Mangrove trees and blue crabs are some of the estuarine species that have adapted to unique environmental conditions. In almost all estuaries the salinity of the water changes constantly over the tidal cycle. To survive in these conditions, plants and animals living in estuaries must be able to respond quickly to drastic changes in salinity.
Plants and animals that can tolerate only slight changes in salinity are called stenohaline. These organisms usually live in either freshwater or saltwater environments. Most stenohaline organisms cannot tolerate the rapid changes in salinity that occur during each tidal cycle in an estuary.
Mangrove trees have become specialized to survive in the extreme conditions of estuaries. Two key adaptations they have are the ability to survive in waterlogged and anoxic (no oxygen) soil, and the ability to tolerate brackish waters.
Some mangroves remove salt from brackish estuarine waters through ultra-filtration in their roots. Other species have special glands on their leaves that actively secrete salt, a process that leaves visible salt crystals on the upper surface of the leaves.
All mangrove species have laterally spreading roots with attached vertical anchor roots. These roots are very shallow. Because the soil in shallow areas of mangal forests is typically flooded during high tides, many species of mangrove trees have aerial roots, called pneumatophores, that take up oxygen from the air for the roots. Some species also have prop roots or stilt roots extending from the trunk or other roots that help them withstand the destructive action of tides, waves, and storm surges.
Many mangrove trees also have a unique method of reproduction. Instead of forming seeds that fall to the soil below and begin growing, mangrove seeds begin growing while still attached to the parent plant. These seedlings, called propagules, even grow roots. After a period of growth, these seedlings drop to the water below and float upright until they reach water that is shallow enough for their roots to take hold in the mud.
Mangrove forests, or mangals, grow at tropical and subtropical latitudes near the equator where the sea surface temperatures never fall below 16°C. Mangals line about two-thirds of the coastlines in tropical areas of the world.
There are about 80 species of mangrove trees, all of which grow in hypoxic (oxygen poor) soils where slow-moving waters allow fine sediments to accumulate. Many mangrove forests can be recognized by their dense tangle of prop roots that make the trees appear to be standing on stilts above the water. This tangle of roots helps to slow the movement of tidal waters, causing even more sediments to settle out of the water and build up the muddy bottom. Mangrove forests stabilize the coastline, reducing erosion from storm surges, currents, waves and tides.
Just like the high and low areas of salt marshes where specific types of grasses are found, mangals have distinct zones characterized by the species of mangrove tree that grows there. Where a species of mangrove tree exists depends on its tolerance for tidal flooding, soil salinity, and the availability of nutrients. Three dominant species of mangrove tree are found in Florida. The red mangrove (Rhizophora mangle) colonizes the seaward side of the mangal, so it receives the greatest amount of tidal flooding. Further inland and at a slightly higher elevation, black mangroves (Avicennia germinanas) grow. The zone in which black mangrove trees are found is only shallowly flooded during high tides. White mangrove (Laguncularia racemosa) and buttonwood trees (Conocarpus erectus), a non-mangrove species, face inland and dominate the highest parts of the mangal. The zone where white mangrove and buttonwood trees grow is almost never flooded by tidal waters.
A unique mix of marine and terrestrial species lives in mangal ecosystems. The still, sheltered waters among the mangrove roots provide protective breeding, feeding, and nursery areas for snapper, tarpon, oysters, crabs, shrimp and other species important to commercial and recreational fisheries. Herons, brown pelicans, and spoonbills all make their nests in the upper branches of mangrove trees.
Blue crabs live in estuaries along the United States' Atlantic and Gulf coasts. They are mobile predators whose salinity requirements change at different stages in their lives.
Adult male crabs live in the low-salinity waters upstream, while adult female crabs live in the higher-salinity waters near the mouth of the estuary. During the crabs' mating season (May to October), the high-salinity preference of the female overlaps with the lower-salinity preference of the male. After mating, female crabs migrate offshore, sometimes up to 200 km, to high-salinity waters to incubate their eggs. The females release their larvae, called zoeae, during spring high tides. The zoeae, resembling tiny shrimp, develop in the coastal waters. Zoeae require water with a salinity over 30 ppt (parts per thousand) for optimal development, which is only found in the ocean. Winds and coastal currents keep the larvae near the ocean shore, until they return to the estuary as young crabs, called megalops.
When the megalops return to the estuary, they swim up and down in the water in response to light and tides. This is called vertical migration. The young crabs use nighttime flood tides to move upriver into the shallow parts of the estuary. Eventually, the young crabs take up life on the bottom of the estuary, seeking out shallow-water habitats like seagrass beds and submerged aquatic vegetation, where they feed and gain protection from predators.
Plants and animals that can tolerate a wide range of salinities are called euryhaline. These are the plants and animals most often found in the brackish waters of estuaries. There are far fewer euryhaline than stenohaline organisms because it requires a lot of energy to adapt to constantly changing salinities. Organisms that can do this are rare and special. Some organisms have evolved special physical structures to cope with changing salinity. The smooth cordgrass (Spartina alterniflora) found in salt marshes, for example, has special filters on its roots to remove salts from the water it absorbs. This plant also expels excess salt through its leaves.
Unlike plants, which typically live their whole lives rooted to one spot, many animals that live in estuaries must change their behavior according to the surrounding waters' salinity in order to survive. Oysters and blue crabs are good examples of animals that do this.