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- Shell growth in estuaries is an aspect of marine biology that has attracted a number of scientific research studies. Many groups of marine organisms produce calcified exoskeletons, commonly known as shells, hard calcium carbonate structures which the organisms rely on for various specialized structural and defensive purposes. The rate at which these shells form is greatly influenced by physical and chemical characteristics of the water in which these organisms live. Estuaries are dynamic habitats which expose their inhabitants to a wide array of rapidly changing physical conditions, exaggerating the differences in physical and chemical properties of the water. Estuaries have large variation in salinity, ranging from entirely fresh water upstream to fully marine water at the ocean boundary. Estuarine systems also experience daily, tidal and seasonal swings in temperature, which affect many of the chemical characteristics of the water and in turn affect the metabolic and calcifying processes of shell-producing organisms. Temperature and salinity affect the carbonate balance of the water, influencing carbonate equilibrium, calcium carbonate solubility and the saturation states of calcite and aragonite. The tidal influences and shallow water of estuaries mean that estuarine organisms experience wide variations in temperature, salinity and other aspects of water chemistry; these fluctuations make the estuarine habitat ideal for studies on the influence of changing physical and chemical conditions on processes such as shell deposition. Changing conditions in estuaries and coastal regions are especially relevant to human interests, because about 50% of global calcification and 90% of fish catch occurs in these locations. A substantial proportion of larger marine calcifying organisms are molluscs: bivalves, gastropods and chitons. Cnidarians such as corals, echinoderms such as sea urchins, and arthropods such as barnacles also produce shells in coastal ecosystems. Most of these groups are benthic, living on hard or soft substrates at the bottom of the estuary. Some are attached, like barnacles or corals; some move around on the surface like urchins or gastropods; and some live inside the sediment, like most of the bivalve species. Minute pelagic species in the phyla Foraminifera and Radiolaria also produce ornate calcareous skeletons. Many benthic mollusks have planktonic larvae called veligers that have calcareous shells, and these larvae are particularly vulnerable to changes in water chemistry; their shells are so thin that small changes in pH can have a large impact on their ability to survive. Some holoplankton (organisms that are planktonic for their full lives) have calcareous skeletons as well, and are even more susceptible to unfavorable shell deposition conditions, since they spend their entire lives in the water column. (en)
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