One of the primary methods of survival for any organism is adaptation. Intertidal organisms are no different. These organisms specifically inhabit the area or zone between high and low tide along rocky coasts, sandy beaches or tidal wetlands/marshes. Some examples of these organisms include: hermit crabs, mussels, sea stars, types of algae, mollusks and many others.
When the tide comes in, the intertidal habitat is immersed with salt water and subject to wave action as well as other conditions. Once the tide recedes, the habitat is partially or fully exposed to the sun and elements. As a result of these challenging conditions present throughout the course of the day, this unique habitat is host to a variety of organisms that have several adaptations in order to better survive.
One of the most crucial problems that intertidal organisms have had to overcome is water loss. When the tide goes out, unless water remains in a depression and keeps them submerged until the tide comes back, they will be partially exposed and could dry out. In order to combat these problems, they have adapted to the conditions that they must face. Many intertidal organisms are able to burrow into the sand, migrate, or move short distances in order find a moist spot and last until the tide comes back in. To further prevent water loss, several organisms in this habitat have also adapted by forming a shell around them or secreting mucus in order to create a protective layer on exposed body areas and prevent themselves from drying out. A select few are even able to survive being desiccated (completely dried out) and revive once the tide comes back in.
Getting oxygen is crucial to most organisms and while people have normal lungs and fish have normal gills, intertidal organisms have adapted a little differently. The organisms that only use gills have likely have developed a way to protect those gills from drying out and are able to close themselves off as well as keep their activity to a minimum and survive until the tide returns. Other organisms have a reduced set of gills and some type of lung that will allow them to continue respiration while out of the water and exposed. There are also some intertidal organisms that partially use cutaneous respiration in order to get about half of their oxygen requirements. This adaptation involves oxygen diffusion through a layer or through a different breathing system instead of using a gill or lung.
With the conditions present in the intertidal zones, the organisms that live in these areas have to deal with a variety of different stresses that normal organism may not have to worry about. These stresses may include: mechanical stress, temperature change and salinity.
The mechanical stresses faced by the intertidal organisms stem from the constant wave action during the transition from low to high tide as well as during high tide. To deal with this, they have developed methods of gluing or attaching themselves to the surface material that they live on in order to not get pulled away by the waves. Their bodies may have a hard shell that protects them in addition to being aerodynamically shaped so that they don’t experience much drag or lift from the waves.
Temperatures can rise or fall by large amounts as the tides changes and while ocean organisms only experience temperature change by a few degrees, intertidal organisms need to survive much larger temperature changes every day. Intertidal organisms may need to be protected from direct sunlight, summer temperatures, as well as survive freezing temperatures in the winter. To survive these conditions, many have developed a unique shell with ridges to diffuse heat or are colored in light colors to prevent heat absorption. Burrowing or closing (clam-up) themselves is also an affective method to protect from temperature change while other organisms have merely developed incredible tolerances for temperature change and temperature extremes.
Salinity is another stress for intertidal organisms as it can fluctuate greatly. A depression can accumulate salt deposits and increase salinity compared to the ocean, while low tide will generally cause a decreased salinity content. Similar to the other stresses, the organisms may burrow down or clam-up but some organisms are osmoregulators or osmoconformers. The osmoregulators are able to keep the salinity of fluids inside their bodies at a certain level while the salinity in their surroundings may change. The osmoconformers keep the salinity of their body fluid at the same concentration as their surroundings.
Given that the tide is always changing, intertidal organisms usually synchronize their reductive cycles with the tides in order to ensure survival of the next generation. Many of these organisms merely release egg and sperm into the water column and form planktonic larvae which will be dispersed with the wave action and tides. The larvae are the early stages of many marine invertebrates. Those organisms which are able to migrate will do so in order to reproduce and deposit eggs in the ocean.
Given that high tide brings more chances for food or a larger food supply, many intertidal organisms only feed during high tide and not during low tide, or at least not as much during low tide. Apart from organisms that are predators or scavengers, some organisms have developed filter feeding systems whereby food is strained or filtered out of the water and consumed by the organisms while it remains in place. This feeding system relies on the wave action as well as other individual adaptations in order to create a current or suction that will capture food particles in the water This is the same system used by whales only on a smaller scale.