Adaptations are generally:

• Structural - eg. thick, impermeable shells to reduce water loss and radiation effects.
• Behavioural - eg. seeking small crevices for shelter to reduce the drying effects of the wind.
• Physiological - eg. an ability to lower metabolic rates during exposed periods to minimise oxygen consumption.

An organism with these types of adaptations (eg the blue periwinkle) is well suited to the stressful upper zones of the rock platform. It should be remembered that it can also cope with the physical parameters which exist at lower levels.

Why then are periwinkles only found at upper levels? The answers lie in the interactions which exist between the periwinkle and the other organisms along the tidal continuum.

If the periwinkle has similar food and shelter requirements as another snail (eg, the conniwink), they will be in competition for these resources. If the conniwink is a more successful competitor it will restrict the numbers of periwinkles in an area or exclude them altogether. Distribution patterns are therefore a result of adaptations and interactions.