Waves crash and tons of water are hurled against the rocky shoreline. The waves recede, water pour over the rocks and back into the ocean. The water prepares, almost crouching like an animal about to jump, as the next wave moves in to strike the rocks again. Repeat this ad infinitum. The spectacle of waves crashing against a rocky shoreline is awe inspiring. That is for us humans who can watch from a safe distance. But depending on our distance, and our familiarity with rocky shorelines, we may not be aware that the rocks receiving that pummeling are usually covered with living beings trying to survive and make a living.
That strip of rocky shoreline between the highest of high tides, and the lowest of low tides is known as the intertidal zone. It is an interesting bit of the planet insofar as the organisms that have evolved to live there full time have to cope with an extreme change in environmental conditions, sometimes as often as twice a day. Probably in no other habitat on Earth are the living members of the ecosystem required to deal with such drastic, and such frequent changes in environmental conditions.
So why exactly is this place we call the rocky intertidal so different, so extreme? First, the frequency of the change that occurs is driven by the changing of the tides, which is driven by the orbit of the moon around the Earth. As the moon’s gravity pulls on the Earth, it also pulls on Earth’s ocean, which rises up along two bands on the surface of the Earth. The first band is more or less immediately below the moon (although there is an offset because of friction and drag) caused by the direct gravitational attraction of the moon on the water. The second band is exactly on the opposite side of the Earth from the first band. The reason it exists is more complicated, but suffice it to say that it is a function of gravity and centripetal acceleration.
As the relative positions of the Earth and moon change, these tidal bulges try to follow the moon’s position. As a result, the vertical height of the ocean at any place on Earth will tend to rise and fall twice a day. This is what we call the tides. The overall pattern of the tides on Earth is quite complex and depends on the shape of the coastline, depth of water and other factors.
The second fact that makes the rocky tidal so different and extreme, is that all of the organisms that are found living there are of marine origin – they are all organisms of the sea. That means that when the tide is low, some or all of the organisms in this strip along the shore will be exposed to the air and, during daylight hours, the sun. Think about what that means to these living things. As marine organisms all get their oxygen from the water that normally surrounds them. When out of the water, they essentially have to hold their breath until the water returns. So you may wonder why they don’t just move down to where the water is found. Some actually do, but it is one of the unique aspects of many of the organisms that live in the intertidal that they are fixed in one location, or what science calls “sessile” (sĕs-sīl). So they can’t move. So “in order to make it, they have to take it,” to paraphrase a line from a popular movie of today.
Thus these species have all evolved adaptations that allow them to cope with being exposed to the air for several hours every day. But these adaptations are not enough, because while they get the organisms through the rigors of low tide, the high tide, as described above, is no cake-walk either. Now, while submerged in water and able to breath and eat, they are subjected to forces that threaten to tear them off the rocks, or crush them beneath floating objects hurled against the rocks by the waves. This, of course, requires an altogether different set of survival strategies.
So the next time you visit the shoreline, or see the action of waves on a video screen or in a photograph, stop for a moment and consider the conditions with which the organisms that live there must contend.
This photo was taken with a Canon EF 28-135 f/3.5-5.6 IS USM lens zoomed to 38 mm on a Canon EOS 5D Mk. III. The exposure was set to 1 sec at f/16 and ISO 200.
To see more of photos and read other blogs, visit www.chuckkopczakphotography.com