The marine intertidal is the region of the shore that emerges with outgoing tides and submerges with incoming tides. Organisms in this habitat must tolerate breaking waves, which impose large and seasonally variable physical forces on the organisms. Exposed intertidal organisms, descendants of entirely aquatic ancestors, must cope with extreme desiccation stress. Tidepool organisms, or those that remain in small remnant pools of water, encounter fluctuations in water temperatures, water quality, and dissolved oxygen in the water. In addition, the longer tidepools remain separated from the ocean, the more conditions deteriorate.
As the tide recedes, organisms in the highest portion of the intertidal or attached to vertical surfaces are literally left 'high and dry' - essentially thrust into a terrestrial environment. Exposure to heat and wind causes water loss, which in turn causes desiccation. Shelled invertebrates close their shells to protect their soft tissues from direct exposure. Small, mobile invertebrates and fishes hide within the intertidal algae, where they are sheltered from direct exposure, in a microenvironment with increased humidity. Many species of algae have evolved thick cuticles to reduce water loss by evaporation; other algal species have evolved the ability to tolerate large losses in water content for short periods of time. This latter group of algae serves as an example of species that do not attempt to maintain a constant internal milleu. Instead, they conform to surrounding conditions and are physiologically tolerant of the concomitant changes in tonicity and osmolarity. There are boundaries to every species' tolerance and zero percent water content would be fatal for these algae, but large changes in water content are tolerated. By either regulating or conforming, intertidal organisms can persist until they are submerged again by the incoming tide.
Cracks and crevices in the landscape often contain pools that persist between high tides, and many intertidal organisms can be found there. However, organisms living in these microhabitats face a different, but no less demanding, set of challenges between tides. These pools are typically small and shallow. During emersion, they are heated by the sun and experience large thermal fluctuations. These pools also have large surface areas, and water evaporates from the pools over time, which causes the salinity in the pools increase. Tidepool organisms also 'pollute' their environment by releasing nitrogenous wastes into the water. Because these environmental challenges will vary depending on the elevation, size, and depth of a tidepool, intertidal fish species tend to distribute themselves according to their ability to either compensate internally or be tolerant of fluctuations in temperature, salinity, and nitrogen. Species that can accommodate wider fluctuations in these environmental variables tend to be found higher in the intertidal, where pools are exposed longer and conditions become more extreme.
Animals that remain in emergent tidepools also consume most of the dissolved oxygen in the water, which creates another physiological stressor. Tidepool algae or plants may exacerbate this problem by consuming oxygen at night, although they release oxygen into the pool during the day (sometimes to dangerously high levels). Because dissolved oxygen levels are typically low in tide-pools, some tidepool fishes have evolved the ability to breathe air. An air-breathing fish that remains in a pool will take an 'air breath' from the surface, and hold the air in its mouth, where oxygen is extracted. Some tidepool fishes also have rigid gills and gill filaments that remain erect when unsupported by water, which allows oxygen extraction to continue while air contacts the respiratory surfaces of the gill filaments. If tidepool conditions become too unfavorable, fishes with these gill modifications can exit the tidepool and potentially move to a new tidepool until the tides return and conditions improve. Emergent tidepool fishes may also extract oxygen through the skin, via a process termed cutaneous respiration. However, aerial respiration does pose a challenge for eliminating acidic and nitrogenous wastes, which normally diffuse readily through the gills into the surrounding water. In addition, because evaporative water loss occurs rapidly in a terrestrial environment, ionic and osmotic regulation are also problematic for these fishes. In fact, evaporative water loss may limit the distance and nature of the terrestrial excursions undertaken by marine intertidal fishes.
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