The seaweeds carry out photosynthesis, and the animals start eating. When the tide goes down, they all stop. You could sit there all day with a stopwatch timing how long they are all covered in seawater. The higher up the seashore an animal lives, the shorter its feeding time becomes. What is much worse than not being able to feed is getting hot and dried up when the tide has gone down.
Spring tides happen every two weeks.
The spring tides in the spring and autumn are very big indeed. Perhaps the most difficult place to live is up in the splash zone. Here is a zone of Pelvetia canaliculata, it is very near the top of the shore. Perhaps there would be too much competition for light, food or space. Some organisms just cannot cope with all that competition.