Amphibious Fish: Aquatic-Terrestrial Adaptors
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Amphibious fish are some of the most remarkable creatures in the animal kingdom, possessing an incredible ability to survive on dry land for short periods.
With special adaptations such as the use of fins as makeshift ‘feet’ for walking and specialized lungs for breathing, more than 11 genera of fish are considered to be amphibious.
In this article, we’ll explore these aquatic-terrestrial fish and uncover their evolutionary journey and extraordinary adaptations that allow them to thrive in diverse habitats.
What are Amphibious Fish?
An amphibious fish is one that is able to survive out of the water for a certain period of time. It is important to keep in mind that these fish will not survive permanently on land, they merely come out of the water as an escape.
Some of these fish will escape the water to get away from predators, whereas other times, they may find themselves in an environment of low oxygen and being out of the water will be beneficial. It may also be too hot in the water, and moving onto land allows the fish some shade. That said, these adaptations also help the fish in the event it should accidentally be washed up on shore.
How did Amphibious Fish Evolve?
Animals throughout the ages have evolved new physical traits and behaviors that help them to adapt to and survive in the ever-changing environment.
It’s thought that the very first land animals evolved from the water around 400 million years ago, the first species, known as tetrapods, left the water. It’s amazing that some of the evolutionary traits we see in modern amphibious fish resemble those seen in the animals that first emerged from the water all those years ago.
During the first part of the evolution of these fish, we see the development of strong, sturdy fins without a lot of joints. Unlike those of something like a goldfish, the fins of fish like the ancient eusthenopteron, could have been used for ‘walking.’ It would have originally used this trait for hunting along the seabed. Over time, fish would evolve these limbs to be much stronger, allowing them to hold their body weight out of the water.
Fossils of another ancient creature known as ichthyostega have been found in Greenland demonstrating that these animals had both a tail suitable for swimming and ‘limbs’ suitable for moving over dry land.
Of course, the most important thing that needed to evolve was how the fish took in oxygen. Fish originally had gills, and this has been seen in fossils. However, through fossils, scientists have been able to determine that, according to the location of the spiracular tract, fish may have been able to breathe on land.
Where this tract is located over the top of the gills, it likely functioned alongside them, meaning the fish would only be able to breathe in water. But some species were seen to have a larger spiracular tract that had moved further up on the head, acting almost as a blowhole.
Adaptations of Amphibious Fish for Surviving Out of Water
Life on land is very different to life in the water, so it’ll come as no surprise that amphibious fish need to have special adaptations to help them survive.
Usually fish breathe using gills which typically come in four pairs on the sides of the head. As the fish closes the gill covers and takes in water through the mouth, this water passes through the gills and small lamella take out the oxygen.
However, amphibious fish need other ways to breathe if they’re going to survive on land. They also have some other, very interesting adaptations.
Lungs
Animals need to breathe in oxygen to survive. While fish in the water use gills for this, amphibious species have special lungs or other air breathing organs.
In many cases, these lungs are derived from the fish’s swim bladder and are, compared to other animals’ lungs, incredibly primitive. However, in addition to gills, they still allow the fish to breathe terrestrial oxygen.
Most fish that have lungs are freshwater species, such as the aptly named lungfish.
Muscular Pectoral, Pelvic & Dorsal Fins
In order to propel themselves on land, fish have the need for much stronger and musclier fins. As such, fish such as the mudskipper are blessed with muscular pectoral fins that connect to the body similarly to human shoulders and also feature an ‘elbow.’ This structure allows the fish to drag itself along the land.
These specially adapted fins are so strong that some species are even able to climb with them.
Certain species of catfish can often be seen moving across roads after floods and have a strong, spiny pectoral fin that gives them an extra boost.
Breathable Skin
The notion of having lungs is one thing, but imagine if fish were able to take in oxygen through their skin! Well, some species can, including the mudskipper that has blood vessels very close to the surface of the skin which can take in oxygen. And they’re not the only species capable of this, with many marine amphibious fish making use of vascular ‘breathing.’
However, the key to this specially adapted skin working as it should is that the fish remains wet. In periods where their homes may dry up, some species, like the mangrove killifish, are able to stay alive for up to two months as long as they keep their skin moist.
Bulging Eyes on Top of Their Head
You may have seen some amphibious fishes and noticed that they have big, bulbous eyes on the tops of their heads. But there’s a reason for this unusual appearance, and it’s all about being able to survive outside of the water.
The way that their eyes have adapted allows them to see clearly both under the water and on dry land. Mudskippers have a special lens that is not as curved as other fish and this actually allows them to see better out of the water, meaning they’re able to remain on the lookout for predators. And so their eyes don’t dry out when they’re spending time on land, there’s a special pouch that holds water.
Types of Amphibious Fish
There are 11 families of amphibious fish, and they all have some amazing adaptations and ways of surviving. Let’s get to know some of these species a little better.
1. Mudskipper (Periophthalmus spp.)
Out of all of the amphibious fish, the mudskipper maybe spends the most time on land; up to 80% of its life, in fact. They are found in Indo-Pacific regions, Polynesia, and Australia and frequent swamps, mud flats, and estuaries.
There are 25 species of mudskipper, some of which can grow up to 12 inches (30 cm) long. They’re known for their ability to skip around land as well as their frog-like eyes on the top of their heads which allow them to see both in and out of the water.
They have pectoral fins that sit slightly more forward than usual, and these strong fins are what enable the animal to move around. Moreover, there have been some instances where mudskippers have been seen to jump up to two feet (61 cm) using the strength of their fins alone.
This species burrows under moist sediment for thermoregulation, but it must remain wet at all times to ensure that its specially adapted skin and mouth lining can function for breathing.
2. Mangrove Rivulus (Kryptolebias marmoratus)
Found in brackish and marine waters along the eastern coasts of North, Central, and South America, the mangrove rivulus can lay claim to being the fish that can survive longest out of the water, up to 66 days!
It does this thanks to its special skin through which it breathes and one of the main reasons that it leaves the water is due to temperature. According to one study, kryptolebias marmoratus reap the benefits of an amphibious lifestyle because they can escape stressful thermal conditions through emersion and evaporatively cool to prevent overheating. In short, they’re better able to control their body temperature when out of the water.
When on the land, these fish become much less aggressive than when in the water. They spend a lot of their time living in old insect burrows. Not only are they able to breathe through their skin, but they adapt their gills to retain nutrients and water.
3. West African Lungfish (Protopterus annectens)
Sometimes known as the Tana lungfish, this species can be found across the western and some middle parts of Africa as well as the northern parts of southern Africa.
Like many amphibian species, the West African lungfish has a set of primitive lungs which enable it to take in oxygen outside of the water. However, they’re unable to do this unless they’re in a state of dormancy. When they enter this state, wrapping themselves in a cocoon of mucus under the mud, they’re able to remain this way for up to 3 and a half years! This is known as estivation and occurs when the African river beds dry up and force the fish to adapt.
4. Epaulette Shark (Hemiscyllium ocellatum)
It’s like something from a horror movie; the shark that can walk on land. But it very much exists and these Australian tropical fish are able to walk owing to their specially adapted fins that can be rotated so they double as ‘feet.’
These are small sharks, usually growing no bigger than a 3.3 feet (1 meter), that live in coral reefs and tidal pools. If oxygen levels in the tide pools are low, the shark can survive for up to two hours.
Sadly, these amazing creatures, which can live for up to 25 years, are now critically endangered due to habitat loss.
5. Northern Snakehead (Channa argus)
The northern snakehead is an incredibly interesting species that are native to places like Russia, China, and Korea. It was discovered in Europe back in the 1950s and has since been in US waters, most notably in Georgia, where it is considered to be a highly invasive species.
The problem with the fish is that it is easily out-competing native wildlife, and the advice from authorities is to kill any northern snakeheads you find on sight. Moreover, there is a risk that these fish will transmit diseases to native species.
Northern snakeheads prefer shallow, muddy waters and have pectoral fins that are more forward, enabling better movement on land. They move their heads to help them move forwards.
Another way that they’re able to survive on land for up to four days is a sac above their gills in which they can store oxygen.
6. Nopoli Rockclimbing Goby (Sicyopterus stimpsoni)
The Napoli rock climbing goby is only found in Hawaii and takes its name from its ability to climb rocks. It does this thanks to suckers both on the body and in the mouth. They do this to escape predators on their journey to their adult habitat.
These fish are primarily found in coastal regions and freshwater streams but they are unfortunately under threat due to a loss of habitat because of things like hurricanes and ray-finned fish taking over.
Napoli rock climbing gobies begin their lives in the ocean, where the eggs are swept out to. Over the first few months of life, they’ll develop here before gradually moving inland by the age of around two years.
7. Climbing Perch (Anabas testudineus)
The climbing perch is native to parts of Asia, stretching from Pakistan to China. However, there are concerns that the species may be migrating toward Australia, where it would be considered highly invasive.
These fish are able to survive on land for as long as six days thanks to specially adapted lungs located near the gills.
They can be found hibernating in dry creeks, where they’ll remain for as long as six months and can be seen ‘walking’ using movements from its tail as well as spines underneath the gills. Although, it isn’t amazingly adapted to walking and will only move short distances on land.
8. West African Bichir (Polypterus retropinnis)
Looking at the West African bichir, you could be forgiven for thinking you were looking at a primitive tetrapod since they look far more like these than modern bony fish. They’re native to the Congo river and the Ogooue river in Africa and are slender fish that grow up to 13 inches (34 cm).
In terms of surviving outside of the water, the West African bichir has special slit-like spiracles as well as lungs, allowing it to breathe on land. It uses this ability when the oxygen levels in the water are too low.
The West African bichir is a popular aquarium fish and will coexist with other large species quite happily.
9. Mosquitofish (Gambusia affinis)
The mosquitofish is a small species that is native to North America. While they’re used to control mosquito populations, they’ve actually had the opposite effect after being introduced to Australia, where they’re now considered a pest.
These fish are able to survive in water where the oxygen levels are low and may also come to the surface to obtain oxygen when the levels are too low.
Studies have also shown that these fish use thrashing movements when they are on land to get them back to their natural habitat. While these movements may seem uncoordinated, it appears that they’re choreographed perfectly for the fish’s needs.
10. Grunion (Leuresthes spp.)
If you’re on a Californian beach late on a summer’s night, you might be lucky enough to see thousands of grunion leaving the water and making their way onto the beach. They do this to spawn, which takes the adults just a few minutes. However, the babies can stay on land for as long as two weeks!
The grunion is a toothless fish that primarily feeds on plankton. However, there’s a lot of research that needs to be done into their feeding habits.
We’ve been able to observe the fish jumping onto the beach to spawn and once the eggs are laid, they’ll remain under the sand until high tide washes them back out to sea. Since copulation and spawning take place on the beach, the species has now been protected through beach closures and limited fishing.
11. Walking Catfish (Clarias batrachus)
While the walking catfish isn’t doing so well in its native Africa and Asia, it seems to be thriving in Florida, where it’s often seen wandering around roads and parking lots. It’s able to do this by moving its body allowing it to slither.
But why does it behave this way? Well, the walking catfish uses this ability to move from one body of water to another and it happens after there has been a period of heavy rainfall. In one ‘out of water session’ that can last up to 18 hours, these fish can cover over 0.75 miles (1.2 kilometers).
They’re even able to breathe out of the water with specially adapted lungs that have a tree-like structure. The gills close when the mouth opens, allowing air in and the oxygen is absorbed by blood vessels within this organ.
12. Eel Catfish (Channallabes apus)
In order to catch prey, the eel catfish can be seen propelling itself out of the muddy waters it inhabits in Central Africa. Just like the walking catfish, this species has a tree-like organ that allows it to breathe out of the water. Despite not having weight-bearing pectoral fins like some amphibious species, the eel catfish has spines that propel and support it out of the water.
Growing up to 16 inches (40 cm) in length, this fish will come out of the water and bend its body so that its mouth lands directly on its prey. However, it’s also able to hunt in the water by simply sucking food into its mouth.
13. Mummichog (Fundulus heteroclitus)
Found in estuaries and coastal waters, the mummichog lives along the eastern parts of North America. It’s a species of killifish, and interestingly, in 1973, it was the first species of fish to be sent into space.
These fish are tough cookies and can survive in waters of varying salinities, whereas most species are either fresh or saltwater tolerant. They can also survive in a wide range of temperatures thanks to an ability to alter their metabolic rate.
While they don’t actually come out of the water, mummichogs are able to ‘breathe’ air on the surface of the water which contains a lot of oxygen, and may survive outside of the water for several hours as long as the air is moist.
