The seashore is ever changing because it is subjected to harsh conditions, extreme temperature shifts occurring during the day and night, as well as in the summer and winter. It is also affected by breaking waves, changes in salinity, the tides and the force of the winds.
On sandy and muddy shores, the majority of the marine organisms are filter feeders. They take in the water around them and filter it, extracting any bits of food the water may contain. In order for filter feeders to survive there must be a bountiful amount of food within the water and the water must flow fast for the food to be dispersed. These marine animals bury themselves in the sand or the mud during the day to protect themselves from the sunlight. Animals will dig themselves deeply into the mud until there is enough moisture for them to survive. As the tide flows outward animals are in danger of drying out. Some animals will make the muddy under-layers a temporary home while others will live there permanently.
The tide brings with it carcasses of animals, decaying seaweed and feces. This organic matter provides food for some organisms such as crabs and sea urchins. However, too much organic matter can become dangerous. The sand grains can bind together, which will stop the flow of water. The deeper layers of mud will lose their oxygen helping anaerobic bacteria to flourish.