What is the intertidal zone?



A tide pool within Monterey Bay National Marine Sanctuary

A tide pool within Monterey Bay National Marine Sanctuary.

Intertidal zones exist anywhere the ocean meets the land, from steep, rocky ledges to long,
sloping sandy beaches and mudflats that can extend for hundreds of meters. Four physical
divisions, each with distinct characteristics and ecological differences, divide the intertidal zone.
They are the:

  • Spray zone: dampened by ocean spray and high waves and is submerged only
    during very high tides or severe storms.
  • High intertidal zone: floods during the peaks of daily high tides but remains dry for
    long stretches between high tides. It is inhabited by hardy sea life that can withstand
    pounding waves, such as barnacles, marine snails, mussels, limpets, shore crabs, and
    hermit crabs.
  • Middle intertidal zone: over which the tides ebb and flow twice a day, and which is
    inhabited by a greater variety of both plants and animals, including sea stars and anemones.
  • Low intertidal zone: virtually always underwater except during the lowest of
    spring tides. Life is more abundant there because of the protection provided by the water.

Sea creatures arrange themselves vertically in the intertidal zone depending on their abilities to
compete for space, avoid predators from above and below, and resist drying out. Residents of the
higher intertidal zones can either close themselves up in their shells to remain moist and ward off
predators, or are mobile enough to retreat to a submerged zone when the tide goes out. In the
lower parts of the intertidal zone, many plants and animals attach themselves in place and are
very sturdy, very flexible, or otherwise well suited to stand up to wave energy.

Larger marine life, such as seals, sea lions, and fish, find foraging for food ideal at high tide in
the intertidal zone, while a large variety of shorebirds, looking for their meals, stroll hungrily
over the intertidal zone at low tide.


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