Intertidal Rocky Shores Interactive Database

Intertidal Life

For an organism to survive in any environment, it must meet its food, oxygen, space and shelter requirements. In addition to these, organisms on a rocky shore must be able to tolerate the physical stresses unique to this environment.

As the marine environment gives way to the terrestrial further up the shore, the severity of most physical stresses increases. See Figure 1 below for details.

Figure 1. Environmental changes on the rocky shore

This variation in the degree of stress along the shore can be related to the movement of the tides up and down the shore. Extreme upper areas of the shore can be exposed to the air for periods of up to two weeks. The higher an organism's position on the shore, the greater the exposure time and stress.

The tide comes in and out almost twice in 24 hours, leaving the higher reaches of the shore exposed for much longer than the lower areas.

Only extreme high tides which occur approximately every two weeks will reach the upper part of the shore. Marine animals and the few plants living there may be submerged for only a few hours every fortnight! In this time, animals such as the blue periwinkle must obtain most of their energy requirements and undertake reproduction.

Blue periwinkles

Out of the water...

Due to the lower density of air when compared with water, temperature fluctuations are far more rapid and extreme for exposed animals. Radiation is more intense and the potential for water loss is heightened with the drying effects of wind.

Under the water...

A high tide brings to the rocky shore a set of physical parameters which are less variable; there is little temperature fluctuation, no wind effects and relatively little radiation reaching the organisms. Waves generated by winds at sea however can be a major survival challenge with the stresses from waves being greatest on the lower part of the shoreline.