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Animals and plants of the intertidal zone have to cope with regular exposure and submersion for many hours at a time.
Photo: Genevieve Johnson

October 21, 2002
Creatures of the Intertidal Zone
  Real Audio

Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey, east of the Amirantes Bank in the Seychelles.

The Amirantes are made up of about ten small islets and cays spread over an area of 150 square miles. Most of these are surrounded by extensive fringing coral reefs that act as important buffers against coastal erosion and offer critically important hiding places for the young of many open-ocean fish when they are at their most vulnerable. The reefs also are exposed to the sun and sea on a daily basis. Because the species that live here have to cope with regular exposure and submersion for many hours at a time, plants and animals have nowhere to hide and those living near the top of the high tide line are subject to the greatest physical stress.

The intertidal shoreline, between the high and low tide lines, is a challenging place to live for plants and animals, yet millions of individuals manage to survive and thrive in this habitat of extremes. Imagine that your home was underwater half the day, being pounded by surf-all of your possessions washed away as you struggled to cling to the rocks or the sand. Now imagine that during the other half of the day you were exposed to the heat of the sun, dehydrating on that same rocky surface or sandy floor-not an easy life. Yet in this intertidal world where space is at a premium, a staggering variety of life forms has evolved. So who is it that lives here, and how do they survive?

Sea urchins survive in the low-tide zone and must move to a rock pool on exceptionally low tides.
Photo: Genevieve Johnson

The intertidal zone is usually divided into three sub-zones (high tide, mid tide and low tide) in which the number of species increases the deeper the zone (because periods of exposure to air and sunlight are shorter the deeper an animal chooses to live). These zones are much more clearly seen on a rocky shoreline where most life tends to cling to surfaces rather than burying itself in mud or sand.

The high tide zone is the most difficult place to make a living as these shoreline communities are out of water almost half the time and this exposes them to the great environmental stresses of desiccation, changes in salinity, changes in temperature and, when it is raining, to osmotic problems from exposure to freshwater. Fewer species live here and their diversity is low. Barnacles, algae and hermit crabs are the dominant organisms in this zone. The hinged shells of barnacles can close so tightly that they form a seal perfect enough to retain a tiny seawater reservoir inside the shell at low tide-a coventry in which the barnacle can continue to live as it waits to be covered again by seawater.

Mollusc species such as chitons cling so closely to rocks that they are able to dissolve and excavate over time a shallow depression in the rock. The depression thus created is the size and shape of their base, which makes their seal with the rock surface more effective. They sally forth at night to forage on algae. But each returns to its own depression at the end of its foraging trip to await the return of the tide. By being out of water chitons stay out of reach of strictly aquatic predators, and by having a tough shell they avoid many potential land predators.

The shells of some snails have special ridges that effectively increase the surface area of their shell. This allows the snail to cool down, the way the large surface area of the radiator in a car dissipates the heat in the engine coolant. While crabs can run and hide from sun and dry air, either in tide pools or under rocks, this is not an option for the slow moving snails. For them and for chitons, it is their color that also helps prevent overheating.

Crabs cope well in the intertidal zone. They are able to run and hide in rock pools or under rocks. This is not an option for most creatures.
Photo: Genevieve Johnson

When running away or clamming up is not enough to prevent overheating and drying out, some species simply allow themselves to desiccate. Chitons can survive losing 85% of the water in their body tissues, and certain seaweeds may lose up to 90% becoming crunchy and dry. Yet, when the sea returns, they quickly re-hydrate, recovering completely-a nice example of the fact that life is simply structure and requires no magic inner process going on at all times to survive. Many seaweeds can dry down to apparently lifeless structures, then resume their active lives, simply by adding water. (Brine shrimp are an example of an animal that can do the same.)

The mid-tide zone is underwater three quarters of the time. It is only exposed at low tide and is therefore home to more species. Sea stars, anemones and rockweed are a few of the residents that live here; all are capable of surviving short periods of exposure as the tide retreats. Filter feeders often cluster in this zone where they are covered by water for at least part of the day. When exposed they withdraw their tentacles, turning into jelly-like blobs. Space can be tight here and there is much competition between residents.

The low tide zone is usually underwater except at extremely low tides. Lobsters, sea urchins, abalones, sea cucumbers, prawns and small fish are common here. In many instances animals are only able to survive by moving to rock pools. These oases are bigger, more plentiful, and are refilled faster in the low tide zone, providing a haven for many creatures that would otherwise suffer from exposure. Here they are able to await the return of the sea.

When the tide is in, all species must make the most of feeding opportunities without becoming prey themselves. When the tide is out, animals and plants not only have to protect themselves from overheating and dehydration, but also from predation from birds and crabs. When the tide returns, the animals that have been taking refuge begin to emerge. Mussels and barnacles open up, seaweed becomes moist and begins waving in the currents and sea anemones unfurl, waiting for its tiny prey to collide with its deadly tentacles. Many of the plants and animals that live within the tidal zone also use the slowest rhythms of the tides to fertilize and distribute their eggs. The highest spring tides are the best time for static coastal creatures such as corals and certain seaweeds to wash their larvae out to sea as the extra water in those impressive tides returns to the open sea from whence it came.

Many creatures such as snails, chitons, and clams choose to close up when exposed to dry air and simply await the return of the sea.
Photo: Genevieve Johnson

We see that the coast is one of the most demanding of ocean habitats and that living here means being able to withstand the constant battering of waves and storms and the ability to cope with continuous tidal changes. Unfortunately, many creatures that struggle to survive under these conditions must now face an added burden. Their specialized marine ecosystem is now presented with a different set of survival challenges posed by humans-things like coastal development, plastic pollution, chemical runoff, sewerage outflows-even overfishing. When we are indifferent to the fate of coral reefs and intertidal zones we contribute to the demise of these coastal habitats. But by making small adjustments to our lifestyles, from disposing of plastic and garbage responsibly to choosing not to build a home along an untouched coastline, all of us can make help preserve these all-important buffers to coastal erosion-these valuable nursery habitats for open-ocean fish.

Log by Genevieve Johnson

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