-being of the organisms, are present here with a generally wider range than in the sea and with more rapid fluctuations. For example, the temperature range in the air is usually much greater than in the water. The salinity of the interstitial fluid within the organism tends to rise with evaporation and to fall when rain floods it with fresh water. There is less protection from insolation, and particularly from ultraviolet radiation, than below the surface of the water. Different species vary in their ability to insulate themselves from such changes, and different habitats, even as close together as one in a crevice and one on the neighboring rock surface, have widely divergent external conditions. Finally, the much more rapid changes in conditions introduce the factor of duration of particular conditions. An elevated temperature or salinity may, for example, be tolerated by a particular alga for a period of 6 hours without ill effect but may prove lethal after 18 hours.
Temperatures on the rocky shore, in general, range between those of the sea and air, although, of course, an object in the sun may become hotter than the surrounding air. The rock and the organism itself act as heat reservoirs and help to reduce the temperature range. Evaporation of neighboring water, as well as that in the tissues, may help also to reduce temperatures. Few measurements of temperature in situ in the tissues have been made, but most of these, made on a hot day and during the low-tide period, showed temperatures well below that of the air.
Hutchins has shown the importance of maximum and minimum temperature in limiting the geographic range of a species, limits being set either to survival or to breeding. Such correlations between geographical distribution and temperature are well known. In the intertidal zone it is necessary to consider not only the sea temperatures but also those of the air, since the latter are likely to be the more extreme. Incidentally, the region of poor fit in the Irish Sea is a region where a marked extension of range appears to have taken place in the last few years. Isotherms such as this do not, of course, represent the true minimum temperatures that would be encountered in the intertidal zone; data on such minimum temperatures are usually unavailable at present. In the same way, available observations on sea temperatures do not usually refer to the actual intertidal zone, often being made in somewhat deeper water.