Tide Pool Ecology: Life on Rocky Shores

The intertidal: a home whose environment flips twice a day

Walk on a rocky shore and you find places that are seabed at high tide and dry land at low tide. This is the intertidal zone. As the tide rises and falls, the same rock moves between sea and air about twice a day. For living things it is among the most changeable homes in the world, with temperature, drying, salinity, oxygen and wave force shifting within hours.

Three zones: supratidal, intertidal, subtidal

For a first look at tide pools, see Tide Pool Exploration; for the ecosystem as a whole, see The Tide Pool Ecosystem.

A rocky shore is roughly divided into the supratidal zone (reached only by spray), the intertidal zone (moving between sea and air between high and low tide) and the subtidal zone (always underwater). Tide pools are hollows in the intertidal rock that keep seawater at low tide.

Tidal height sets the environment

According to materials from the Kanagawa Prefectural Museum of Cultural History, at low tide summer heat, winter cold, rain, river water and waves act directly on the intertidal zone, and the substrate and interactions between organisms then determine distribution over small areas. Even on one rock, a difference of a few tens of centimeters in height greatly changes the time spent underwater. This height is the main reason intertidal organisms line up in stripes.

Tide pools as small seas: water that changes within hours

At low tide, seawater is left in hollows in the rock. Unlike the open sea, a tide pool holds little water, so conditions become extreme. The main players are four: temperature, oxygen, pH and salinity.

Oxygen and pH rise by day and fall by night

A study on San Juan Island in the United States reported that in tide pools at low tide, dissolved inorganic carbon and alkalinity decreased while pH, temperature and dissolved oxygen increased. By day, photosynthesis by seaweeds and microalgae takes up carbon dioxide and releases oxygen. At night photosynthesis stops while animals and microbes keep respiring, so carbon dioxide rises, pH falls and oxygen declines.

Seawater is well buffered, so pH rarely reaches extreme values, but a study of summer salt marsh ponds reported daily swings in a range of about pH 7 to 8.5. Some tide pools have also been observed with pH above 9 in summer. Results vary greatly with pool size, height and sun exposure.

Salinity changes, but less than temperature

According to a report by WHOI researchers, the daily change in tide pool salinity is smaller than that of temperature or oxygen. However, salinity moves greatly for a time when heavy rain brings fresh water, or when strong sun evaporates water and makes it saltier. Pools higher on the shore are more affected.

Height and size create different worlds

A link has also been noted between pool elevation and the layering (stratification) of temperature, salinity and oxygen. A shallow pool high on the shore has a long wait until the next high tide, so it is exposed to extremes. A pool low on the shore is flushed quickly and stays close to the open sea. This is why pools on the same shore have different residents.

Enduring drying and heat: physiology and body design

The higher an organism lives, the longer it is exposed to air at low tide. A 2022 review by Leeuwis and Gamperl in Oceanography and Marine Biology: An Annual Review summarizes how animals of the upper intertidal cope with stresses of air exposure such as drying, heat, freezing, hypoxia and salinity change, through both evolutionary adaptation and flexible (plastic) responses.

Closing the shell to keep water in

Barnacles, mussels and limpets reduce water loss by closing their shells or plates. Some snails seal the shell opening to reduce evaporation. During low tide they stay still and lower their metabolism to save energy.

Heat shock proteins and antioxidant enzymes

It is known that heat shock proteins and antioxidant enzymes help organisms tolerate heat. Heat shock proteins protect proteins that become fragile in heat and help repair them, acting as molecular chaperones. Species living in more stressful places tend to ramp up such molecules faster.

Seaweeds that revive after drying out

Some seaweeds can recover water and restart photosynthesis when they are submerged again after severe drying. Sea lettuce (Ulva) and laver (Porphyra) are known examples. They survive low tide by drying out and photosynthesize and grow rapidly at high tide, a two-step way of living.

Tolerance has a cost

Raising stress tolerance has a price: less energy for growth, reproduction and immunity. Adapting to harsh conditions is not free. Because of this trade-off, no all-purpose species can endure everything, and different species divide up the shore.

Moving away: using the environment through behavior

When body design alone is not enough, mobile organisms get through by changing place. Unlike fixed barnacles and mussels, snails, crabs and fish can move with the tide and the sun.

Taking shelter in damp refuges

Mobile animals such as snails and crabs move at low tide to refuges such as rock crevices, under seaweed and damp shade. This avoids direct sun and drying. In the intertidal, the choice of microhabitat can decide life or death.

Vertical movement with seasons and waves

A study in Taiwan reported that a species of limpet has its distribution limited by physiological limits, namely summer heat and drying at low tide, and moves up and down seasonally with wave height to reduce drying stress. When waves are high, spray keeps higher levels damp, so the animals can reach higher positions.

Limpets that go home

Some limpets go out at high tide to graze algae on the rock and return to their own "home spot" before low tide. The strategy is to fit the shell tightly to the rock and protect water. This homing is also thought to rely on an internal rhythm that anticipates the tide.

• Fixed organisms: close the shell and cluster to hold water
• Mobile organisms: shelter in damp refuges
• Moving up and down with the tide to avoid drying

Why zones form: competition and predation set the lower limit

Why intertidal organisms line up in stripes has long interested ecologists. One famous answer is that the upper limit is set by physical stress and the lower limit by relations between organisms.

Connell's barnacle experiment (1961)

In a paper published in 1961, the American ecologist Joseph Connell studied two barnacles (Chthamalus and Balanus) on Scottish rocky shores. The fast-growing Balanus is weak against heat and drying and cannot live high on the rock. Chthamalus can live high up, but is excluded from the lower zone when Balanus takes the space. Connell moved Chthamalus to lower positions and showed that it survived without competitors but died when they were present.

However, this "physical above, biological below" pattern does not always hold, and later studies have revised and debated it. Conditions change with wave exposure and the presence of predators on each shore.

Paine's sea star removal and the keystone species

Another famous study is the experiment by the American ecologist Robert Paine. On a rocky shore in Washington State, he removed the predatory sea star (Pisaster ochraceus). Its prey, the mussel (Mytilus californianus), increased and nearly monopolized the rock surface, and seven other attached species were pushed out. Reported in a 1966 paper, this result showed that a predator near the top of the food web supports diversity of the whole community.

In a 1969 paper Paine proposed calling such a species a keystone species. A keystone is the stone at the top of an arch; if it is removed the whole structure collapses. At the same time, there has been critical examination of how the 1966 paper is interpreted and how the keystone concept spread, and in 2016 Lafferty and Suchanek discussed how the concept became widespread.

Organisms to see on Japan's rocky shores

If you go at low tide, Japan's rocky shores are an excellent place to check these ideas with your own eyes. Here are organisms that are easy to observe, seen through the lens of adaptation.

Common species in the upper to middle zone

Materials from the Kanagawa Prefectural Museum of Cultural History introduce the striped barnacle and a white-striped barnacle as typical of the upper to middle intertidal of inner bays, and note that another barnacle tends to be common in the lower part. Even close relatives live at different heights, because each species has different limits of drying and heat it can tolerate.

Safety and manners when observing

• Check low tide times and tide levels in a tide table, and always turn back before the tide rises
• Wet rock is slippery, so wear non-slip footwear
• Return organisms to where you found them, and put rocks back if you turn them over
• Follow the rules of places where collecting is banned or restricted, and where fishing rights apply

When observing, notice not only the stripes but also that the shell color and shape of the same species differ by place. In wave-exposed places some species have lower, flatter bodies, a clue to shape differences matching the environment.

Climate change and tide pools: beyond the extremes

Intertidal organisms that live in extreme environments are often assumed to be very tolerant. But many species are also thought to already live near their limits. So when water temperature rises or heatwaves occur, even a small change may have a large effect.

Living near the limit

As the Taiwan limpet study shows, summer heat and drying at low tide set the upper limit of distribution. If summer air or sea temperatures rise, the upper limit may move down and the habitable range may shrink. However, actual effects differ by species, place and wave exposure and cannot be generalized.

For sudden rises in sea temperature, see Impacts of Marine Heatwaves.

Combination with ocean acidification

Tide pool pH swings widely between day and night, so tide pool organisms may be more used to variation than open-sea organisms. Even so, it has been pointed out that acidification combined with heat could be a harsh condition for shell building in shellfish and barnacles. Research in this field continues.

Protecting the sea close to us

The intertidal is also the sea people can reach most easily. Trampling, overcollecting, hardening of shorelines and litter affect tide pools. The more people observe, the more important manners become.