In order to avoid being washed out to sea, sea anemones have developed the ability to shrink in order to reduce the force of water on their bodies. As they shrink, the force from the flowing water becomes smaller and will not dislodge them. They also have the ability to expand their bodies to increase the area exposed to the flow. When it is feeding time, this expansion maximizes food intake as the anemones filter small food particles from the flow with their tentacles. Sea anemones do not feed when the water flow becomes too fast.
Sea anemones can change their shape because of their skeletal system.
When anemones shrink, they reduce the area exposed to the water flow and the drag force the flow creates on their bodies. Imagine moving your hand through water. If you try to pull your open palm against the water, you can feel the drag force created by your hand coming in contact with the water. Now if you pull the edge of your hand through the water, you will feel less drag. That's because drag force is directly related to area.