The Effect of Temperature on Shore Organisms
The temperature around an organism, such as a limpet or a serrated wrack, might seem like an obvious factor to suggest as being important in controlling where it lives. How can we find out? First of all, how do the temperatures vary from place to place, from time to time and in the sea and the air?
Water retains heat much better than the air and therefore organisms at the bottom of the shore are buffered against major changes, those at the top have to tolerate daily swings in temperature which can be many tens of degrees. The limits are; very high temperatures in summer at the top and very low ones in winter in both the sea and on land.
All rocky shore organisms are restricted by their ability to withstand, at the one extreme, frozen water, and at the other, high summer temperatures
Seawater remains at a far more constant temperature than the land, (seawater varies between 5° and 15° Celsius, whereas the land temperature varies between below freezing in winter and 30° C plus in summer) so species that are immersed in seawater for long periods of time are buffered against large temperature changes. The temperature of the surroundings also affects the rate of metabolism; very cold conditions will slow it down, whereas very high temperatures may denature vital enzymes. Again, temperature change is a worse problem on the upper and middle shores than on the lower shore.
So, our thoughts really need to centre around the temperature tolerance of upper shore species, and the upper limits of others. What do we know about this?
An AWFUL lot is the simple answer! Much experimentation has been carried out on the temperature tolerances of shore organisms. More details can be found in the species specific pages and in some references below.
Newell, R.C., Biology of Intertidal Animals (1970). A comprehensive book on this topic at the time, chapter 9 deals with temperature. Sadly out of print and somewhat dated.
Along with thermal stress goes the risk of water loss by evaporation and consequent desiccation. This a complex issue because the more water an organism can loose the more it will cool down. Thus it has been shown in the rocky shore woodlouse (Ligia oceanica) that it can keep its body well below environmental temperature in dry air, but not in saturated air. This is a classic example of living things having to strike a balance between different and conflicting activities. Another would be in leaves where the opening of stomata lets in Carbon Dioxide but also lets out water. Further information about this can be found on species specific pages.
The majority of the work on desiccation and its role in zonation has been carried out on algae of the intertidal zone. In its simplest form it shows us that organisms at the top of the shore seem to be able to withstand desiccation much more readily than those lower down. For example, the high shore periwinkle Littorina neritoides has been shown to be able to withstand desiccation for as long as 5 months!
Of course, to suggest that upper shore species will be resistant to desiccation and actually showing it has an bearing on their distribution are two different things. Experiments by Broekhuysen in South Africa (1940) confirmed people's suspicions about this relationship however. Similar experiments by Micallef, (1966) on the mollusc group called trochids showed similar results, but there are some interesting lessons to be learnt about how complicated real life really is from this study!
One of the most interesting and widely quoted studies of distribution/zonation on a rocky shore is that shown by the barnacles. Here, clearly littoral species close their valves and withdraw the cirri, but sub-littoral species do not and soon dry out when in the air (for a diagram of barnacle anatomy click here).
Another genus which ranges across the shore is Patella. A study of the upper and lower shore Patella vulgata and the lower shore P. aspera was done by Davies and gave very interesting, although quite predictable, data.