The upper limit of the barnacle zone is clearly determined by food availability. Barnacles live in this zone because, even during neap tides, the tide brings them food every day, twice a day. They must be able to sustain themselves on the amount of food that is delivered to them by the tide. The period of time that they are submerged is also important as this determines how long they can collect food. Multiple factors must be considered in the equation for success for barnacles - suitable substrate is a necessity, and speed of water movement is another important factor. Let's get to know the barnacle better...