One of the challenges inherent in a marine lifestyle is in maintaining an internal balance against external osmotic pressures. Cell membranes are permeable to water, and water tends to flow from areas of low ion concentration to areas of high ion concentration (which is called ‘osmosis’). Though the cell is incredibly complex, from an osmotic perspective it is basically a small sack of water with some ions in it. If cells aren’t isosmotic (i.e. containing the same concentrations of ions) to the surrounding environment, then water will flow across a cell membrane. Depending on the relative ionic concentration of the cell to the environment, water may flow either into or out of a cell. Either way, this water flow is bad for the organism and may result in cells shriveling up or bursting.
Different organisms solve this problem in a variety of different ways.
Hagfish and many marine invertebrates are osmoconformers and ion conformers. They simply keep their body fluids isosmotic with seawater by using the same ions found in seawater. If there is no osmotic difference between the seawater and their body fluids, then water won’t flow one way or the other.
Most teleost fish are osmoregulators and ion regulators. They keep their body fluids osmotically distinct from seawater and actively work to counter the effects of osmosis. Since there are fewer ions in fish body fluid than there are in seawater, fish are constantly losing water. To deal with this, marine fish are “drinking” seawater almost constantly. Since they only want the water and not the associated salt, they have special cells called chloride pumps that remove the extra salt.
In contrast, sharks (along with amphibians and coelocanths) are osmoconformers and ion regulators. Their body fluids are almost the same concentration of ions as seawater, but they use different ions. Sharks do have to deal with a slight influx of salt, which is excreted by a rectal gland.
One of the ions that sharks use is urea. Urea is relatively easy to produce (most organisms already make it in some form, they just excrete it), and works just fine as an ion from an osmotic perspective. The main downside is that urea is that it has a destabilizing effect on many enzymes, which is countered with the use of another ion: tri-methyl amine oxide (TMAO). After a shark dies, the urea in their body fluids converts into the foul-smelling and toxic ammonia.
Sharks are often thought of as “primitive” organisms, but they have a complex and effective method for living in salt water. Sharks’ use of a waste product to maintain osmotic balance is yet another amazing thing about these animals.
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