Surviving in Salt Water
Part of the Water: H2O = Life exhibition.
No matter how thirsty you are, drinking seawater will only make you thirstier. Seawater is too salty for humans and most land animals--it's about 3.5 percent salt by weight. Seawater dehydrates you because the amount of water needed to flush the excess salt from your body would be more than what you drank. But many animals that live in or near the ocean have evolved ways to pump out the extra salt while keeping their water levels in balance.
A wandering albatross spends months at a time flying or floating on the open ocean, far from any source of fresh water. So albatrosses have evolved a way to drink seawater, which is too salty for most birds and land animals. To get rid of excess salt from the water and food they ingest, albatrosses have salt glands just behind their eye sockets. The glands excrete a highly concentrated salt solution that drains out through the tip of the beak.
Soaking in Salt
Most fish that live in the ocean tend to lose water--the high salt content of the ocean causes water to constantly flow out through the fish's gills. So fish need to drink lots of seawater to stay hydrated. And because seawater is so salty, they also must pump out the excess salt, both through their kidneys and using specialized cells in their gills.
Bony Saltwater Fish
Water naturally seeks a chemical balance, or equilibrium. That means water flows from areas of higher water concentration to areas of lower water concentration to equalize the system. Water concentration inside a fish is higher than in the ocean itself because the ocean is so salty. As a result, most saltwater fish constantly lose water through their gills and skin.
Because the fish is losing water, it must drink a lot to stay hydrated-but salty seawater is the only water around.
To get rid of excess salt, the fish's kidneys pump lots of salt into its urine.
Sharks don't lose water the way bony fish do--their bodies stay in balance with the ocean in a different way, thanks to the chemical called urea. There's essentially as much urea and other chemicals in water inside a shark as there is salt in seawater. So the shark stays in balance with the saltwater outside its body and water doesn't constantly flow out.
Instead of drinking water, the shark absorbs some seawater (and salt) through its gills.
A glandin the shark's digestive system gets rid of excess salt.
In Balance With the Ocean
All the salt in the ocean can make life complicated for animals living there. Sharks cope with the salty water by generating lots of the chemical urea. This substance, produced throughout the shark's body, counterbalances the salt in the ocean water. In other words, there's as much salt in the seawater as there is urea (and other chemicals) in the water inside the shark's tissues. So sharks don't lose water the way fish do. The shark gets rid of excess salt using a salt-excreting gland near its anus.
When you're splashing around in the ocean, you'd never guess that water molecules cling together like socks in the dryer. These weak forces are easy for humans to ignore, but for very, very small creatures water is almost like a thick syrup. Instead of swimming around, diatoms--a type of single-celled algae--simply drift wherever the water takes them. To keep from sinking, some oceanic diatoms trade heavier salt particles, like calcium, for lighter ones, like sodium.