- Science: Explained
Why can some animals live in fresh and saltwater?
Fish that break all the rules by living in the Ocean and streams:
Look into a river and you will find very different animals to the Ocean, even if they are just a few miles apart. Why are these wet worlds so different? In short – it is all about osmoregulation (explained below).
Whether you live in the Ocean, in a river, on a mountain or in Thneedville (yes, that’s a Lorax movie reference) – everywhere has its challenges. One of the main challenges of living in saltwater (read more about why the Ocean is salty here) is maintaining the balance of water in an animal’s cells.
What is osmosis?
Water will travel from areas of high concentration to low concentration in an attempt to balance them (this is called osmosis). Salty water has lower water concentration than freshwater.
Join us to explore the difficulties of swimming between the two worlds, some of the incredible journeys of the fish, like salmon, eels and bull sharks, that overcome them and crown the winner of the wet.
What is osmoregulation?
All living things need water, and they need salts. Maintaining the balance of both is tricky – too much or too little of either is fatal.
Imagine a balloon full of water, but this balloon can let water in and out of it. This is our cell. The water in the balloon (our cell) has a little bit of salt in.
If you put the balloon in a bucket of freshwater, water enters the balloon (by osmosis) to balance the concentration. This could end up bursting the balloon. Alternatively, putting the balloon in salty water will lead to the water leaving the balloon, shrivelling it.
Animals living in these environments have to adapt to avoid bursting or shrivelling – neither sound particularly fun.
Fish living in freshwater have to hold on to their salts and avoid water intake. Saltwater fish take in as much water as they can and excrete the extra salts.
How do freshwater and saltwater fish stay hydrated AKA: do fish drink the water they live in?
To maintain a good balance of water and salt, fish in different environments alter their drinking, gill function, kidneys and excretion (waste removal).
Marine fish will constantly drink sea water, getting as much water in as possible. They actively remove salt from the water through cells in their gills. The pee of marine fish is highly concentrated urine (if you get dehydrated, your body does the same – your pee will be very yellow, with little water diluting it), minimising water loss.
Freshwater fish, on the other hand, don’t drink water. They don’t need to. Think of the balloon example – they are saltier on the inside, so water wants to move in. Freshwater fish easily absorb water through their gills. They use energy to pump salts in, against the concentration gradient – they actively ingest salts. Their pee is very diluted, ensuring they don’t become swamped by discarding lots of water.
Why can’t freshwater fish live in the Ocean?
Knowing all that, let’s see what would happen now if we put a marine fish in freshwater. A marine fish wants to lose salt from its body and keep water. Think of the balloon – a marine fish invites water in, pushing salt out. This means the balloon will lose all its salts and get over filled with water. That is one unhappy fish.
For a freshwater fish in the Ocean, the opposite happens. Used to a world with plenty of water and little salts, the balloon will shrivel as it is filled with more salts and loses water. The bottom line is the same – an unhappy fish.
Are there fish that can live in both freshwater and salt environments?
Amazingly, yes. There are examples of fish that can live in both marine and freshwater all over the world. There are two main types.
- Anadromous fish are born in freshwater, spend most of their lives in the Ocean and then return to freshwater to spawn.
- Catadromous fish live in freshwater most of their lives, returning to the Ocean to spawn.
We will explore these groups through a couple of their most famous members, as well as a shark that makes it all look easy.
How do salmon survive in both fresh and salt water?
Salmon are incredible fish.
Not all salmon are anadromous: there are Atlantic salmon in North America called sebago, named after one of the lakes they are found in. More Atlantic salmon live in Norwegian, Swedish and Russian lakes; and yamame are a landlocked Japanese masu salmon.
But some salmon travel thousands of miles between fresh and salt water over the course of their lives.
Chum salmon have been estimated to complete total migrations of over 10,000km (6,200 miles) across the North Pacific from feeding grounds to the Yukon river or streams in Japan.
Anadromy appears to have evolved at a similar time that the Ocean cooled and became richer in food. This, coupled with the existence of landlocked variations, suggest that salmon were a freshwater fish that took to the sea, although this is not confirmed. What is certain is the incredible journeys and transformations many salmon go through to mate.
For salmon, the movement from river to Ocean and back to river is integral to their life cycle.
The first few years of a salmon’s life are spent growing in the river, before they go to the food-rich Ocean. Here they gorge themselves, growing very quickly. After travelling many thousands of Ocean miles, they will return to the rivers they were hatched in, to spawn themselves.
But how do they manage to conquer both environments?
Through hormonal changes, salmon make behavioural and physiological changes to the ways they manage their osmotic balance. In freshwater, they won’t drink water – when in salt water they will drink a lot. Hormones change the fish’s physiology, increasing the number of ion transporters in the gills and kidneys to process the salt balance.
The change is a costly one though, as salmon won’t feed during their return to freshwater, relying on fat reserves built up through years in the Ocean. They battle up their rivers, overcoming waterfalls, predators waiting on the banks and their own failing bodies to reach the same spot they hatched in. Here, they will spawn.
For most of these fish, it is the last thing they do.
What can salmon teach us?
Just as rivers are the connection between us and the Ocean, salmon are among the clearest species to bridge that gap. And they feel the impact of people more keenly. Rivers are the frontlines, and salmon are in the trenches.
Salmon rivers are best in old forests, as the tree roots hold the banks together and keep the rivers form – holding it narrow and fast flowing. Where forests are lost, the river can widen and the salmon population diminishes. And as we build dams to harness the power of the rivers, we block the salmon from getting home.
The populations of salmon in different rivers show far more genetic variation than between people. Each salmon is genetically coded for its river home. Climate change, pollution, human development and fishing – salmon deal with a lot.
But their adaptability is shown time and again. The genetic diversity they show allows them to overcome the challenges they face. Just as they are able to thrive in these two different worlds, they can take on the new world we are shaping.
What do you call returning to the Ocean to reproduce?
Catadromy is the mirror of salmon – starting life in the Ocean, living in freshwater and returning to the salt to spawn.
European eels begin life as eggs riding Ocean currents, drifting through the Sargasso Sea near the Bahamas. They hatch into small, transparent, leaf-shaped larvae called leptocephali. Like so many (hungry, well-teethed) leaves in the wind, the Ocean carries the eels to the coasts of Europe, a journey taking 2-3 years.
On reaching the coast, they metamorphose (change body shape) into glass eels – still small and see-through but eel-shaped. After up to a year, they change again into elvers (juvenile eels) and begin to travel up rivers. Here, they change again into yellow eels and can remain in freshwater for up to 20 years until they reach sexual maturity.
This means an eel can be 23 years old before reproducing. When their time comes, they become silver eels, migrating down rivers towards the Ocean.
How do eels switch between living in fresh and saltwater?
For the elvers and glass eels, they need to do the same as our salmon. They alter the salt uptake of their gills via specialised cells, increasing it in freshwater and decreasing in saltwater.
For many years, European eels were characterised by mystery. They were well known in the rivers of Europe, yet no one could find baby eels. Greek philosopher Aristotle suggested that they just appeared out of the mud, and this was the general belief for almost 2,000 years.
It was Johannes Schmidt, a Danish biologist working in the early 20th century, who began to unveil the elusive eels. By finding progressively smaller eels across the Ocean, he followed the trail of breadcrumbs to the Sargasso Sea. He couldn’t find any spawning, but he drew the metaphorical arrow.
It wasn’t until 2022 that we found the first direct evidence of adult European eels travelling to and reaching the Sargasso Sea. This study also shows us just how far the eels travel – up to 8,000km. If you’re ever lucky enough to see one of these eels, appreciate just how far it has come, and how far it still has to go.
The eels adapt twice, changing their whole bodies to swap salt for fresh and back. On the way back, the silver eels don’t feed. As with salmon, they rely on fat reserves for their journey, and their bodies slowly disintegrate, and once they have reached their spot, they reproduce and then die.
But the switch doesn’t always have such dire consequences.
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Are bull sharks the best sea-swappers?
Yes. Bull sharks (Carcharhinus leucas) are the aquatic conquerors supreme. As we have seen, moving between fresh and salt water is tough. Most salmon only do it once, some can manage it a couple of times, their bodies failing them under the stress. Eels change their whole bodies when they make the switch. Yet bull sharks can move between fresh and saltwater with apparent ease.
They have been found in unexpected places. In Africa, bull sharks are known as Zambezi sharks as they are found deep into the Zambezi river. They were initially described as a different species* – because no one expected a bull shark there. In Brisbane a bull shark was spotted swimming the streets after flooding in 2011, and they have been to Baghdad up the Tigris. They have even been found in Alton, Illinois, 2,800 km (1,740 miles) from the Gulf of Mexico.
The ultimate tourist? A bull shark was reported in the upper reaches of the Amazon, in the Ucayali River, Peru. Nearly 5,080 km (3,157 miles) from the Ocean.
How do bull sharks do it?
Ready for some high-density science?
They change how salty they are (in the balloon). In the Ocean, bull sharks’ blood is at least as salty as the water they are in due to the levels of urea and trimethylamine oxide (TMAO). But when in freshwater they excrete much more urea, lowering the salt concentration of their blood to minimise the gradient (the difference in saltiness).
However, they are still more salty than freshwater, so they absorb water and lose salts through their gills. They change their salt and water processing to match their environment. All sharks have rectal glands through which they excrete excess salt when in the Ocean. When in freshwater, bull sharks reduce the activity of their rectal gland to preserve these salts. The kidneys of bull sharks in freshwater go into overdrive, producing large amounts of dilute urine to avoid the balloon-popping scenario. Both the kidneys and the gills are triggered to actively uptake salts, while the liver changes urea production.
On top of all this, bull sharks have to deal with the different densities of salt and freshwater. As anyone who has visited the Dead Sea will tell you, more salt = floaty (scientific term). So, bull sharks in freshwater reduce the densities of their livers to counter their reduced floaty-ness. Still, living in freshwater really does drag them down, which may be why they still mostly prefer the Ocean (can’t say we blame them).
Why do these species matter?
These fish, as with our rivers, are great connectors. Their journeys are important to all those they encounter. By travelling between the separate biomes, they transport nutrients and link ecosystems, strengthening them.
They are used as indicators of water quality and ecosystem health and provide food sources. Something harder to measure is their cultural importance, which resonates through many different social histories – they bring us closer together as well.
The challenges of moving between the Ocean and its fresh-water fingers are staggering. Yet these fish tackle it head on.
Which do you find more impressive; the salmon battling bears and waterfalls to return to its river home; eel larvae drifting thousands of miles and swimming back, changing their whole body to tackle each step; or the bull shark that is just as at home in the heart of the Amazon as a reef in the Indian Ocean?
From the legend of the salmon, to the mystery of the eels, to the euryhaline hero the bull shark, these fish are truly conquerors of the coast.
Book recommendations from our Marine Scientist
When I am writing my articles, I use a variety of sources. One of the most engaging are the books. Here are a few I used for this article, do let us know if you have a read, and watch out for more recommendations.
Blowfish’s Oceanopedia
Salmon by Mark Kurlansky
Sharks of the World
Additional sources:
*Peters, W. C. H (1852): Hr. Peter legte einige neue Säugethiere und Flussfische aus Mossambique vor. – Bericht über die zur Bekanntmachung geeigneten Verhandlungen der Königlich Preussischen Akademie der Wissenschaften zu Berlin. Aus dem Jahre 1852. Berlin, pp. 273–276. (not available online)
