Adaptation to varying salinity

Changes in the osmolarity of the environmental medium are a physiological stress faced by a variety of aquatic organisms. The physiological ability to cope with this environmental variability allowed a number of aquatic organisms to colonize osmotically changing habitats such as brackish waters or to span from fresh water (FW) to sea water (SW) environments. Many fishes undertake vast migrations between rivers and oceans in order to find food or breeding sites. Eels are one of the most well-known euryhaline migrating fish. They are born in sea water and pass through the leptocephalus stage, an extended larval phase, in the open ocean. Then, they undergo a juvenile stage that is a smaller version of the adult stage. The young eels swim upriver where they gradually develop into adults. They stay in the fresh water for several years before they become sexually mature, at which time they begin swimming down river towards the sea. Therefore, during their lifecycle the eels experience environmental osmolarity changes twice ranging from about 1100 mOsm in SW to nearly 0.5 mOsm in FW. They are able to maintain body fluid osmotic pressure and electrolyte concentration at levels independent of ionic concentration and osmolarity of the environmental medium. This is achieved by balancing the gain and loss of water and ions. In fresh water environment the animal continuously faces the osmotic gain of water and constantly needs to excrete the excess water. In sea water environment the eel needs to constantly acquire water to compensate for the osmotic water loss across the body surfaces.