Don’t pee in the water

8–15–16, Lefko

So my FFOTD is based on a very basic, but oft-ignored question: what the hell is the difference between freshwater and saltwater fish, and why can’t they just switch between environments? The answer had to do with the ion concentrations in their blood. A la Reddit: “Freshwater fishes tend to have much higher concentrations of ions (like sodium) in their blood compared with the concentrations in the water. Their bodies are designed to expel large volumes of very dilute urine frequently. This works to their advantage in a freshwater environment because they are surrounded by water with low salt concentrations. So, just pee a lot and hang onto what little salts you have. They also have specialized cells in their gills to allow them to directly take up sodium and chloride from the water to fine-tune the salt balance in their blood and cells. Saltwater fishes face the opposite problem. They need to maintain salt concentrations in their blood that are much lower than the surrounding environment. To do this, they actively drink water and form a highly-concentrated urine to expel the excess salts. They also actively expel salts at their gills. So the basic freshwater strategy is to pee like hell and absorb salt. The basic saltwater strategy is to drink and hold it so they can absorb as much of the water (while leaving behind the salts) as possible. Put either of these fishes in the opposite environment, and these critical systems fail to function. The “pee like hell” strategy will quickly deplete cells of water in a saltwater environment, while the “drink and hold it” strategy will completely water-log them. These salt concentrations are critical to many bodily functions. Just think about what happens to people when they get dehydrated or, in some cases, drink TOO MUCH water. They are at real risk of death. Same for these fish.”

A very interesting situation comes up when you start looking at sharks — they can go between environments! Wtf, how, you ask? The answer is urea, a nitrogen based compound that is highly concentrated in shark blood. We [humans] also produce urea as a metabolic waste product that we excrete in our urine (its decomposition into ammonia is what makes pee smell). Sharks store enough urea in their blood to bring their ion concentrations roughly equal to the ion concentration of sea water. When they move into fresh water they are able to expel urea directly through their skin to match their blood ion concentration to that of the surrounding fresh water — thus preventing the shark from ballooning up as water rushes into its system. This is actually the reason that most people don’t eat shark (other than Mako)! If you ask a salty weathered captain why you don’t eat shark, you’ll often hear “they piss through their skin, ya don’t want to eat that!” He’s mostly right — you would be eating high concentrations of the compound that is really the hallmark of your piss…Just some food for thought *heh puns*

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