Most people know fish from the dinner table, or from their aquarium. Not many people know that fish form an extremely diverse group of animals and have evolved into many species that are as different from each other as night and day. Many of them have unique ways to see the world around them or to hunt prey. Others change their appearance or their physiology so dramatically over their lifespan, that it is hard to imagine that one is looking at the same fish at different phases of its life cycle. 

Sacabambaspis, a 460 million year old jawless relative of modern jawed fishes, swims in the shallow coastal waters of modern day Bolivia. Image: Nobumichi Tamura
Evolutionary origin of fishes has been localized to shallow coastal waters - fish science explained
All fish that we know today find their evolutionary origin in shallow coastal seawater, between 480 and 360 million years ago. Here, some fish developed into good swimmers with flexible bodies that conquered the deep seas. Others became armored and inflexible, limiting their swimming abilities, remaining confined to shallow waters. Stillothers moved to freshwater, and some even made it onto land, giving rise to the terrestrial vertebrates that are currently populating our planet including ourselves, humans. [1

Fish have thus had a long evolutionary history, starting from one habitat, and evolving into many different species, living in essentially every aquatic environment. They have developed into animals with stunning strategies to survive in their ecological niche, some so bizarre that it is hard to believe that they are real.

 

10. Living in the freezer: icefish

Icefish have antifreeze proteins in their blood. Image: Valerie Loeb [Public domain], via Wikimedia Commons
Icefish - fish science explained
Fishes have colonized the harshest of aquatic environments, some of them living happily in the ice-cold seawater at the North and South Pole. Due to the high salinity of seawater, its melting point is well below 0 °C (or 32 °F). In fact, it is -1.9 °C.  Fish blood is not that salty, so one would expect the blood to be frozen at ambient water temperature. However, Arctic and Antarctic fish have found a solution to the problem. Their blood is loaded with unusual but simple proteins that function as antifreeze. These proteins consist of 4 to 55 repeats of only three amino acids, the building blocks of proteins, and bind to the surface of ice crystals that start to form in the blood. As a consequence, the crystals cannot grow any further and the blood stays liquid. [2] [3] [4]

Although the antifreeze proteins are important for fish to survive in hostile arctic environments, fish still face the problem of viscosity of their blood. Under low temperatures, blood gets thicker and might clog veins and arteries. Icefish increase blood fluidity by lowering their number of red blood cells. Their hematocrit is only 16% (for comparison, ours is about 45%). A group of icefish species known as the crocodile icefishes went so far in their adaptation to low temperatures that they gave up their red blood cells completely! They are the only vertebrates known today that do not have red blood cells or hemoglobin. To compensate, and to allow enough oxygen to reach their tissue, they have exceptionally large gills for respiration, and can take up oxygen from the seawater through the skin. Crocodile icefishes have a bigger heart and a higher blood volume in comparison to their counterparts who do have red blood cells. [5]

 

9. Mexican cavefish: diabetic fish living in the dark

Mexican cavefish have adapted to living in the dark and little food supply. Image: Cavefish H. Zell [GFDL (http://www.gnu.org/copyleft/fdl.html) ou CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], Wikimedia Commons
Mexican cavefish - fish science explained

Astynax mexicanus is a tropical freshwater cavefish native to Mexico. A few million years ago, some of these fish presumably got trapped in dark caves and this gave rise to completely different varieties of cavefish. These fish are really special. They have undergone many physical, behavioral and physiological changes, one of which seems to have evolved as a result of living in complete darkness: the loss of eyes. Interestingly enough, Mexican cavefish eye development starts normally early in their lives, but development stops within a few days. This is due to chemical changes of certain genes that control eye development. These genes cease to be expressed, and eye development stops. Incidentally, some of these silenced genes are also found in humans and are dysfunctional in color and night blindness. [6]

Mexican cavefish have high blood sugar levels and are insulin-resistant, a condition very similar to diabetes type II. While human diabetics can develop serious health problems, cavefish are not bothered by high blood sugar, and instead use it to their advantage. As they live in caves that only receive new freshwater in spring, they eat as much as they can during this period to fatten up. They then use their reserves during the rest of the year. Thus, these fish have adapted their physiology to extreme living conditions with limited access to food. For this reason, they have developed a type II diabetes-like energy system. It is currently not known why cavefish do not develop diabetes-related health problems. [7]

 

8. Anglerfish: intimate lovers that fish with light 

Anglerfish fish with light and have a remarkable reproduction strategy. Image: Masaki Miya et al. [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
Anglerfish - fish science explained

Anglerfish comprise a highly curious group of about 160 different species living in the deeper parts of vast oceans. They live in obscurity, at depths where sunlight does not penetrate and mates and food are scarce. Some anglerfish species have gone to great lengths to deal with these particular conditions.

Female anglerfish have a large head that bears an enormous mouth full of sharp, inwardly pointing teeth. This allows them to consume prey up to twice their size, so that they have something to live on when food is not available. They have a very unusual way of attracting prey. Protruding from the front of the head is an appendage that moves back and forth, and… gives light! This is the only light animals can see in the deep sea. It is generated by bacteria that have been included in the tip of this “fishing rod”, and is effective in attracting prey: small fishes and crustaceans. It might be that this bioluminescent light attracts partners as well, but male anglerfish mainly use another strategy to find females.

Young male anglerfish have a very sensitive nose. And that is a good thing, because it is essentially the only thing about these fish that works well. They are small, about one-tenth of the size of a female, and do not have a bioluminescent fishing rod, so they have difficulty feeding. Their only purpose in life is to find a female fast, and they use their superior sense of smell to do so. Once it has found a female, it will bite into her skin. Then it starts to secrete an enzyme from its mouth that digests the skin of her body and his mouth. Next, the blood vessel systems of both fish fuse, with the male thus becoming a parasite of the female, as he now depends on her for nutrients. In return, the male gives sperm to the female for reproduction. [8]

 

7. Archerfish – Spitting hunters

Archerfish shoot insects out of the air. Image: Joseph Bylund through (CC BY-SA 2.0)
Archerfish - fish science explained

In 1764, the first report of unusual insect hunting fish native to the Dutch colony of what is now Indonesia arrived in Amsterdam, capital of The Netherlands. The report described a fish that could shoot a fly out of the air with a drop of water expelled from its tubular mouth, and this with cunning precision. Naturalists were skeptical. Spitting fish feeding on flies was too bizarre to believe and went against the generally accepted view at the time that airborne animals feed on fish, and not the other way around. And that without ever leaving the water!

Now, of course, we know better. These water-spitting fish are for real and are known as archerfish. They can adjust the power of their water missile to adapt for the distance and size of the prey to be shot down into the water. The secret to making their water missile so powerful lies in the higher velocity of the last water droplets in the beam. These catch up with the droplets that left the fish’s mouth just before, and fuse with them, so that the tip of the water beam contains all the energy and accelerates. Compare this with a water beam from a child’s water pistol. Here, all water droplets travel at the same speed, so that the water beam will eventually break up into smaller droplets, and energy is lost. Archerfish have thus solved this technical problem, allowing them to become successful hunters of insects above the water surface. [9] [10]

 

6. Flatfish: wandering eyes

Flatfish undergo an extraordinary metamorphosis during their lives
Flatfish - fish science explained
Flatfish are commercially interesting species. Sole, place, turbot, and halibut find their way into the kitchen of many restaurants around the globe. What most of the dining guests do not know is that flatfish are not born flat. In their larval stage, they are symmetrical, just like any other fish. But at the end of the larval stage, these fish undergo an extraordinary metamorphosis that dwarfs that of caterpillars turning into butterflies.

Metamorphosis is driven by ossification of what will become the blind side (the downward facing side) of adult flatfish. Bone formation on this side will push the eye away to the other side (the side facing up). This process is regulated by thyroid hormone, the very same hormone that plays an important role in human metabolism, growth and development. As a result of bone formation on one side only, the skull becomes asymmetric, and one of the eyes moves from one side of the head to the other, to end up next to the other eye. Also, the jaws are remodeled so that flatfish can live their lives on the bottom of the seas. [11]

 

5. Peters’ elephantnose fish: seeing the world through electricity

Electric fish sense their environment through the electric field they generate. Image: spinola [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Electric fish - fish science explained
Peters’ elephantnose fish live in muddy, slowly moving rivers and pools in West and Central Africa. Visibility in those waters is poor, and, on top of that, the elephantnose fish is a nocturnal animal. Eyesight is thus not of much use. How do elephantnose fish then sense their environment? The answer: with the aid of electricity.

Elephantnose fish generate a weak electric field around them with their electric organ. This is a modified muscle at the base of the tail. The electric field is disturbed by any object or creature that is sufficiently nearby. The fish constructs a mental image of its environment on the basis of these disturbances. It detects disturbances with electroreceptors on its body, especially on the “trunk”. Although this appendage looks like an elephant’s trunk, it is not really the nose, but actually an extension of the lower jaw. Elephantnose fish use this organ to look for worms and insects to eat. [12]

The information about the electric field is then relayed from the receptors to the brain, first to a specialized little lobe, and from there to a giant cerebellum that covers the entire brain. The mass of the brain comprises 3.1% of the body mass, and is responsible for 60% of whole-body oxygen consumption. The corresponding values for the human brain are 2.3% and 20%, while those for other vertebrates are considerably lower. It takes an exceptionally large and active brain to get an idea of what is happening around you when you use electrosensing instead of vision! [13]

 

4. Carp and catfish: swimming tongues 

Carp and catfish taste their environments with special receptors on their skin
Carp and catfish - fish science explained
Benthic fish species, those living on the bottom of a lake or the sea, are swimming constantly through clouds of sand, debris, and, hopefully, food. Carp and catfish look for food in these clouds, but instead of tasting everything they find by taking things in their mouths, as a human baby does, they detect edible things by tasting their environment with parts of their bodies.

Carp and catfish have taste buds, much like the ones we have on our tongues, all over their head, barbels, and fins, and catfish also have them scattered all over their body. These fish constantly taste the waters, and realize very quickly when they have bumped into something edible.

However, the mouth is still the most important region for tasting. The taste buds on the body give a general idea of whether the fish is swimming through something it can eat; the taste buds in and around the mouth give much clearer signals about the nature of the food.

Finally, carp have a taste filter built in: the palatable organ. This is a fleshy, muscular area on the roof of their mouths that is covered with millions of taste buds. When carp suck in food, they use this organ to taste it. The palatable organ automatically grips the edible particles of the food. The fish can now freely reject the non-edible parts of the food by blowing them out of their mouths, and then swallow the edible particles. [14]

 

3. Salmon and trout: living in sea - and freshwater

Salmon and trout have to go through great lengths to make migration from fresh to seawater and back possible
Salmon and trout migration - fish science explained
For a fish, migrating from fresh- to seawater and back is as challenging as going to the top of Mount Everest and back to a valley for us. The two aquatic environments pose completely opposite requirements for the physiology of the fish. One cannot catch a salmon in the sea, release it immediately in a river and expect it to survive. In reality young salmon, born in the rivers, go through a process of smoltification to prepare them for their new environment.

In freshwater, the blood of fish is saltier than the water the fish swims in. Fish have enzymatic pumps in their gills that pump salt from the water into the body. Also, they pee all the time, to get rid of the water that diffuses into their bodies due to osmotic pressure. Think about that when you see your fish swimming in your aquarium!

In seawater, the situation is exactly the opposite. The fish blood now contains less salt than their environment, so that the fish have to drink constantly to make up for the water leaving their body due to osmotic pressure. The pumps in their gills pump the salt out of the body, instead of pumping it in, as freshwater fish do.

During smoltification, young salmon and trout undergo many hormonal changes. Most notably, levels of the steroid hormone cortisol and thyroid hormone go up. These hormones make sure that the enzymatic pumps in the gills reverse, and adapt the kidney for their new job in the sea; water retention.

On the way back, when salmon and trout migrate from the sea back to their place of birth in rivers, these changes reverse. They also prepare themselves hormonally for reproduction. These changes throughout life are not always visible on the outside, but they are in fact as spectacular as the morphological changes during flatfish development. [15] [16]

 

2. Coral gobies: male or female?

Coral gobies can change their sex. Image: LASZLO ILYES (laszlo-photo) from Cleveland, Ohio, USA [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons
Coral gobies: male or female? - fish science explained
For many fish, it doesn’t matter whether you are born a boy or a girl. This can still be changed later! Let’s take coral gobies as an example. These monogamous fish are amongst the few species that can repeatedly change sex in either direction (a male becoming a female, or a female becoming a male).

This remarkable capacity of changing sex is thought to be an adaptation to coral life and their sedentary lifestyle. They don’t have many mating opportunities, and are at risk of predation when they move from one patch of coral to the next. The ability to change sex repeatedly in either direction allows any two fish to form a pair and to breed.

The brain gives the signal to change sex, but it is still unclear what the exact trigger is. The brain brings about a plethora of hormonal changes to induce the formation of either testes or ovaries, and to induce the degeneration of the previously present gonads in one of the two partners. [17

 

1. Mudskippers: stepping on land

Mudskippers are fish that spend most of their time on land
Mudskippers: stepping on land - fish science explained
Mudskippers might arguably form the top of the pyramid of fabulous fishes. Living in mangroves and other intertidal zones of the African Atlantic, and African and Asian Indo-Pacific coasts, these fish spend most of their adult lives on land. Here they hunt, fight and court, and communicate by sound. Like frogs, they have their eyes on the top of their heads to have an all-round view of their environment to detect predators. While on land, they breathe through their skin, and walk on their pectoral fins (at the front) and pelvic fins (at the back of their belly). However, their fins lack the bony structure of the fish that colonized the land millions of years ago, and gave rise to all terrestrial vertebrate life on Earth.

Although they feel comfortable living on the land, they still depend on water for their survival. They have to keep their skin and eyes wet. Mudskippers have a sac filled with water beneath their eyes, which is known as the dermal cup. They regularly pull their eyes into the cup to moisten them. It looks like if they are blinking when they do this. Also, they fill their gill chambers with water, so that the gills keep functioning and take up oxygen.

Mudskippers make nests in burrows under the ground, which they actively supply with oxygen by gulping air, storing this in their mouths, and exhaling it in the burrow where the eggs develop. As these amphibian-like fish are very sensitive to environmental changes and pollution, they are currently being recognized as potential bio-indicators in the environmental monitoring of coastal water and intertidal ecosystems. [18] [19] [20] [21] [22]


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