ASCDC
E-NEWSLETTER

If one were to look for the most unstable environment on earth that poses the greatest challenges to survival‚ the best choice would be the intertidal zone.

The definition of the intertidal zone is the area between the high tide mark (the highest point to which the tide rises) and the low tide mark (the lowest point to which the tide falls). The area above the high tide mark lies beyond the sea's reach‚ except in the case of natural disasters‚ but the area below the low tide mark is forever immersed in sea water. The intertidal zone is what lies between dry land and sea – a strip of land that is constantly transformed by the changing tides. In reality‚ the term 'intertidal zone' includes numerous distinct ecosystems – rocky coastlines (such as Taiwan's East coast) giving rise to tidal pools‚ sandy coastlines with beaches stretching to the horizon (such as Taiwan's West coast)‚ or mangrove forests‚ if the terrain is suitable (such as the mouth of the Danshui River). Nevertheless‚ the natural environment of all intertidal zones shares a common trait – volatility.

The Mudskipper (Periophthalmus cantonensis) mainly lives in the semi-salty waters of river mouths and mangrove forests. Pictured above is the Danshui River mangrove environmental conservation area.

Because of the tidal pull between the earth‚ moon‚ and sun‚ the sea level rises and falls twice a day. Intertidal organisms must therefore survive in an environment that is constantly changing between dry and wet. In addition‚ the strength of the sun's evaporative effect can severely affect the living conditions of these organisms‚ either in terms of a difference in temperature or osmotic pressure. As a result‚ most organisms capable of surviving in intertidal zones have developed special skills for such a harsh environment. Below are several summarized methods by which organisms can endure the changing environment.

Adapting to Heat and Aridity

Marine organisms‚ when faced with aridity‚ often choose to lie in narrow crevices to reduce exposure to the sun and contact with the air; examples include the snails and chitons that inhabit rocky coastlines. Some shellfish even increase the density of the ridges on their shells‚ thereby allowing them to release heat absorbed from the sun into the surrounding air. Some Cnidarians such as the Sea Anemone‚ on top of withdrawing their tentacles into their bodies and shrinking their body surface area as small as possible‚ also secrete mucus to reduce water evaporation. On the other hand‚ intertidal organisms such as the Fiddler Crab and Mudskipper‚ which live on sandy coastlines‚ retreat into caves to escape the sun's heat. As for the organisms that stay in tidal pools‚ as long as they remain in the pool‚ they are underwater and will not have to face the problem of aridity.
 

Mudskippers can be found throughout coastal Northern Taiwan‚ Penghu Island and Xiao Liuqiu Island.

 

Respiration Adaptability

When the tide falls‚ many gilled organisms face the threat of suffocation. In the case of shellfish‚ many close their shells and hibernate‚ reducing the body's metabolic rate and allowing them to get through the drought.

Meanwhile‚ mudskippers have developed an additional method of breathing; they are able to absorb and release gases through their skin and mouth‚ thus preserving the stability of their blood oxygen level and maintaining their metabolic operation.

Adaptation and Specialization in Finding Food

Due to the instabilityof the intertidal zone‚ the overwhelming majority of organisms that live within it have developed scavenging strategies according to the feeding patterns of the environment. To take the example of the Mudskipper‚ it has developed specialized dorsal fins so that‚ when the tide falls‚ it can climb or jump along the muddy sands in search of algae or other organic detritus. Crustaceans such as the Soldier Crab ^[1] normally hide in caves‚ emerging in groups during low tide‚ using the brief period when the sand is still wet to find organic detritus for additional nutrition. Bivalves ^[2] use other mollusks‚ such as the Axe shaped musclular ^[3] ‚ to bury themselves in sand. The rising tide brings them the greatest joy‚ as it allows them to filter out the organic detritus from the sea‚ their source of nutrition and growth.

Egrets (Egretta garzetta) are one of the predators of the Mudskipper.

As for the many organisms that live in tidal pools‚ although the time available for hunting for food is relatively limitless‚ when the tide rises‚ it directly exchanges pool water for sea water‚ not only taking away organic waste‚ but also bringing with it organic debris. High tide is therefore perfect for searching for food - so much so that certain fish species also use this time to enter tidal pools to find food and lay eggs.

The intertidal zone's unstable nature forces organisms to develop specific evolutions and ways of living. Because the intertidal zone is the closest to human activity‚ it is the easiest of nature's “classrooms” to observe‚ but at the same time it is the most susceptible to damage by human destruction‚ sewage‚ and waste‚ all of which makes it more difficult to survive. In truth‚ the ability of intertidal organisms to change and adapt‚ maybe even evolve‚ greatly protects such a special environment‚ which in the future may bring further revelations.

[1] Soldier crab (Mictyris longicarpus): a small crab that inhabits sandy beaches and mud flats.

[2] Bivalves: Bivalvia‚ the main distinctive feature is they have two shells.

[3] Axe shaped musclular: A special muscle that Bivalvia mollusks have that is shaped like an axe‚ used to move the body by contracting and expanding.