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An estuary is the wide part of a river where it flows into the sea. It is affected by both marine influences (tides, waves, and the influx of saline water) and riverine influences (flows of fresh water and sediment) and is therefore not a perfectly stable environment. As a result, an estuary contains fewer resident species than the nearby marine or freshwater ecosystems, resulting in less competition for food and space. Because there is less competition, many estuarine species tend to be generalists; that is, they are able to consume a variety of foods, depending on what is available. Species that can tolerate the salinity and temperature changes in estuaries can exploit the area's high productivity, grow rapidly, and multiply into enormous populations.
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Many marine animals have body fluids that contain about the same concentration of salts as seawater and that are essentially isosmotic to the surrounding water, that is, the pressure of their body fluids is equal to the pressure of the seawater, and they neither gain nor lose water. Because the marine environment remains relatively constant, they do not have a problem maintaining water balance. Animals that live in estuaries, however, must have some physiological mechanism for dealing with the varying salinity: otherwise, their tissues and cells would absorb water and lose salts as they encountered an environment with lower salinity than the sea. Thus, estuarine animals are either osmoconformers, which survive by having tissues and cells that tolerate the loss of salts through dilution, or osmoregulators, which maintain an optimal salt concentration in their tissues regardless of the salt content of their environment.
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Animals such as jellyfish are unable to actively adjust the amount of water in their tissues. When their environment becomes less saline, their body fluid gains water and loses ions until it is isosmotic to the surroundings. These organisms are examples of osmoconformers. The ability of osmoconformers to inhabit estuaries is limited by their tolerance for changes in their body fluid.
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In contrast to osmoconformers, osmoregulators employ a variety of strategies to maintain a constant salt concentration in their bodies. Osmoregulators that live in estuarine waters concentrate salts in their body fluids when the concentration of salts in the surrounding water decreases. For instance, some crabs and fish regulate their salt content in less-saline water by actively absorbing salt ions through the gills to compensate for salt ions lost from their body. This helps them to maintain a relatively constant body fluid. Some animals can either concentrate salts when their environment is less saline or excrete salts when the environment is extremely salty. The latter are generally animals that live partly on land or in areas such as salt marshes and mangrove swamps that occasionally receive large amounts of rain. Other animals, such as the blue crab, are osmoregulators at lower environmental salinity and osmoconformers at higher environmental salinity. Many fish species are osmoregulators that can adjust to both high-salt and low-salt environments.
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Some estuarine organisms wall themselves off from their external environment to decrease water and salt exchange with their surroundings. Many estuarine animals have body surfaces that are less permeable than those of purely marine forms. This decreased permeability can be the result of increased amounts of calcium in the exoskeleton (outer skeleton) or increased numbers of mucous glands in the skin.
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In addition to changes in salinity, the problem of remaining stationary in a changing environment affects the distribution of organisms in estuaries. The more or less constant movement of water in an estuary makes it difficult for some organisms to remain stationary long enough to feed and carry on other vital functions. Because of this, survival favors organisms that are benthic-those that live at the bottom. Marine plants and algae in estuaries have substantial root systems, or holdfasts, to prevent moving water from pulling them up and carrying them out to sea. Animals live attached to the bottom, either in the available spaces around other sedentary animals and plants or buried in the small crevices between sediment particles.