Surviving in the Intertidal Zone: The gateway to the Ocean 

The intertidal zone is the gateway to the Ocean

The intertidal zone is the dynamic interface between land and sea, which is constantly shifting and changing with the tide.

This extreme ecosystem is divided into four vertical zones, based on the amount of time each section is submerged. There are the low tide zone, middle tide zone, high tide zone and splash/spray zone. 

The intertidal ecosystem in the Ocean is divided into four zones
Image credit: Science Learning Hub

Anything that lives in the intertidal zone must withstand dramatic changes in moisture, temperature, salinity, and wave action.  

Marine life and ecosystems respond to these challenges either by short-term, reversible adjustments (phenotypic plasticity), or by long-term adaptations that involve heritable genetic changes (evolution). 

Let’s explore the challenges of living in this dynamic environment, and some of the ingenious ways that intertidal animals have adapted to survive here.

Intertidal animals are exposed to air for a large portion of their lives

The drying out effect of air exposure poses a challenge for these animals, and they must find ways to reduce water loss to survive here.

The most common adaptation is to avoid water loss altogether. Intertidal molluscs (like mussels and oysters) retain water within their shells and tightly seal them shut. Others seal off their shell opening with a door-like structure called an operculum (snails do this).  

Limpets seal themselves against the hard substrate using suction and produce a mucous layer at this interface to create a watertight seal.  

Limpets seal themselves agains hard substrate in the intertidal zone

Air exposure leads to greater temperature fluctuations and extremes. 

Mobile animals such as crabs avoid the largest temperature changes by shuttling between cooler and warmer environments. This technique is called behavioural thermoregulation.

For less mobile animals, thermal regulation becomes more challenging. 

More sensitive animals such as sea stars/ starfish are limited to the low tide zone where they’re more frequently submerged.

Others, such as some species of intertidal mollusc, have evolved internal mechanisms such as heat shock proteins and a high freeze tolerance. These help the animals to cope with extreme temperature variations in the high tide zone. 

Tidal pool at the beach. Posted by Ocean Generation.

The intertidal zone is characterised by severe changes in oxygen availability. 

Particularly in overcrowded tide pools, when many animals are aggregated together during low tide, increased respiration can reduce water oxygen content to critical levels. 

In response to this, some intertidal animals can switch to aerial respiration and take in oxygen from air instead of water.  

For example, the high intertidal porcelain crab (Petrolisthes) has an aerial gas exchange organ on each of their walking legs for air-breathing during periods of emersion.

Porcelain crabs can take oxygen from air instead of water. Posted by Ocean Generation.

On sandy shores, some species of mudskipper will repeatedly emerge at the surface of their burrows and take in mouthfuls of air. They deposit this air into a specialised chamber within their burrows to protect themselves and their eggs against hypoxic (low oxygen) conditions. 

On sandy shores, activities of some intertidal creatures can influence local oxygen availability.  

Lugworms burrow deep into the sediment, feeding on organic material in the sand and creating U-shaped burrows deep below the surface. These burrows help to rework and ventilate the sand. This process is known as bioturbation, which increases localised oxygen availability.

They spend their lives filtering out organic material from the sand, pooping out wiggly mounds of undigestible sand to the surface, known as ‘casts’. 

Lugworms burrow deep into the sediment in intertidal zones.

All intertidal habitats are impacted by fluctuations in salinity 

Heavy rain or river inputs can make coastal water fresher, and evaporation and droughts can cause hypersaline (saltier) conditions.

On sandy and muddy shores, intertidal worms move vertically up and down their burrows along a salinity gradient until they reach more favourable conditions.

Other animals maintain their internal osmotic (salt and water) balance in the same way that they protect themselves from water loss. Some clamping to the substrate (limpets); others closing their operculum (snails); or moving to more favourable environments (crabs).

The shallows are exposed to ultraviolet (UV) radiation from the sun. 

This can cause DNA mutations and damage to molecules needed for key biological pathways.  

To combat this, the aggregating anemone (Anthopleura elegantissima) contracts during peak levels of UV radiation. It attaches debris to its column for extra protection against harmful sun rays: Just like putting on a sunhat.

Other intertidal animals have even evolved ‘sunscreens’. These are absorptive, reflective, or light-scattering pigments in their skin and mucus which help to protect them from the damaging effects of sun exposure. 

For example, Irish moss (Chondrus crispus) is the reddish leafy seaweed that can be found on rocky shores and tide pools across the UK and Ireland. If you look closely, you’ll see that it looks slightly iridescent in the light.

This is because the tips of the growing fronds are covered in multiple, transparent layers. When sunlight hits these layers, it’s reflected away from these delicate regions, protecting this alga from harmful UV rays.

Irish Moss reflects sunlight to protect from harmful UV rays

To avoid being swept away by wave action, intertidal animals must hold on tight. 

Barnacles secrete cement, while mussels produce a sticky thread called Byssus threads to attach themselves to the rock. Once attached, these sessile animals remain in position for most of their lives.  

Other intertidal creatures rely on suction. Sea stars (starfish) have rows of tube feet on their underside. Each of their feet has a sticky, suction cup at its end that help it hold on to the rocky substrate. These also come in useful to prise apart the shells of bivalves to eat.

Sea stars have rows of tube feet to stay in place

Many adaptations are shared among diverse animal groups.  

This is an example of convergent evolution: when similar features independently evolve among different species under the same pressures.

Understanding the challenges overcome by animals living at the interface between land and sea may allow us to better understand our own historical transition from Ocean to land, millions of years ago.  

So next time you’re at the coast, see if you can spot some of these creatures and their adaptations. (Remember, they’re under enough stress already, so be respectful and don’t touch, poke or prod at them).

Take a moment to appreciate the incredible feat of survival achieved by the Ocean creatures that not only survive but thrive in this extreme environment.

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The motion of the Ocean explained: Waves and tides  

Motion of the Ocean explained by Ocean Generation.

The Ocean spends its life in perpetual motion.

From the smallest ripple to the largest storm wave, energy from the Ocean is one of the most powerful forces on our planet. They have carved the shape of our coastlines over thousands of years. 

Many of us observe the constant motion of waves and tides, but few of us ever stop to consider how these not-so-simple certainties actually work. 

What are Ocean waves, and where do they come from 

First things first, Ocean waves are the transfer of energy across a body of water, not the movement of water itself.  

Surface waves are caused by wind out at sea. As the wind blows across the Ocean, particles near the surface are disturbed. Friction and pressure generate ripples, and this wave induced pressure causes each individual water particle to push and pull on its neighbour.

The water molecules begin to move up and down in a circular orbit, creating a wave crest. This motion propagates energy through the water in the direction of the wind. 

How waves are created in the Ocean

Once they have enough energy from the wind, these wave crests spread out and begin their journey across the open Ocean as “swells”. These swells can travel uninterrupted for thousands of miles, until they reach the shore and meet their dramatic end. 

As the wave approaches shallower water, the circular orbits of the water molecules in the lower part of the water column are disrupted by the seafloor and get slowed down by friction.

The water molecules closer to the surface are less effected by friction, so the energy continues to move through them at its original speed.  

The wave grows in height but is left unsupported as the lower part is dragged along the seafloor. Eventually, the wave finds itself with nothing underneath it, and collapses in a dramatic fashion, known as the wave “break”.  

A wave break is when a wave collapses. Posted by Ocean Generation, leaders in Ocean education.

Another form of Ocean waves that move across our planet are tides.  

The predictable rise and fall of the Ocean along our shores is as certain as the sun rising in the East and the stars coming out at night.

For centuries, humans have learned to predict the tides for navigation, fishing and other recreational activities.

But to fully understand how tides work, we must look up to space.   

The moon and Earth both exert a gravitational force and are constantly accelerating towards each other in orbit.  

As our planet accelerates towards the moon, the water on the side closest to the moon accelerates faster than the solid rock in the middle and accumulates to form a slight bulge.

This is known as tidal bulge.

As the Earth rotates, this watery swell stays in the same position relative to the moon. The land rotates into this bulge at high tide, and out of it at low tide.

So, when we stand on the beach and watch the tide going out, what we’re actually observing is the Earth rotating away from the Ocean.  

Ocean waves and tides have been shaping the universe. Posted by Ocean Generation.

But wait a second, why are there two high tides per day? 

This is where things get a bit more complicated. Put your scientist hats on, and imagine the following:

While the water on the near side bulges towards the moon, the water on the far side bulges away from the moon.

Remember that the moon and Earth are constantly accelerating towards each other in orbit.

A centrifugal force (a force which acts on an object that’s rotating) acts as a result of this spinning.

On Earth, this centrifugal force is strongest at locations facing away from the moon, causing the water to bulge away from the moon at the far side. 

Earth therefore rotates into two tidal swells each lunar day of 24 hours 50 mins. 

What is a lunar day?

A lunar day is the time it takes for a specific point on Earth to rotate from an exact point under the moon to return to the same point under the moon.

This explains why there are two high and two low tides per day, and each high tide occurs 12 hours and 25 minutes apart.  

To understand how Ocean tides work we have to look up to space.

The sun also has a gravitational tidal force on our Ocean: It’s called a solar tide 

However, it’s much smaller since the sun is much further away.  

When the sun and the moon are aligned, their lunar and solar forces combine to create a larger tide, known as spring tide.

In contrast, when the sun and moon are at a right angle, their opposing tidal forces partially cancel each other out, creating a smaller tide. This is known as neap tide.

Spring tides and neap tides explained.

Back on Earth, the shape of the coastline can have a dramatic influence on tidal magnitude 

For example, the highest tides in the world can be found in the Bay of Fundy, Nova Scotia, Canada. The size, depth and unique funnel-shape of this coastline causes a natural oscillation (a back-and-forth movement in regular rhythm) of the water in near-perfect sync with the tide, which has an amplification effect.  

So next time you’re taking a stroll along the coast and listening to waves crashing against the shore, take a moment to consider the forces in play to make it all possible.

Waves and tides are all part of the continuous movement of energy that has formed and shaped our universe since the beginning of time.  

The highest tides in the world 
can be found in the Bay of Fundy, 
Nova Scotia, Canada.

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