8. Coastal Creatures


Have you ever noticed how a beach changes throughout the day? High and low tides change the environment for many plants and animals living in these tidal zones. Join us in exploring our B.C. coast with a beach walk, where we will discover our local intertidal creatures and discuss some of the challenges of living in a tidal zone!

How does a beach look so drastically different in a matter of a day?

Photo: Anthony Watts from the University of Southampton

The answer: the gravitational forces of both the moon and sun upon the earth.

It is this magic-like phenomenon in which gravitational forces override the centrifugal forces of the earth’s rotation. The tides are being pushed more so than pulled to create a high tide or a "bulge" of water due to the forces of the moon and sun that are affected by the lunar cycle . In short, it's an interplay of gravity between the moon, earth and sun that produces a force that create 2 high tides and 2 low tides each day (aka the tidal cycle).

These two interplanetary bodies pull our planet‘s water toward them, creating oceanic bulges on the Earth‘s surface. When the sun and moon are pulling in the same direction, the sun behind the moon, their pull is the greatest, and the tidal fluctuations most extreme.

What is the lunar cycle?

Photo: WikiHow to Make a Moon Phases Chart

The lunar cycle is the monthly pattern of the changing appearance of the moon from earth. The phases are new moon, crescent, quarter, gibbous and full. All of which are connected to the relative positions of the sun.

These patterns are simply changes in the portion of the lit side of the Moon that you can see from Earth, but also connect to our tides.

For example, during the new moon, combined gravitational pull of the sun and the moon generate even higher tides ad correspondingly lower tides. This is because molecules of water near the poles are pulled mostly straight down, those on the face of Earth closest to the moon experience the strongest pull toward the moon, and those on the opposite side of Earth feel the weakest acceleration.

Does this happen at lakes too?

Photo: www.wallpaperawesome.com

Well the difference in distance from the Moon to various points on a surface of the water is much smaller than radius of the Earth, but tides of amplitude of about two inches could be still observed. But local lakes are so small that all points on the surface of the water are practically at the same distance from the Moon, hence no observable tides exist. Likewise there are no tides in a swimming pool or bathtub.

What are the different types of tides?

Photo: Zonation of the Benthal

Spray Zone: The uppermost zone of a rocky shore is also called the supralittoral zone. This transition area between land and sea is exposed to air, but regularly receives the ocean‘s spray. This zone appears almost barren when compared to the diversity of life at the lower zones of the shore. However, the animals and plants of the spray zone are almost terrestrial and even some cannot survive for long periods of time in seawater. Some examples of animals that live in the spray zone are periwinkle snails, black encrusting lichens and orange lichens.

Photo of periwinkle snail by Genny Anderson Photo of black encrusting lichens by dugwin2By: dugwin2 Photo of orange lichens by E-Flora BC

Intertidal Zone: Within the intertidal zone at low tide, light and temperature levels may exceed or fall below those of the sea. A heavy rainfall can lower the salinity of any nearby saltwater pools forcing animals living there to adjust their body chemistries. Herons, gulls, and other air-based and land predators can access exposed seashore inhabitants. Mobile species such as crabs, limpets and snails hide under rocks, in moist crevices or under seaweeds can hide from the sun and prevent them from drying out. Some attached animals like barnacles and mussels can save a bit of water seawater inside their shells, and shut up tightly during low tide. Many non-motile (sedentary) species are adapted to withstand high and choppy waves. For example, kelps attach to the substrate with strong holdfasts, barnacles stick to rocks by secreting cement, and sea stars have thousands of tube feet that hug the shore with tiny suction cups. Mussels secrete strong pliable “byssus” threads that anchor them to the shore.

Photo of limpets by The University Times Photo of a sea star by Beach Combers Education Kit Photo of barnacles by Wikipedia

Subtidal Zone: The lowest zone, the subtidal zone, is found below the low-tide line. It has more stability where there are small changes throughout the day. Wave-action effects are minimal, temperatures change only seasonally, and interactions between creatures are more permanently established. The subtidal fringe has the most diversity of species of all the intertidal zones, and includes those that can tolerate only a short exposure to air. Some of the many types of species in this zone include kelp, eelgrass, many species of red algae, sea slugs, sponges,bryozoans, , snails, limpets, chitons, crabs, shrimp, sea anemones, sea stars, sea urchins, sea cucumbers, moon snails as well as fish such as sculpins, clingfish and gunnels.

Photo of a shrimp by MyFWC. Photo of bryozoans by University College Cork, Ireland Photo of a sea anemone by Ocean Treasures

What does it mean to adapt?

As mentioned above, each animal in a specific tidal zone is able to be able to live in the conditions of their environment. The adaptations of an animal cannot be interpreted except in relation to the environment with which it interacts.

An animal must be able to adapt in two different ways to its environment:

1) the animal must be adapted to live in a biotope (=area of uniform environmental conditions) defined by such environmental factors as temperature, moisture, water, height, depth, etc. This is the habitat.

2) the animal must be adapted to interact, live, and survive interdependently with the assemblage of other animal and plant species etc. in the community in light of pressures such as competition, predation etc. This is the niche.

Photo: The Wildlife Trusts

To adapt to an environment animals have evolved their own specific methods to sustain themselves. As discussed above, there's a range of ways in which an animal can acclimatize themselves to fit their environment such as, crabs being able to move around to find crevices for moisture when the tide is low. Or the subtidal animals typically being static as they can only handle short periods of exposure to air. These instances of adapting to their environment moreover highlights the importance of how the tides shape the homes of our marine animals.

Questions to ask you Junior Biologists

1. How is a low tide created?

The gravitational force caused by the positioning of the sun and moon is what generates the patterns of the tides (i.e creating a high and low tide). For instance, high tide is created when the side of the earth is nearest the moon, which has a stronger gravitational effect than the sun because it is so much closer to the earth. And reverse, when the tide is high in one area, the displacement of water causes a low tide in another area.

2. What are the differences between the spray zone and the subtidal zone?

The key difference between the spray zone and the subtidal zone is that the spray zone is the uppermost part of the rocky shoreline whereas the subtidal zone is essentially the opposite as it is the lowest. Moreover, the animals that are present can be different as well due to their differing living conditions. For example, the animals found in the spray zone will be able to withstand a long period of time in the open air while the animals in the subtidal zone thrive underwater and are typically unhappy being out of the water for a prolonged time.

3. What kind of adaptations do animals in the intertidal zone have?

As this zone is the middle ground between the spray zone and the subtidal zone, there is a lot of diversity. You could find animals that are mobile like crabs, limpets and sea stars. Or you could find animals that aren't particularly mobile, but have the capacity to spend a shorter periods of time out of the water like mussels, clams and barnacles. It really does range on what you could find as it overlaps with the animals of the spray zone and the intertidal zone.

4. Why is it important for an animal to adapt to their environment?

Having adaptations to an environment is a crucial component for the survival of an animal. It is how an animal's body is evolves to best live in an environment that could have predators, changes to temperature, water level and irregular access to food. These are simply a few examples as to why an animal would adopt certain adaptations (such as having claws, a hard shell or being able to hold water inside their body) to live in a habitat.

See your Jr. Biologists in action here!

Do you have any questions for the Club Volunteers or our Coordinator, Jordan? Ask away on our Discussion Board!


Next to you go to the beach see what kind of coastal creatures you can find!

Ask yourself the following questions when you discover beach critters to compare how different animals adapt:

-Can it burrow?

-Can it camouflage?

-Can it bite?

-Does it pinch?

-Does it have a shell?

-Can it close its body to prevent it from drying out?

-Is it close to food?

-Can it hide in tight spots?

See what you can find and identify why they picked their home!

Learning Objectives

-Comprehend the relationship between the sun, moon and earth that facilitate the highs and lows of the tides.

-Discuss how the moon phases relate to the changing tides.

-Observe the differing environments between the spray zone, the intertidal zone and the subtidal zone.

-Identify adaptations of marine animals to survive in their environment.

-Recognize how the physical characteristics of marine animals directly reflect the state of their environment.

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