Finding Nemo introduced millions to the technicolour world of coral reefs.  

But beneath its heartwarming tale of family reunion lies a treasure trove of marine biology – some spot-on, some wildly imaginative. Let’s dive in and separate the science from the storytelling. How accurate is Finding Nemo?  

Let’s start by identifying some of the main characters.  

Who are the fish in Finding Nemo?  

The clownfish 

Nemo and Marlin are orange clownfish or clown anemonefish (Amphiprion percula), and their home-bound lifestyle is spot-on. Unlike their cartoon counterparts gallivanting across the Ocean, real clownfish are the ultimate homebodies. Adult clownfish rarely venture more than a few metres from their host anemone, making Marlin’s anxiety about Ocean exploration biologically justified rather than neurotic. 

What type of fish is Dory? 

Dory goes by a lot of names: regal tang, palette surgeonfish, blue tang, royal blue tang, flagtail surgeonfish, regal blue tang to name a few (Paracanthurus hepatus).  

Regal tangs like Dory are common throughout the Indo-Pacific, so her presence on the Great Barrier Reef checks out perfectly. However, her famous memory problems contradict everything we know about fish cognition. Studies show that P. hepatus can remember spatial layouts for months and demonstrate complex social learning. More on fish brains later.  

How accurate are the fish in Finding Nemo? 

Mr Ray the spotted eagle ray (Aetobatus narinari) makes a charismatic teacher, though real eagle rays are typically solitary creatures who’d probably skip group activities in favour of a solo swim.  

Gill the Moorish idol (Zanclus cornutus) represents one of the aquarium trade’s biggest challenges. These stunning fish are notoriously difficult to keep alive in captivity due to their specialised diet of sponges and tunicates (a group of marine invertebrates that include sea squirts which look, to non-divers like coloured blobs on the reef). This explains Gill’s dissatisfaction with captivity and desperate escape plans.  

The film shows a fish dropping their kids off to Mr Ray’s class using their mouth, representing one of nature’s most devoted parenting strategies.  

Cardinalfish (Apogon species) are the most common marine mouthbrooders, with males incubating eggs in their mouths for 8-10 days. This explains why they seem unable to speak clearly – try having a conversation whilst holding 200 delicate eggs in your mouth without swallowing. The cartoon, however, doesn’t look much like a true cardinalfish. 

Crush and Squirt are green turtles (Chelonia mydas), shown as current riding nomads, which is entirely accurate. Green turtles have been tracked making migrations of almost 3000km (1,864mi)!  

Our current estimates are that green turtles live to approximately 80 years old, so the claim that Crush from Finding Nemo is 150 is a bit steep. Turtles aren’t known to travel in family groups, but Squirt does show the independence of a baby turtle. Right from the egg, turtles are fending for themselves, which Squirt shows they are more than capable of.

Do sea turtles really cruise the East Australian Current? 

The East Australian Current (EAC) serves as nature’s highway in Finding Nemo, and this isn’t just Pixar imagination. The EAC is a genuine part of the Oceanic conveyor belt (global network of currents circulating water), flowing southward along Australia’s eastern coast at speeds up to 1.5 metres per second. 

Crush’s “express lane” concept isn’t pure fantasy either. Ocean currents do have acceleration zones, particularly near topographical features like seamounts and continental shelf breaks. These current jets can provide genuine fast-track transport for marine life, making the turtle highway a plausible, if simplified, representation of oceanic dynamics. 

Green turtles (Chelonia mydas) really do use these currents for epic migrations, though their navigation system is far more sophisticated than simple current-following. The sea turtles use magnetic field detection to create internal GPS systems, imprinting on magnetic signatures as hatchlings and using these for navigation throughout their lives. 

Are the vegetarian sharks possible? 

Bruce and his gang’s “fish are friends, not food” philosophy in Finding Nemo might seem biologically ridiculous, but nature occasionally surprises us.  

Bonnethead sharks (Sphyrna tiburo) can derive up to 62% of their nutrition from seagrass, making them the Ocean’s most successful vegetarian predators. These remarkable sharks have evolved specialised digestive adaptations to break down plant cellulose – essentially becoming underwater cows with teeth.

Whilst no shark is completely vegetarian (they still eat crabs, especially when they are older), the bonnethead’s plant-munching abilities suggest that Pixar’s gentle giants aren’t entirely impossible – just highly evolved.  

Other creature features in Finding Nemo 

Pixar’s attention to detail shines with creatures like the Spanish dancer (Hexabranchus sanguineus) – a spectacular sea slug that really does inhabit the Great Barrier Reef and can reach 40cm in length. These crimson beauties are nature’s underwater flamenco performers, funky reef rugs on a magic carpet ride over the reef.  

However, some characters are biogeographical impossibilities. They wouldn’t be in the same scenes.  

The anglerfish is most likely a black sea devil (Melanocetus johnsonii), the same species filmed swimming to the surface in early 2025. Whilst visually terrifying, the encounter represents a fundamental ecological error. These deep-sea specialists live 200-2,000 metres (656 – 6561ft) down, where they’d never encounter shallow reef fish. Our clownfish friends don’t usually stray below 15m (49,2 ft). The poor blobfish is a good example of what happens when you take an animal out of the pressure range it’s adapted to.  

Similarly, Nemo’s classmate Pearl is a flapjack octopus (Opisthoteuthis californiana). These are usually hanging out at depths of 200-1,500 metres (656 – 4,921 ft). These adorable cephalopods (who had a new species found in 2025) are built for life under crushing pressure and would be about as comfortable in shallow reef waters as a penguin in the Sahara.  

Let’s really get stuck in. Pearl talks about one of their arms (they say tentacles, but we know octopus have arms) being shorter than the rest. This means two things – that Pearl is a male octopus, and that arm is their hectocotylus, or an arm shorter than the rest that’s specialised to store and transfer sperm during mating.  

Finding Nemo got it wrong? Let’s talk clownfish reproduction and genders. 

Since we are ruining childhoods, let’s address the elephant seal in the room. Brace yourself for the biological bombshell that completely rewrites Nemo’s story. 

Clownfish live together in anemones, with the largest individual as the matriarchal female. The largest male mates with her, with other smaller males helping with the chores and waiting their turn.  

When Coral, Nemo’s mum, died in that barracuda attack, the real biological story would be different. Within 10-18 days, Marlin would undergo a complete sex change, transforming into Marlina – the new dominant female clownfish of the anemone. This isn’t just changing wardrobes; it’s a full hormonal makeover involving suppressed testosterone and elevated oestrogen. 

But would Marlina then mate with Nemo, as some marine biologists suggest? (Because Nemo was the only clownfish in the anemone.) Probably not. Studies show that clownfish larvae typically disperse 7-12 kilometres from their birth sites, and genetics prove most anemone families aren’t actually related. Marlina would more likely wait for a wandering young male to join the family and restart the dynasty properly. Thank goodness.  

Does Mr Ray actually teach anything? 

We love that Mr Ray’s impromptu biology lessons contain genuine scientific gems, though we do have notes. His Ocean zone definitions are accurate – the mesopelagic (200-1,000m or 656 – 3,280ft), bathypelagic (1,000-4,000m or 3,280 – 13,123ft), and abyssopelagic (4000m+ or 13,123ft+) zones represent real oceanographic divisions with distinct communities. 

His species song (it’s called ‘Let’s name the species’, if you want to look it up) is catchy and gives a fun overview of the species you can find on a coral reef.  

“Cnidaria” would be more accurate than “coelentera”. Coelentera is an old term grouping a lot of the animals he goes on to name: hydrozoa (hydriods like the Portuguese man-o’-war), scyphozoa (true jellyfish), anthozoa (coral and anemones) and ctenophora (comb jellies). Add in the porifera (sponges), byrozoa (colonies of moss animals), echinoderma (urchins and sea stars) and “some fish like you and me” and you have a pretty comprehensive overview of life of the reef.  

Mr Ray’s excitement about “stromalitic cyanobacteria” is understandable and surprisingly sophisticated for a children’s film. These layered rock formations, created by ancient cyanobacteria, represent some of Earth’s earliest life. They were crucial in the Great Oxygenation Event 2.4 billion years ago. We can thank them for introducing oxygen to the atmosphere! Even now, the Ocean provides around half the oxygen we breathe.  

Fish cognition: Smarter than we thought 

Dory’s memory issues might be Hollywood fiction, but fish intelligence is no joke. Recent research has revolutionised our understanding of piscine cognition. Fish can recognise individual faces, remember complex spatial maps, use tools, and even show signs of self-awareness. 

Cleaner wrasses (Labroides dimidiatus) pass the mirror test – a cognitive benchmark previously thought exclusive to mammals and birds. Meanwhile, archerfish demonstrate remarkable learning abilities, accurately spitting water at insects with ballistic precision that would make a sniper jealous. 

The idea that fish have three-second memories is complete codswallop. Goldfish can remember things for months, whilst cichlids can recognise their offspring years after separation. If Dory existed, she’d likely be suffering from a very specific neurological condition rather than general fish amnesia. 

(additional note – read What A Fish Knows By Jonathan Balcombe for more) 

Scientific pet peeves in Finding Nemo 

The blue whale 

The film shows Marlin and Dory falling to the back of the throat, to be blown out of the blowhole into Sydney harbour. But blue whales can’t blow something out of its blowhole from its mouth.  

A whale’s blowhole is linked to the lungs, nothing else. It isn’t spurting water out, it’s a mix of mucus and water on its skin (think blowing your nose when you’re wet). Scientists can actually find out a lot from a whale from its snot, and they use ‘SnotBots’ – drones to collect whale blowhole bits.  

The jellyfish 

The jellyfish in Finding Nemo aren’t really any specific jellyfish, just mash of a few features to create a generic jelly. The closest real-life versions are the maeve stinger (Pelagia noctiluca) or the Amakusa Jelly (Sanderia malayensis), but neither are a perfect fit.  

Despite the sound effects, they don’t electrocute their prey – they have small cells firing tiny needles loaded with venom into anything that touches them.  

Marlin claims “I am used to it”. There isn’t much science to say that would help. Remember Nemo brushing in the anemone before school? That is science! Clownfish avoid being stung as they have a protective mucus layer similar to the anemone (it has to avoid stinging itself). They brush up against the anemone to coat themselves in the mucus, keeping them safe from stings. But this is specific to their home anemone and wouldn’t help much against a smack of generic jellyfish. Marlin isn’t any more jellyfish-proof than any other reef resident. 

So, is Finding Nemo accurate? 

Finding Nemo succeeds brilliantly in capturing the wonder of marine life whilst taking considerable liberties with biological reality. Its greatest accuracy lies in depicting clownfish territorial behaviour and anemone relationships, whilst its most glaring errors involve biogeographical impossibilities that would make any marine biologist wince.

We can’t not mention how clownfish would actually react in Marlin’s situation – a biological reality that completely transforms the story’s foundation. It’s a perfect example of how nature’s truth can be stranger and more complex than fiction. 

Perhaps the real magic lies not in perfect scientific accuracy, but in inspiring curiosity about the Ocean’s genuine wonders. After all, reality is often far more extraordinary than anything Pixar could animate. 

FIN. 

Post by

Will Steen

Or more accurately: Why do blue-footed boobies have blue feet?

The Galápagos Islands are full of the weird and the wonderful. One of their most iconic species is the blue-footed booby (Sula nebouxii), a marine bird that is found all along the Pacific coast. It is best known for its blue feet (which it was very creatively named after), making it really cool to look at but, why are they blue? To answer this question, let’s break it down into the how and the why.

The how: What are the mechanisms that cause blue-footed boobies’ blue feet?

Our first answer is potentially a killjoy one; they’re not actually blue, they just appear that way! 

Colours most often come from pigments, which absorb specific wavelengths of light, but the blue in blue-footed boobies doesn’t seem to come from a blue pigment. Blue pigments are actually very rare in nature – the funky blue mandarin fish is one of the few examples of an animal that makes any.

Instead, the blue in blue-footed booby feet is likely a structural colour which comes from light being reflected. The way different structures on animal surfaces are organised can affect the way that wavelengths of light reflect off their surface – like in the diagram:

The blue-footed booby has a layer of collagen under its foot skin, and the structure of this collagen likely makes them appear blue by reflecting only blue light wavelengths. This is the case for the majority of blue animals.

Pigments might still play a role though because blue-footed booby feet sometimes appear slightly greener which might be due to yellow pigments (blue + yellow = green!). These pigments are carotenoids which animals can’t actually make themselves. Instead, they get them from their diets and the food they eat can influence what colour they are.

For example, flamingos aren’t born pink – they’re born grey but become pink because of the carotenoids in their food. Similarly, if you eat too many carrots you could turn orange (but please don’t, that would be bad for you).

In blue-footed boobies, these carotenoids from their diet can affect how bright their feet are. Biologists tested this by changing how much food blue-footed boobies were given – when they didn’t get food, their feet became duller but when they were fed again with fresh fish, their feet became brighter.. This is super cool and also super important to keep in mind for our next question…

The Why: Why have blue-footed boobies evolved blue feet?

The reason that anything in nature looks or works the way it does is because of evolution. Darwin’s theory of natural selection says that traits that increase survival will be passed on so the fact that blue-footed boobies have evolved blue feet suggests they might be helpful in some way. But what advantage do blue feet give them? To answer this we need to understand sexual selection.

What is sexual selection?

Great question! It can be thought of as a special type of natural selection where traits that increase reproduction (instead of survival) will be passed on. This can include animals choosing a mate based on preferences for certain traits, which increases the chances of animals with those traits reproducing and so, the trait is passed on. For example, peahens prefer peacocks that have larger, more colourful tails which means large colourful tails get passed on over time!

As it turns out, female blue-footed boobies prefer brighter feet! 

We know this because biologists carried out some fun experiments – they used make-up to make male feets look duller (who says biology isn’t a very serious science?). When males had duller feet, the females were less likely to mate with them. Brutal!

Biologists also did this to males that had already mated with females (because blue-footed boobies don’t lay all their eggs at once). When the feet of these males were made duller, females actually made their second eggs smaller so that they’d hatch smaller chicks. Even more brutal!

This suggests to us that females are deciding who to mate with based on foot colour and if the blue isn’t as bright as they like they want to reproduce with those males less. So, foot colour might be a sexually selected trait because it increases the chances of reproduction for the males!

Why do female blue-footed boobies like blue feet?

As lovely as the blue is, it’s not just that they really like the colour. The stakes are quite high for them; they are choosing males to father their offspring and want to make a good decision. So, if they are basing this off foot colour, foot colour likely contains information that is quite valuable. It is likely a signal.

What are signals?
Signals are behaviours or structures that have specifically evolved to change the behaviour or state of others by conveying information. The response of the receiver must also have evolved due to signals – it is important to understand what receivers have to gain from responding to signals.

As we already know, carotenoids from their diet can influence blue-footed booby foot colour. What’s even more interesting is that these carotenoid changes can influence the immune response and foot colour also correlates to immune response.

Females might also be interested in how good of a parent males might be and foot colour might also signal this. When biologists swapped baby birds between nests, they found that the foot colour of the foster father was a pretty good indicator of condition (even though they weren’t genetically related).

So, to summarise: blue feet have potentially evolved because male foot colour might signal their condition, females want to reproduce with good condition males so they choose males based on foot colour.

What about female blue-footed boobies’ foot colour?

Male blue-footed boobies also seem to prefer brighter feet on females but the story is slightly less straightforward.

When female feet were made duller, the effects are mostly after they’ve already formed a pair, Blue-footed boobies form pairs then lay eggs but there is a courtship period before they lay an egg. In this period, females with duller feet received less courtship nest presentation (when birds add materials to nests) both from males they were paired with, and other males. 

In the period after egg-laying, making the female foot colour duller also had impacts on how much males incubated eggs. However, this was also affected by egg colour and size, which can indicate offspring quality. When females had duller feet, males incubated more in nests with a large egg but when they had colourful feet, males incubated both small and large eggs. Males also spent less time incubating small-dull eggs than small-colourful and large eggs.

So it seems that female foot colour is signalling something about their condition (and as an extension, the offspring’s) and the way males respond depends on the phase of reproduction they’re in. However, it also seems that foot colour isn’t the only useful indicator; egg size also is. 

One explanation might be that females have to decide between investing in their offspring (egg size) and signalling (foot colour) so foot colour might not give the whole picture. The mechanisms and reasons for this aren’t completely understood yet which just means there’s more left to learn about blue-footed boobies – exciting!

It’s not just blue-footed boobies that use signals

Nature is full of quirks! Beyond blue-footed boobies, evolution has brought about an array of interesting signals throughout nature. Animal signals are incredibly cool and incredibly diverse; they come in all shapes and sizes (literally) and some of them are also less honest than others. For example, some mantis shrimp display a claw that looks threatening to scare off intruders even when they are actually weak.

The signals that animals display will evolve to reflect the context in which animals live, which might be the environment or how they interact with species.

For example, yellow warblers (a species of small bird) produce alarm calls to scare off parasites. However, the yellow warblers that live alongside blue-footed boobies on the Galapagos don’t produce these calls. Why? They don’t need to!

They have been geographically isolated from their parasites for thousands of years, while the warblers on the mainland have not. These alarm calls either evolved before the Galapagos warblers became isolated, and they then lost the behaviour, or the calls evolved only in the mainland warblers afterwards. Either way, they reflect the lack of parasites on the islands!

Understanding why animals look or behave the way they do could be very valuable to us. For example, in the blue-footed boobies, their feet could show when their condition is declining. Since we know feet colour has likely evolved to signal their condition, if the foot colour changes, this could be because their food quality and availability has changed. This could signal to us that they’re in trouble!

So, as well as being incredibly interesting, learning more about nature and how species have evolved could also be important to protecting them.

Post by

Agata Czernecka

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.  

1. Anadromous fish are born in freshwater, spend most of their lives in the Ocean and then return to freshwater to spawn.  
2. 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 20^th 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.  

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)

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Will Steen

Our planet is known as the blue planet, over 70% of it is covered in water, most of which is the Ocean.

This water shapes our landscapes, influences where life thrives, affects the health of our Ocean and the weather in our skies. 

The Ocean is always closer than you may think (not in a sinister, about-to-jump-out-at-you way. It’s more of a realising-it-is-Thursday-and-the-weekend-is-only-round-the-corner-kind-of-way). 

Take a moment, think: what is different from the water you drank this morning (if you haven’t had any, this is your reminder to drink some) and the water lapping up a warm tropical beach? Every drop of water, from what’s come out of your tap to the water five kilometres deep in the middle of the Pacific, is connected.   

It is all just at different points in the water cycle. 

How does the water cycle work? 

There are four processes that drive the water cycle: evaporation, transpiration, condensation and precipitation.  

Water is warmed and evaporates, becoming water vapour. Amongst the many good things plants do, they release water into the atmosphere through transpiration. These two processes are responsible for putting water vapour in our air, our atmosphere.  

Water vapour is invisible.  

The steam we see when we boil the kettle (or the clouds in the sky) is water becoming liquid again, on contact with the cooler air. That is condensation, the transition back from gas to liquid. When enough of this cloud cools and turns to water, it will clump together and fall as precipitation (snow, hail and rain).  

How is the Ocean connected to the water cycle? 

This water then starts its journey back to the centrepiece of the cycle: the Ocean. 

The Ocean holds 97% of the Earth’s water – approximately 1.34 billion cubic kilometres. 86% of evaporation is from the Ocean, and 78% of precipitation re-enters the Ocean, directly. You can’t have the water cycle without the Ocean. 

Ice holds 2% of global water and just 0.001% is in the atmosphere – that is all the clouds in the sky.  

But if we add all that up, there’s a little still on the table – or more accurately, on land. That is the groundwater, lakes, swamps and the rivers. Rivers make up only 0.0002% of the total water on Earth.

What does the water cycle do? Why is it important? 

There are five main points of importance for the water cycle: 

1. Regulating Climate:  
The water cycle helps distribute heat around the globe, influencing weather patterns and climate conditions. It absorbs and releases energy during evaporation and condensation, which affects temperature and weather. 

2. Sustaining Ecosystems:
The water cycle provides the water necessary for plant growth and supports all forms of life by delivering freshwater to ecosystems through precipitation.

3. Shaping Landscapes:  
The water cycle contributes to erosion and sedimentation, reshaping geological features over time. 

4. Circulating Minerals and Nutrients:
Water transports minerals across the globe, enriching sea and soil and supporting plant life. 

5. Maintaining Freshwater Supplies:
The cycle replenishes freshwater sources, such as rivers and lakes, which are essential for human consumption and agriculture. 

Imagine a world without a water cycle – what would it look like?  

How is the water cycle changing?  

Human activity is interfering with the hydraulic cycle at every stage.  

Deforestation means less trees to transpire and absorb rainfall. Urbanisation interrupts drainage and can increase surface runoff. When it rains, the water that would have been absorbed by the ground now hits tarmac and runs down the road. 

The single greatest threat to the water cycle, and therefore to all life on Earth, is climate change.  

How is climate change impacting the water cycle?  

Climate change is intensifying the hydraulic cycle. Higher temperatures lead to more evaporation, more water vapour in the atmosphere, which results in more intense storms and rainfall. At the same time, droughts are becoming harder to predict and more severe.

These changes directly threaten our lives. 

Water is the life blood of our planet, and the water cycle is the pulse that keeps it alive. 

The hydraulic cycle regulates our climate, fertilises and maintains our ecosystems and shapes our world. We are changing it through our actions and activities.  

Understanding this cycle is the first step, but acting to protect it is the most important. The question is: what will we do to safeguard the blue heart of our planet? 

Post by

Will Steen

Everyone knows the Ocean is salty. But how did it get salty? Where did the salt come from? Is it getting more salty?  

These are all great questions to bring up as you ask for the salt over dinner. And after this explainer, you can answer them.  

Why is the Ocean salty: Explained 

Imagine a bowl. Now pour some slightly salty water into the bowl and put it into the sun on a hot day. Eventually, the water will evaporate, leaving that little bit of salt behind.  

Now, add some more slightly salty water to the bowl. The left-behind salt dissolves and mixes, making saltier water. Leave it in the sun again, the water will evaporate and leave salt behind. If you now attach a constant stream of slightly salty water into the bowl, you have a little model of our Ocean.  

Just like in the above example, rivers (the stream) bring tiny amounts of salt into the Ocean (the bowl) and the sun evaporates the water, leaving behind the salt. These amounts have built up over huge periods of time.  

Where does the salt in the Ocean come from? 

Most of the salt in our Ocean comes from rocks on land. Carbon dioxide in the atmosphere dissolves into rainwater, making it slightly acidic. When it rains, this slightly acidic water can dissolve the rocks it falls on and over, in the form of ions (charged molecules).  

These ions, mostly sodium and chloride, are carried into rivers, which carry them into the Ocean.  

Some salt also comes from volcanic activity, where elements from the Earth’s core can be released into the Ocean through underwater volcanoes and hydrothermal vents.  

Why aren’t rivers salty? 

Rivers are, very slightly salty. Depending on what the water has run over on its journey (rocks, decomposing plants, your ex’s belongings etc.), the contents of each river, including salt levels, varies. 

For water to be considered freshwater, its salt content must be less than 0.05% salt (saltiness is also commonly shown as parts per thousand or ppt: 0.05% is 0.5ppt). This means freshwater can still have a little salt in. 

Between 0.05 and 3% salt content is brackish water, and saltwater is between 3 and 5% salt. Above 5% (or 50ppt) salt is brine.

What are the saltiest bodies of water in the world?  

One of the saltiest bodies of water is the Dead Sea. It was cut off from the river Jordan by damming in the 1950s, so there is no significant freshwater input. This means the water is gradually disappearing, as the water level drops close to 1.21 metres (4 feet) every year.  

Think back to our bowl of water example – the water evaporating leaves its salt behind, so the sea is getting more salty. Salinity is roughly 337ppt (33.7% salt) – ten times the average of the Ocean.  

But it isn’t the saltiest – that title goes to the Gaet’ale Pond in Ethiopia. It is a volcanic spring, with a salinity of 433ppt. You wouldn’t want to swim there – CO2 bubbling up presents the risk of suffocation and the hot, acidic water could leave painful burns.    

Does the Ocean vary in saltiness? 

Yes. Where there is more freshwater entering the Ocean, it’s less salty. This can be in places that rain a lot, have lots of rivers entering, or have ice melting. On average the Ocean has a salinity of 35 ppt. 

Have a look at the picture below. The red areas show high salinity, purple areas are low salinity. Try and work out why each area is the colour it is.

The Baltic Sea is very enclosed, has lots of river input and rain, and little evaporation, so the Ocean surface can be around 10ppt. The Red Sea is much higher, 40ppt. This is due to very little rain or river input, and high evaporation.  

Is the Ocean getting saltier? 

The Ocean is now more in balance. If we go back to our bowl example, there is a bit we didn’t tell you about. Salt can mineralise at the bottom of the bowl – solidifying into rock, leaving the water. The amount of salt that mineralises is the same as the amount entering, so the Ocean stays the same level of salty.

So, with the next salty mouthful of Ocean water you get – thank the rocks and the rivers (and the rain and the sun).

Post by

Will Steen