Intertidal Biology – A Description and Analysis of the Intertidal Zone

The intertidal zone is a complex marine ecosystem. Often called the “littoral” zone, it is the area where land meets the water. This marine zone experiences the greatest amount of light penetration and is also the closest to the shore. The intertidal zone experiences a variety of environmental extremes and exhibits a vertical zonation due to the rise and fall of the tides. This is very prominent on rocky shores, where the different species can be see clinging to rock surfaces. On sandy substrates, the intertidal zone is not as clearly stratified. The changing water level leaves some areas of the intertidal zone exposed to the air for a prolonged period of time, while others are left submerged. This often creates a shore pattern of distinct parallel bands of organisms. Because of this, the intertidal zone can be divided into six different sub-zones. These are the supralittoral fringe, upper midlittoral zone, midlittoral zone, lower midlittoral zone, infralittoral zone, and the sub-tide zone.

The supralittoral zone is one of the harshest environments for animals that have come from a marine origin. Also called the splash-fringe zone, this level is found immediately above the highest tide level reached in any month6. Receiving the least amount of water than any of the other zones, this area may only become wetted by spray or mist from the sea, but may also receive moisture from precipitation during storms. On some wave-smashed rugged shores the supralittoral zone may extend for a few hundred meters up the shore or up a cliff face, however on calm water shores, this zone may not even exist. It is dependant on the terrain that encompasses the area. Here the greatest environmental danger for shore animals is being dried out by the sun. Most animals that live in this zone have adapted ways conserving water, such as an operculum to seal the shell entrance from water loss. Because of the limited supply of water, the most commonly found organisms in the supralittoral zone are mollusks, oysters, and barnacles.

Underneath the splash-fringe zone lies the midlittoral zone, or tidal zone. This region may be further divided into “high-tide”, “mid-tide”, and “low-tide” levels, as regulated by the rise and fall of the tides. The tides are caused by the gravitational pull of the moon and the sun. Although the moon is much smaller than the sun, it plays a much larger effect upon the Earth’s tides because of its relative location. About 70% of the Earth is covered by water, so when the Moon orbits the Earth, there is a bulge of water underneath it. On the other side of the Earth is also another high-tide water bulge caused by the centrifugal force of the Earth. When this occurs during a new or full moon, when the Earth, moon, and sun are all in line, it creates spring tides. Because all three are in a line, the sun and moon’s gravitational effects are supplementing each other, creating extra high and extra low tides. During a first or third quarter moon, the sun and moon’s gravitational effects are working against each other causing more median neap tides.

The tidal zone is the area most affected by the changing of the tides. It is divided into three subsections, the upper midlittoral zone, midlittoral zone, and lower midlittoral zone. For any organisms that can withstand the wave energy, drying, and temperature extremes, this is a great place to live. The area is rich in nutrients an abundant supply of oxygen. The environment changes greatly with the tides. During high tide, the entire area is immersed in water and the inhabiting organisms must deal with water currents, the amount of substrate in the water, and the salt content of the water. At low tide, organisms must face heat and desiccation stress. All of these environmental factors vary greatly at different times during the day. Most of the effects of the tides on organisms living in these areas are due to the exposure to air. One of the main problems is dehydration.

Some of the strategies for survival by organisms in this area include moving to a moist area in order to maintain homeostasis. During low tide they can move to a sheltered area protected from the sun and the crashing of the waves. The organisms with a bivalve shell keep it closed so as to not dry out7. Snails have evolved an operculum, a tiny door-like structure on the entrance of the shell to close themselves up inside and prevent moisture loss. Some organisms, such as sea anemones, secrete mucus that covers their body and holds in water. The shape and color of the shell also helps some animals adapt to the changing temperature. Organisms that have light colored shells don’t absorb the heat as fast but rather reflect it. Having ridges on the shells also keep the organisms cooler because the texture of a ridged shell helps reflect most light and sun rays. Organisms that live in the pounding surf have also adapted to their environment. Algae have a structure on their body called a holdfast that enables them to cling to rocks and remain stationary. Mussels attach to physical objects using byssal threads, while gastropods use their foot to secure them in place. The clingfish use a modified fin on their pelvic area like a suction cup. Having a compact size and a hydrodynamic shape can also help to hold colonies of organisms like barnacles and chitons in place.

As the tide falls, food availability becomes a factor for many creatures. Because many of these organisms are filter-feeding, they must seek a home that is not left dry for prolonged periods of time. Many find refuge in shallow tide pools in the midlittoral zone6. These small pools collect water and remain submerged even during low tide. However, they are subjected to the same environmental extremes as the rest of the intertidal zone. The temperature and salinity in the tide pools can increase and decrease quickly. As the water warms and evaporates, the salinity level goes up. There can also be an increase in the UV light when the organisms are out of water.

As you go deeper and deeper into the intertidal zone, the level of environmental variability becomes less and less. This is because as depth increases, so does the amount of time the area is submerged in water. As a result of this, biodiversity drastically increases as you enter the lower midlittoral zone6. In this zone you can find anemones, sea stars, sea urchins, chitons, tritons, whelks, limpets, barnacles and crabs. Arthropods and mollusks thrive here. It is here and in the infralittoral zone that you will find most of the plant life in the intertidal zone.

The infralittoral zone is the area that is covered by the waves during a low tide. Organisms in this zone are not adapted for prolonged periods of exposure out of the water. The behavior and lifestyle of organisms at this depth closely resemble that of marine animals. Most of the animals and algae partially exposed at low tide at the infralittoral level are typically marine in lifestyle, with some of them being exposed to the air, more or less accidentally, when the tide is at its lowest. This is the region where intertidal algae flourish. In areas where rocks and boulders are embedded in sand or sandy mud, usually covered with abundant algae in swiftly flowing water, an amazingly wide range of animals may be found. These animals are not truly intertidal, but may be accidentally exposed at low tide by a human turning over rocks. Here we may find colourful sponges, various types of colourful seastars, an amazing variety of chitons, colourful flatworms, spiny worms, and peanut worms. The abundance of different animals here is absolutely amazing.

Underneath this zone is the sub-tide, or marine zone. This is where you start to move away from the neretic zone into the oceanic zone. The tides do not play a large role in the lives of organisms living in this area because they are at too great a depth. These organisms are not exposed to the air with the changing of the tides and spend their whole lives submerged in water. Some animals come in at high tide as predators of intertidal species such as barnacles, mollusks, or other sessile animals. Many carnivorous fish and crabs fit into this category. Also, a large range of algae can be found in the here.
Humans have had a devastating impact on intertidal ecosystems. The recreational use of ocean shores has caused a severe decline in the numbers of many intertidal species. Incoming tides carry in polluted water and other types of refuse and debris that can harm and kill wildlife. The most dangerous intertidal pollutant is oil. Oil and oil spills not only harm birds and marine mammals, but also intertidal algae and invertebrates. It reduces species diversity and increases the population of a few oil-resistant species, such as barnacles.

The intertidal zone, where the land meets the sea, is a place of great environmental variability and species diversity. To thrive here, an organism must be able to adapt to many different environmental factors and extremes. Organisms must be able to survive both in and out of water. They are able to do so through a series of adaptations and behaviors that help them to maintain homeostasis. The intertidal zone is one of the most complex and diverse marine ecosystems.