Tidepools, also known as rock pools or tidal pools, are scattered along rocky shores in the intertidal zone—the area between low tide and high tide. They form when receding seawater from a high tide fills shoreline depressions. Each complete tide cycle comprises two high tides ("high high" and "low high") and two low tides ("high low" and "low low"). With each incoming tide, water replenishes these isolated pools with fresh oxygen and food.

Looking in on a tidepool, one can observe various hunting, feeding, fighting, and other survival behaviors of its inhabitants. It quickly becomes clear that life in a tidepool is tough. To survive, plants and animals must keep from being washed away by the waves at high tides, withstand the warming and drying effects of the Sun, and avoid being eaten.

Tidepools are divided into categories according to their position on shore relative to sea level. Pools in the lower two zones tend to be deeper and exhibit a greater variety of life. The low-tide zone gets exposed only a few hours every few weeks during special "minus" tides (at the time of the full Moon and the new Moon). Because this zone stays covered by water about 90 percent of the time, these tidepools offer inhabitants a more stable environment in terms of temperature and salinity. Urchins, abalone, and brown algae, all of which can survive minimal exposure to air, are common here.

The mid-tide zone, which spans in elevation from sea level to a couple of feet above, gets fully covered and uncovered each day. This is where sea anemones and different seaweeds and algae are most visible. These organisms, like most tidepool inhabitants, have no backbone. Their soft, flexible bodies absorb wave impact.

The high-tide zone gets pounded first by water, when the tide rolls in, and then by Sun, after the tide recedes. Tidepools that form here contain organisms specially adapted to high-stress conditions. For example, so as not to wash away with heavy waves and changing tides, mussels anchor themselves to surrounding rock, secreting a glue-like liquid thread that hardens so strong that they become nearly impossible to pry away.

Tidepools rarely form on shores exposed to heavy wave action, where spray from breaking waves extends the intertidal zone above the high-tide line. To tolerate the mostly-dry conditions of the splash zone, the smaller barnacles that live there retract their feeding appendages and seal themselves off to conserve moisture.

Apart from grazers, such as periwinkles, which feed on algae and other plant life, predation in tidepools is rampant. Certain whelks use acid and a sharp tongue to penetrate the otherwise protective shells of snails. Sea stars almost single-handedly control the mussel population. And, despite their delicate appearance, sea anemones kill small fish and crustaceans using deadly stinging cells from within their sticky tentacles.

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