Kathie Fiveash, Island Naturalist columnist for the Island Ad-Vantages
Tide pools are like natural aquariums left in the rocks when the tide ebbs away. They are full of life. Tide pools are home to many different seaweeds and animals.
If you look carefully you might encounter clusters of snail eggs attached to the rocks, slow-moving starfish, scuttling crabs, strange-looking worms covered with tiny scales, mussels with their shells open exposing their fringed siphons, barnacles combing the water with rake-like organs to collect food.
How do these organisms adapt to their continually changing world?
Here on the island of Isle au Haut we have 10-foot tides. When I look out my window, the view between me and the island a half mile distant is completely different depending on where we are in the tide cycle.
At high tide I see an expanse of water broken only by a few knobs of gray rock. At low tide I see myriad ledges covered with rockweed, some rising from sandy beaches and some containing pools of seawater. There’s a lot of water moving in and out twice a day. The area that is covered and uncovered in the regular rhythm of the tides is called the intertidal zone.
Looking at the expanse of the intertidal area at low tide one can see different life zones, which are determined by the amount of time each zone spends underwater as the tide rises and falls.
The zone closest to the high tide line is the most stressful environment, and fewer animals and plants can survive there. In this high intertidal zone you’ll see rocks darkened by a slick of algae.
Below the high intertidal zone is a band of barnacles which escape predation by living where no predator can get to them. Below the barnacle zone, a brown expanse of rockweed covers the rocks. The lowest zone in the intertidal is a band of reddish seaweed called the red zone.
The plants and animals that live in the rocky intertidal have adapted to withstand the rigors of a challenging and highly variable habitat. Imagine living in a place where twice a day you are flooded and twice a day you are left high and dry. You face extremes of temperature—both daily and seasonally. You also face strong drying winds during the hours when you are high and dry. When you are flooded you need to hold on for dear life to avoid being swept away by waves and currents.
But the intertidal zone does offer shelter in the form of tide pools. Tide pools form anywhere in the intertidal where rocky hollows trap seawater when the tide ebbs. Tide pools can be shallow puddles, deep clefts, or large troughs. They provide living space for many plants and animals that can’t survive on exposed rock.
Tide pools provide a degree of shelter from the harshness of the elements. Tide pools provide plants and animals with a place to stay wet, regulate their temperature, find food, and hide.
The diversity of life in a tide pool depends on where it is in the intertidal zone. The tide pools with the greatest biodiversity are down in the red zone. These pools are completely submerged for long periods twice a day as inrushing seawater, oxygenated by wave action, floods and overwhelms them.
Exploring a big tide pool in the red zone is a fascinating pursuit. There you will find a variety of seaweeds and animals—and sometimes it is hard to tell which is which. As in any ecosystem, there are plants, the primary producers, making their own food from water, air, and sunlight. In this case all the plants are marine algae, better known as seaweeds. They come in three types—green, red, and brown—and exhibit an amazing variety of size and structure.
There are flat delicate green sea lettuces, heavy brown ribbons of kelp, long brown strands of rockweed, red fronds of dulse, and many kinds of filamentous algae in all three colors. Pink patches called crustose algae encrust rocks and shells. Coralline algae, which look like tiny branching corals, wave gently among the bigger seaweeds.
All seaweeds in the intertidal have a leaf-like part and a structure called a holdfast with which they cling to the rock or other substrate so that they will not be swept away.
There is a profusion of animals that feed on the seaweeds and on each other. Herbivores like periwinkles, limpets, and sea urchins graze peacefully over the rocks and seaweeds. Filter feeders like sponges, mussels, and clams draw seawater into their bodies and strain out tiny particles of nutrition. Predators like starfish, crabs, anemones, and dog whelks stalk different favored prey species. Scavengers, including multitudes of tiny shrimplike creatures called amphipods, zip through the water looking for dead or dying animals and seaweed.
Each one of these animals has a unique way of life, but they all have at least one thing in common. They all must attach themselves to the rock or hide in a deep crack to avoid being carried off by waves and currents as the tide rises and engulfs the tide pool.
The best way to observe a tide pool is to sit and watch it for a while. You may see a hermit crab pick its way over the bottom in the shell of a periwinkle, or an anemone open its frilly mouth as it waits for small prey.
You will certainly see thousands of tiny amphipods swimming busily about. If you venture into the pool and feel in the cracks in the rock, you may find a sea urchin. Or you can lift the curtains of rockweed to look for crabs.
Just be sure to arrive before the tide turns, watch for incoming waves, and skedaddle when the ocean begins to surge into the pool. You are not as well adapted to tide pool life as the plants and critters that live there.