Tidal Pools, Intertidal Zone and Life Found There

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20120517-NOAA ocean intertidal_100.jpg
intertidal zone
Tidal pool are isolated pockets of seawater found in the ocean’s intertidal zones, areas where the ocean meets the land — from steep, rocky ledges to long, sloping sandy beaches and vast mudflats. The ones that get the most attention form in zones of rocky shoreline, sometimes only a few meters wide, where everything is covered and uncovered by seawater as tides ebb and flow each day. [Source: NOAA]

Formed in depressions along the shoreline of rocky coasts, tide pools are filled with seawater that gets trapped as the tide recedes. While these small basins at the ocean’s edge typically range from mere inches to a few feet deep and a few feet across, they are packed with sturdy sea life such as snails, barnacles, mussels, anemones, urchins, sea stars, crustaceans, seaweed, and small fish.

As ocean water retreats outside the tide pool during low tide, the resident marine life must endure hours exposed to the sun, low oxygen, increasing water temperature, and predators such as wading birds that specialize in dining in these shallow pools. At high tide, the pool’s plants and animals are bathed in fresh seawater, but must endure the pounding of crashing waves and foraging fish with temporary access to the shoreline.

To survive in this rugged environment, tide pool inhabitants often cling very tightly to any rock to which they can adhere. Barnacles, for example, produce a fast-curing cement that lets them stay put. This natural substance is among the most powerful glues known to exist. In fact, researchers are trying to figure out if and how it can be harvested or reproduced for commercial use.

The space in a tide pool may be limited, but the food there is plentiful. Every wave at every high tide delivers fresh nutrients and microscopic organisms, such as plankton, to support and replenish the pool’s intricate food chain. Washed in by the waves, these organisms nourish the smallest animals, which, in turn, sustain the larger ones.

Tide Pool Tips: 1) The best time to visit tide pools is at low tide. 2) Bring a bag with you to pick up any plastic, paper, glass, or metal trash on the beach. 3) Find footholds on bare rocks, which are less slippery than those colonized with algae and other sensitive sea life. 4) If you peek under a rock, put it back where and as you found it. Leave the animals and plants alone. 5) Do not collect intertidal species. It is illegal to do so in many areas. Marine biologist Sarah Ann Thompson wears thick overalls and sturdy knee pads, which offer protection from the sharp shells of mussels and barnacles, when she searches for invertebrates living amid kelp on rocks.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org

Intertidal Zone

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fireworm, sometimes found in tidal pools
The intertidal zone is the area where the ocean meets the land between high and low tides. 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. [Source: NOAA]

Four physical divisions, each with distinct characteristics and ecological differences, divide the intertidal zone. 1) Splash Zone (Spray Zone) is dampened by ocean spray and high waves and submerged only during very high tides or severe storms. It is exposed to wind and sun, and home to the fewest species. 2) High Intertidal Zone: floods during the peaks of daily high tides but remains dry for long stretches between high tides. Most often comprised of bar rock with patches algae and barnacle, it is inhabited by hardy sea life that can withstand pounding waves, such as marine snails, mussels, limpets, shore crabs, and hermit crabs. 3) Middle (Medium) Intertidal Zone: is alternatively exposed and submerged each day by tides that ebb and flow twice a day. It is inhabited by a greater variety of both plants and animals, including sea stars and anemones. Low Intertidal Zone: is almost always underwater except during the lowest of spring tides. The greatest diversity is found here. Life is more abundant there 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.

Sea Life Found in Rocky Shorelines

Short wrote in the Daily Yomiuri, “Way up on the rocks, safely above the actual breaking waves but where the rocks are regularly wet by flying spray, is an area called the splash zone. Several species of periwinkle snail can usually be found grazing here. Further below, in the actual intertidal zone (the stretch of rocks that get exposed when the tide is out), live two other types of grazing mollusks, the chitons and limpets.

Grazing mollusks are equipped with an incredible feeding device known as a radula. This is a tough, flexible belt studded with hard sharp teeth of chitin. The belt is extended from a protective sac in the mollusks' mouth, and forcibly dragged across the wet rocks, mowing and scraping off the edible algae.

Rocky shore grazers enjoy access to a reliable crop of excellent food, but have to deal with two major challenges as well. The first of these occurs when the tide is out and the rocks are exposed. The sun beats down mercilessly, threatening to desiccate the animals. The second challenge occurs as the tide returns, when pounding surf can crush the animals or sweep them away. Generally speaking, the higher up on the rocks the longer the time exposed, and consequently the greater the danger from desiccation; while the lower down the greater the threat of being crushed or swept away.

C) Splash, Spray Fringe Zone
D) High Tide Zone
A) Mid Tide Zone
B) Low Tide Zone

The periwinkles, living way up in the splash zone, are seldom threatened by waves, but must deal with the longest periods of exposure. As the tide recedes and the rocks they graze on begin to dry out, the snails fill their body cavities with seawater, then pull their little trap door, or operculum, tightly shut. Tucked away in their own little self-contained cosmos, they can survive for long periods of time.

The periwinkles' ability to survive inside their shell is truly amazing. Once, upon arriving home from a rocky coast foray I discovered a periwinkle that had accidentally fallen into my backpack earlier that day. I wound up keeping the snail for a whole month before a chance to return to the coast presented itself. I thought for sure the little mollusk must be dead by then, but when placed in a small pool of seawater it quickly opened up and crawled away.

Limpets are closely related to snails, but have lost their both the coil in their shells and their operculum. They have instead evolved an umbrella-like shell and an extremely wide foot that serves like a powerful suction cup, pulling the shell down tight against the surface of the rock. So powerful is this grip that the limpets cannot easily be pried off. The shell will often crack before the foot releases its hold. Their suction cups allow the limpets to hang on even when battered and blasted by the most powerful waves. In addition, they can also seal some seawater inside their shell before hunkering down, and wait out fairly long periods of exposure.

Chitons have taken the cling-to-the-rock act even a step further. Among the most primitive of mollusks, the chitons' shell consists of eight interlocking valves that fit together like plates on a suit of armor. The plates are embedded in a tough but flexible girdle, an arrangement that allows these mollusks to twist and curl freely. Limpets require a flat piece of rock but chitons can cling tightly to even irregular surfaces.

Limpets and chitons that live higher up in the intertidal zone are usually well camouflaged. When exposed to the sun, they cling so low and tight that most people think they're just part of the rock. When moving, however, they glide smoothly and effortlessly over the surface like miniature spacecraft.

Tidal popol in Porto Covo, Portugal

Tidal Pool Life

John Steinbeck famously described tidal pools as "ferocious with life." Mel White wrote in National Geographic: The observation applies spatially — lots of things are happening in a relatively small area — but also temporally: Things happen fast between tides. [Source: Mel White, National Geographic, June 2011]

“The rocks and pools here create an abundance of opportunities and host a diversity of life to rival any rain forest. Pisaster is just one of scores of species that have adapted to innumerable micro-habitats with a seemingly endless variety of physical shapes and lifestyles. One little worm can shoot a harpoon out of its head to stab its prey. A limpet tends and guards its own farm plot. A seaweed releases acid for defense when it's injured. A nudibranch (which looks like a gussied-up slug) eats poisonous creatures and implants stinging cells under its own skin to repel predators.

“Why all the aggression? It's simply the result of lots of plants and animals competing for resources in a highly productive but limited space. In nature, as in real estate, location is everything, and the intertidal zone is Park Avenue.

“Many creatures in the intertidal zone show exceptional longevity, both individually and as species. Sea anemones have lived decades in laboratories without showing any discernible signs of aging, and some in the wild are believed to be 150 years old or more. "Short of predation or other fatal accidents," one reference book states, "anemones are potentially immortal."”

“Biologists value the intertidal zone as an easily observable model of ecological processes that happen on much larger scales. Those who study life zones — the way flora and fauna change from the desert up to alpine peaks — must traverse many miles of landscape to experience a wide range of habitats. The intertidal strip displays zonation — from the sea grass at the bottom up through strata of sea anemones and mussels and barnacles to the limpets at the top — all within a few steps.

When a tornado rips through a mature forest, and growth begins anew, centuries will pass as grasses give way to shrubs and pioneer trees eventually yield to the species of an old-growth forest. When a wave-tossed log scrapes away a patch of intertidal life down to the bare rock, biologists can watch mature life return practically before their eyes, the cycle of succession lasting just a few years.

Tidal Pool Creatures

Among the tidal Zone rocks and pools is an array of fancifully-named creatures that come in all shapes and colors. They include red rock crab, calico sculpin,, red sponge nudibranch, sea clown nudibranch, red sea fan, monkeyface prickleback, bat star, chink snail, woody chiton, nereid worm, syllid polychaete, glycerid worm, sea palm, red gunnel, tinted wentletrap, surf grass, red sea cucumber, red abalone, San Diego lamellaria, purple sea urchin, hammerhead doto, leather star, peanut worm, Cockerell’s dorid, ringed nudibranch, variegate amphissa, grainyhand hermit crab, ochre sea star, cabezon, red octopus, opalescent nudibranch, mermaid’s cup, smooth iridescent seaweed, splendid iridescent seaweed, colorful dendronotus, stubby frond nudibranch, rough limpet, Farlow’s soft seaweed, blood star, six-armed star, Pacific sea comb, brown turban snail, fern, shield limpet, green rope and flat porcelain crab. [Source: National Geographic, June 2011]

Tidal pool creatures in Santa Cruz, California

According to National Geographic: Covered and uncovered by tides each day, violet coralline algae, sea stars in orange and lavender, spiny purple sea urchins, and green sea anemones have adapted to lives of constant change. Some creatures are easy to spot, but it takes a sharp eye to notice some small sea spider species, less than half an inch long. The tide’s ebb and flow transforms a seascape. At low tide on the coast, rocks and pools emerge, teeming with life. Bright green sea grass and shiny kelp hold fast to rocks as waves crash over them.

Crustaceans evolved shapes and colors to blend in with the blades of sea grass where they live. Gooseneck barnacles occupy rock faces in the middle intertidal zone, while sea anemones thrive on the low shore. Predatory sea stars move actively in between. Residents of California’s intertidal zone range in size from one-quarter inch to one and a half inches. Among them are shrimp, sea anemones with young, feather duster worms whose tentacles breath and catch food, crustaceans with seven pairs of legs, sea star with tube feet, used for locomotion, lined chiton mollusks, nudibranchs (sea slug) and clusters of fish eggs.

The deepest parts of tidal pools nurse strange creatures like this California stickyhydroid (Eudendrium californicum ), only a few inches long. Although it resembles a plant, those pink “blossoms” are jellyfish-like animals that breed and collect food for the colonial organism. A small crustacean ( Caprella kennerlyi ) has attached itself near the hydroid’s tip. A decorator crab disguises itself by hooking on bits of algae. The penpoint gunnel comes in colors from reddish to green to brown, depending on its surroundings. If the camouflage works, intertidal zone predators won't notice these potential meals. Giant green sea anemones and purple sea urchins remain mostly submerged when the tide goes out but organisms higher in the intertidal zone — like the sea star ( Pisaster ochraceus ), whose soft body stiffens protectively when out of the water — will be more exposed.

Diversity of Life in a Tidal Pool — Based on the Number of Phylum

Mel White wrote in National Geographic: A coincidence of geology and climate makes the northwestern coast of North America one of the world's most diverse and productive intertidal regions. Near-shore upwelling of cold Pacific Ocean currents provides nutrient-rich water, winter freezes and rock-scraping ice are rare, and abundant fog softens the drying effect of sunlight on marine animals that must spend half their lives or more out of the sea...Juvenile development of intertidal creatures varies nearly as much as their physical forms. Many go through a free-swimming larval stage lasting weeks or months, venturing into the immense ocean before settling down as adults on a patch of rock. [Source: Mel White, National Geographic, June 2011]

Eric Sanford likes to perform a kind of magic trick for his students on the rocky coast. First, the audience must understand the concept of phylum (plural, phyla): the organizing principle for classifying the entire animal kingdom. The German word for phylum, bauplan (body plan), is helpful here in that all animals are grouped according to their physical structure. For instance, everything with a notochord (which would be a spine for sharks, pythons, pelicans, you) belongs in the phylum Chordata. Butterflies and shrimps and other animals with jointed legs belong to Arthropoda. Depending on who's doing the classifying, biologists list around 33 phyla.

sea anemone eating a wind sailor in a tidal pool

Next, Sanford leads his audience on the short walk to Horseshoe Cove, another indentation in the rugged shoreline of Bodega Head in northern California . After a little exploration through the thick growth of kelp and sea grass, he picks up a promising-looking rock about the size and shape of a slightly melted bowling ball. "Let's see what we can find," he says. "This yellow crust is a sponge, in the phylum Porifera. This sea anemone is in Cnidaria, and this tiny sea star is in Echinodermata. This snail and this nudibranch are both in Mollusca, and here are several tube worms, in Annelida. "We've got a couple of things here that look something like sponges, but they're in Bryozoa. Here's a tunicate, also called a sea squirt, which is in Chordata, and a crab, in Arthropoda." It takes some searching, but he comes up with one more: "And this is a flatworm, in Platyhelminthes."

So there's the magic. Eric Sanford is holding, in one hand, representatives of more than one-fourth of all the animal life on Earth: nine phyla on one rock. In comparison, the entire land surface of the planet, from Poles to Equator, is home to only about a dozen phyla. Sanford is actually a little crestfallen because he can't find a peanut worm, an odd thing in the phylum Sipuncula that would give us an even ten. "A lot of this diversity occurs because life first evolved in the sea," Sanford says. "And intertidal systems are really a microcosm of the ocean in general. In a tide pool, for the few hours that the tide is out, you're able to access it all."

Threats to Tidal Pool Creatures

Mel White wrote in National Geographic: Biologists question, though, how even highly adaptable intertidal plants and animals will respond to threats they've never known before. Local issues range from pollution and siltation due to coastal runoff to an increase in the harvest of some seaweeds, fueled by the demand for natural food. [Source: Mel White, National Geographic, June 2011]

Of far greater significance is ocean acidification caused by higher levels of atmospheric carbon dioxide. Mollusks, crustaceans, and even coralline algae are among the living things that use calcium in their structures, a process that could be impeded by higher acidity levels in seawater. Rising ocean temperatures are also a threat: Warm water holds less oxygen than cold water. National and state marine reserves can help protect against overharvest of ocean resources, but they're just as vulnerable as the rest of the seas in the face of global climate change.

In a meditation on the interconnectedness of life, Steinbeck wrote, "It is advisable to look from the tide pool to the stars and then back to the tide pool again." As a microcosm of the ocean — the nursery of all life including our own — the intertidal zone serves as another galaxy in the universe, one easily within our grasp.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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