Salt is everywhere on our planet, but some animals evolved to live in it while, others didn't; what happened there? What caused this split?
Sup salty dogs, thanks for watching DNews, I'm Trace. Water is everywhere and is one of the main reasons life was able to evolve on Earth. It's pretty great. Salt is also everywhere. Salt and water together, make up most of the water on our planet and it's a major component of the beginning of life as we know it!
Organisms need salt to survive. Salt draws water out of cells in process called osmosis. Osmosis is the tendency of water to flow across a membrane to balance salinity. Essentially, nature wants to make sure there's a balanced level of salt everywhere. This is why if you drink saltwater you can die of dehydration -- the water-salt balance gets out of whack and water gets pulled out of your cells. It's pretty serious.
And yet, as boney fish moved from a salty sea into freshwater (or sweetwater), they had to eat salts to maintain that salt-water balance. They still do this today, urinating any excess salt -- sometimes up to a third of their body weight a day. We do this too, which is why we crave (and love) SALT. But this brings us to the big question -- if saltwater fish are drinking saltwater constantly, how do they avoid dehydration due to osmosis? !
Well, there are two answers. One comes kind of built in. The gills of some saltwater species, like the flatfish: turbot, have adapted to carry more of a special enzyme called " gill Na+/K+ ATPase." This allows their gills to leach salt from their bodies back into the ocean. Without this adaptation they would die because of that high salinity.
But, some species of eel, salmon, bass, and flounder have adaptations that let them move between fresh and saltwater! Species which can survive in a variety of salinities are referred to as euryhaline; some even actively adapt their gills or their kidneys… There are a lot of species osmoregulating in a lot of different ways. This is a great example of how evolution solves a problem along many different paths, something scientists call convergent evolution! SO AWESOME.
So, lemme give you a couple examples: anadromous fish, are born in freshwater and live there for months before they make for saltwater. In Atlantic Salmon, an enzyme called type 2 deiodinase is produced in response to the longer daylight hours in spring. Puberty ain't just weird for humans. This enzyme alters the salmon's gills, initiating osmoregulation so they can safely swim into salty water! This is reversed when they return to rivers and streams to breed and start the process with thier own lil' salmons.
And a 2013 study in Molecular & Cellular Proteomics, detailed how a DIFFERENT adaptation evolved in tilapia. When placed into salted water gradually, the researchers found tilapia gills adapted, producing a specialized protein called NDRG1. This let the tilapia survive in a saline environment -- as long as they had time to slowly adapt.
A lot fish evolution has happened in the last 400 million years. Fish have spread into pretty much every water-based environment on the planet. After the Great Dying 250 million years ago which wiped out 95 percent of all marine species, evolution had to figure out how to osmoregulate again. Through an assortment of convergent evolutionary adaptations FRESHWATER fish repopulated the oceans -- learning to deal with the super-salty water. Eventually, they made it to land as well, when a fish named eusthenopteron evolved into amphibians -- and tetrapods -- like you and me! Without salt, and being able to regulate it, life as we know it wouldn't exist. Evolution is the best.
Salt water fish diversity is pretty incredible, thanks to evolution. And this year, on animal planet, River Monsters, it's all about salt water baby. Don't miss new episodes of river monsters every Wednesday at 9/8c on Animal Planet.
