Tidal Pools
Scattered thoughout the saltmarsh are a number of tidal pools which vary in size and depth. These pools are formed when blocks of ice are carried onto the saltmarsh with a high tide in the winter. As the tide recedes the blocks of ice are left behind and subsequently freeze to the ground. On a later high tide, these blocks of ice are lifted from the ground and moved, taking the soil with them and leaving a depression in the ground. The process is self-perpetuating; the depression in the ground may fill with water which will freeze into an ice block that may be picked up, along with soil from the bottom of the depression, at a later high tide.
Just as the size and depth of the pools may vary, so too do the number and kind of organisms living in the pools. The most common organisms encountered in tidal pools include mosquito larvae, water boatmen, salt marsh snails (Hydrobia minuta; small snails up to 4 mm), mud snails (Nassarius obsoletus; larger snails, up to 2.5 cm), various species of fish (mummichogs, common silversides, sticklebacks), small anemones (Nematostella vectensis) and crabs.
Depending on the amount of flooding by the tides the community in any one tidal pool can be very dynamic. The ever changing conditions can lead to a number of problems by the residents of these pools. For example, when these pools are flooded, the fish extend out over the entire marsh in search of food. As the tide falls, they congregate once more in these pools (not necessarily the same one) where food availability decreases between flooding and where all species are more susceptible to predation. As well, there may be tremendous variation in salt concentration within the pools. When flooded by tides (probably two or three times every two weeks), the salt concentration is similar to that of seawater. As water evaporates, the salt concentration can become much higher than seawater. After a rain, the salt concentration approaches that of fresh water. This creates serious water balance problems because of the movement of water into and out of the tissues due to osmosis.
Many species of animals are able to tolerate wide fluctuations in salinity. In some animals (osmoconformers) the concentration of the extracellular fluid changes when environmental concentration changes, while in others (osmoregulators) the extracellular fluid concentration is maintained close to a set point regardless of enviromental concentration. A number of intertidal and estuarine species (marine mollusks, many annelid worms) are osmoconformers. However, most crustaceans and vertebrates that are able to tolerate wide fluctuations in salinity are osmoregulators.
Marine teleosts, for example, are hypotonic to their environment and continually lose water by osmosis. To compensate for this water loss, these fish drink seawater and dispose of NaCl via special chloride cells in the gills which actively transport Cl- ions out (Na + follows passively). Divalent ions (magnesium, calcium and sulfate) which make up roughly one tenth of the salts found in seawater are disposed of via the excretory system while excreting very little water. When faced with conditions of increasing salinity, the number of chloride cells in the gills actually increases, enabling the fish to pump out more NaCl. When environmental conditions become more dilute, the number of chloride cells decreases.
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