Twice a day the sea gives the shore back, and twice a day it takes it again. The intertidal zone is the strip of coast between high and low tide, and the animals and plants that live there have to survive both worlds: pounded by water at one hour, exposed to air the next.
A tide pool in Monterey Bay National Marine Sanctuary. Credit: NOAA.
Where it is found
Intertidal zones exist wherever the ocean meets the land — from steep, rocky ledges to long, gently sloping sandy beaches and mudflats that can stretch for hundreds of meters.
Four zones, from dry to wet
| Zone | How often it is underwater | Who lives there |
|---|---|---|
| Spray zone | only during very high tides or severe storms; otherwise dampened by spray and high waves | — |
| High intertidal | at the peak of daily high tides, dry for long stretches in between | hardy life that withstands pounding waves: barnacles, marine snails, mussels, limpets, shore crabs, hermit crabs |
| Middle intertidal | tides ebb and flow over it twice a day | a greater variety of plants and animals, including sea stars and anemones |
| Low intertidal | almost always, except during the lowest spring tides | the most abundant life, protected by the water |
How life sorts itself out
Sea creatures arrange themselves up and down the shore according to how well they can compete for space, avoid predators from above and below, and resist drying out. Higher up, residents either close themselves inside their shells to stay moist and keep predators out, or move fast enough to retreat to the water when the tide goes out. Lower down, many plants and animals attach themselves in place and are very sturdy, very flexible or otherwise built to withstand the force of the waves.
Visitors on both tides
At high tide, larger animals such as seals, sea lions and fish come in to forage. At low tide, a great variety of shorebirds walk the exposed zone looking for their meals.
Sources
Based on "What is the intertidal zone?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from oceanservice.noaa.gov
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