The seashore

One of the most striking features of the shore is the rich diversity of plant and animal life to be found there. A wide range of invertebrates, some highly mobile, others fixed or sedentary, and shore fishes, are a characteristic feature. Brightly coloured lichens often form distinct bands on the high shore; seaweeds may be present in abundance, and on mud-flats flowering plants often dominate. Physical factors change rapidly and it is here that the student has the opportunity to observe and study the fascinating adaptations to this environment shown by both plants and animals.

Tides

The dominating force on the shore is the rise and fall of the tide. Tides result from the gravitational forces between the Moon and Sun and the seas and oceans on the Earth's surface. The tides with which we are most familiar in north-west Europe are semidiurnal, that is, there are usually two high tides and two low tides each day. This can be appreciated if the Earth is pictured revolving on its axis during the course of a day and passing through a water envelope which has been distorted by the gravitational forces of the Moon and Sun as inFig. 1.1. Because of the constantly changing positions of the Earth, Sun and Moon, the length of time between successive high tides is not exactly 12 hours but is about 12 hours 25 minutes.