Saltwater fish osmoregulate by actively managing salt levels and water intake to maintain a stable internal environment despite living in a high-salt external environment.
Living in the ocean presents a challenge for fish: the surrounding seawater has a much higher salt concentration than their internal body fluids. This difference causes them to constantly lose water through osmosis and passively gain salts. To survive, saltwater fish must employ specific physiological mechanisms to counteract these passive movements and maintain a stable internal solute concentration, typically around 400 mOsmol.
Based on the provided reference, "The Gill's Role in Osmoregulation in Saltwater Fish," saltwater fish manage this balance through two primary actions:
- Stopping Salt Inflow and Active Secretion: Saltwater fish must stop the inflow of salt and actively secrete it at the gill. The gills, which are crucial for respiration, also play a vital role in osmoregulation. Specialized cells in the gills, often called chloride cells, pump excess salts from the bloodstream back out into the surrounding seawater, effectively removing the salt that is constantly entering their bodies. This active transport requires energy.
- Drinking Seawater: To compensate for the water loss due to osmosis, saltwater fish drink seawater and hydrate themselves with it. While drinking saltwater might seem counterintuitive because it contains salt, their digestive system and kidneys are adapted to process this water. They absorb the water from the ingested seawater and excrete the excess salts, often concentrated in small amounts of urine or via the gills.
These two processes work in tandem to help the fish maintain their internal water and salt balance. The gills are central to both preventing salt gain and actively expelling it, while drinking seawater is necessary to replace lost water.
In summary, saltwater fish combat dehydration and salt gain by:
- Using their gills to block salt entry and pump out excess salt.
- Consuming seawater and processing it to obtain water while eliminating salts.
This complex system allows them to maintain their internal environment against the osmotic gradient of the ocean.
