Pacific Northwest Coast Intertidal Zones

Diversity of Tidepool Life Changes as Low Tide Ebbs

Jun 26, 2009

As the tide moves toward the minus point, barnacles and starfish appear first, followed by extreme low tide sightings of sea palms and purple urchins.

These animals and plants must adapt to a harsh life battered by pounding surf, followed by low tide exposure to sun, wind and predators. For some, the transition occurs daily. But for others, exposure comes only at the lowest of tides. The changing conditions give rise to life adapted to the different exposure conditions.

Spray Zone

Before the tide even begins to recede, exposed rocks may be covered with acorn barnacles, distinguished by their small white cone-shaped shells with a hole at the tip. These barnacles cluster in colonies and survive on the sea spray and occasional drenchings of high tide waves. Interspersed among the barnacles may be limpets, also small with a single conical shell.

High Tide Zone

A common seaweed seen when the tide is only beginning to ebb is rockweed. Rockweed is known as a brown seaweed, but appears olive- to yellow-green. It’s plentiful, covering rocks with pliable leaf-like blades. Swollen tips keep it afloat.

Mid-Tide Zone

Clinging to rocks are the starfish or “sea stars.” Generally yellow, orange or purple, starfish can be plentiful, often huddling together.

Green sea anemones also inhabit this mid zone. These beautiful flower-like animals with waving tentacles attach themselves to the bottom and sides of tidepools, where water remains captured in rocky depressions. If they’re left in a dry spot, they pull in their tentacles and close into a soft stubby ball. Anemones can grow quite large.

Little hermit crabs scamper through the tidepools, seeking out hiding places. Tiny fish, most likely sculpins, can also be seen darting about.

Beds of the bigger gooseneck barnacles often cover large expanses of rock, along with California mussels.

This zone also supports an array of seaweeds that can make the rocks extremely slippery.

Low Tide Zone

Sea urchins, looking like round pincushions with their purple or pink spikes begin appearing in tidepools as the tide ebbs even lower.

The beautiful, but less commonly visible, nudibranches may occasionally be glimpsed in tidepools here. The nudibranches’ common name “sea slugs” doesn’t do them justice, as they are decorated in colors from brilliant yellow to orange, red and white, often with markings or plumes of a contrasting color.

Seaweeds, including feather boa kelp and sea palms grow at the lowest exposed levels. Feather boa looks like the boas ladies once wore over their shoulders.

Beware of the sea palms, though. These graceful, but sturdy seaweeds look just like little palm trees, but they grow where surf is roughest. A stand of sea palms signals dangerous territory for beachcombers.

Exploring Pacific Coast tidepools and rocky shores can be exciting. Rules of the journey include caution when walking on rocks to avoid injury and to avoid damage to tidepool animals. In most places, tidepool animals are protected and should not be removed or moved.

Sources:

Tidepool & Reef: Marinelife Guide to the Pacific Northwest Coast, byRick M. Harbo, Hancock House Publishers Ltd., 1999.

Seashore Life of the Northern Pacific Coast: An Illustrated Guide to Northern California, Oregon, Washington, and British Columbia, by Eugene N. Kozloff, University of Washington Press, Seattle.

National Audubon Society Field Guide to the Pacific Northwest, by Peter Alden and Dennis Paulson, Alfred A. Knopf, New York, 1998.

The copyright of the article Pacific Northwest Coast Intertidal Zones in Marine Habitats is owned by Linda McDonnell. Permission to republish Pacific Northwest Coast Intertidal Zones in print or online must be granted by the author in writing.