How Do Tide Pools Work? Zones, Tides and the Animals Inside

How do tide pools work? A tide pool is a hollow in solid rock that keeps a pocket of sea water after the tide drops, so the animals inside stay wet while the rock around them dries in the air. The whole shelf between the highest and lowest tide lines is called the intertidal zone, and everything living on it is sorted by how long it can survive out of the water.

That sorting is why a reef looks striped. Barnacles up high, a black band of mussels in the middle, sea stars and anemones below them. For the best spots and timing, read our guide to tide pools in Marin. This page explains the biology underneath them.

What Makes a Pool Hold Water

Not every rocky shore has pools. You need rock that does not drain, a surface close to flat, and a low tide that falls below the rim of the basin.

Flat shale and sandstone benches do this well. The rock splits into ledges, channels and shallow basins, and each basin keeps whatever water sat over it when the sea fell away. The shelf below Agate Beach is a good example of that shape. A sand beach cannot hold a pool at all, because the water sinks straight through.

Depth and shade decide how hard life gets inside. A deep pool under an overhang barely changes between tides. A shallow one in full sun can heat up, lose water to evaporation and turn noticeably saltier in a couple of hours.

The Intertidal Zone, Band by Band

Biologists divide the shore into four bands. The intertidal zone runs from the highest line that spray reaches down to rock that only the lowest tides uncover.

Zone	Time under water	Who lives there
Splash zone	Wet from spray, rarely submerged	Periwinkle snails, limpets, lichens, shore crabs
High zone	Covered at high tide, dry for hours	Acorn barnacles, limpets, rockweed
Middle zone	Covered and uncovered twice a day	California mussels, goose barnacles, aggregating anemones, hermit crabs
Low zone	Uncovered only on the lowest tides	Ochre sea stars, giant green anemones, chitons, urchins, surfgrass, nudibranchs

The height of each band is not a fixed number of feet. It depends on the local tide range and on how much wave spray the shore gets, so the bands ride higher on an exposed outer coast like Point Reyes and sit lower inside a sheltered bay.

Why the bands have such sharp edges

Two different forces draw the two lines.

Drying sets the top. An animal can only live as high as it can survive without water, because once the tide leaves it has to shut tight and stop feeding. Above its ceiling it overheats or dries out.

Other animals set the bottom. Lower on the rock the conditions are easier, so space is crowded and predators are busy. Mussels smother barnacles. Sea stars eat mussels. The ochre sea star works along the bottom edge of the mussel bed and holds that line, which is why the black band stops where it does.

The top of each band is set by physics, the bottom by biology.

How the Animals Survive the Hours Out of Water

Most of them simply shut and wait.

Barnacles close the plates on top of their shells. Mussels clamp shut and keep a film of sea water inside. Limpets press down onto the rock, and many return to the same spot every time, where the shell has worn a groove that seals well.

Aggregating anemones pull their tentacles in and stick sand grains and shell fragments to their column, which shades them and slows evaporation. Seaweeds like rockweed go limp and leathery, then rehydrate when the water comes back.

Animals that cannot dry out at all stay in the pools. Hermit crabs, shore crabs, small sculpins and sea slugs ride out the low tide in whatever water is left.

What Happens Inside a Pool While the Tide Is Out

A cut-off pool is not a calm aquarium. Everything in it starts drifting away from ocean conditions the moment the sea leaves.

Sunlight warms the water. Evaporation raises the salt. Rain does the opposite and dilutes the surface. Algae pump out oxygen through the day and consume it after dark, so the same pool can be oxygen rich at noon and short of it before dawn.

High pools swing the hardest, which is part of why they hold only a few tough species. Low pools spend most of the day under the sea and keep the crowded, colorful life that people come to see.

Which Tides Actually Open the Pools

The low zone is the reward, and it only appears on the biggest lows of the month. A minus tide drops the water below the zero mark on the chart and uncovers rock that stays submerged on ordinary days.

Those lows arrive on a cycle. Spring and neap tides covers the fortnightly rhythm: the range is widest near the new and full moon and narrowest in between. If the numbers on a chart are new to you, start with how to read a tide chart.

Weather shifts the real water level away from the prediction. Wind and air pressure can hold the water a little higher or lower than the table says, which our guide on tides and weather explains. On a borderline low that difference decides whether the low zone shows at all.

Tide Pool Marine Life Worth Knowing

The giant green anemone gets its color from single-celled algae living in its tissue, so the brightest ones grow in well-lit pools and shaded ones look pale.

Chitons are flat oval molluscs with eight overlapping plates, and they graze algae off bare rock at night. Ochre sea stars come in purple, orange and brown on the same reef, and the color has nothing to do with age or sex.

Look for surfgrass and pink coralline algae at the lowest edge. Where you find them, you are standing at the top of the low zone, and anything below is usually under water.

Where to Go Next

Pick a date with a minus low, then check the hour on the spot page for your reef. The Duxbury Reef tide chart is the one to start with, since it is the largest and most forgiving tidepool shelf in the county: see Duxbury Reef tides for the next week of highs and lows.