18 Fascinating Fish Facts From The Underwater World

Embarking on the journey of exploring Fish Facts reveals the incredible diversity in the world of aquatic species. From tiny gobies to enormous whale sharks, fish play essential roles in Earth's many ecosystems, supplying valuable resources in the form of food, sport, and natural beauty. 

With around 34,000 known species, fish exhibit a vast range of characteristics, allowing them to inhabit nearly every body of water on our planet. 

Did you know that certain fish species, such as eels and knifefish, generate and detect electric fields for various purposes like navigation, communication, and hunting prey? This unique ability sets them apart in the diverse underwater environment, where an astonishing array of colors, shapes, and sizes converge. 

Read on for some of the more fascinating facts about fish, and you'll come to appreciate the endless array of information these aquatic inhabitants have to offer.

Or for more from our underwater friends, click on to our compilation of fish quotes for more words of fishy appreciation or dive deeper with our ocean facts. 

18 Fish Facts From The Deep

1. Fish are masters of camouflage

We know that fish breathe through gills. But did you know they can change color and patterns to blend in with their surroundings? This vital survival strategy helps them evade predators and sneak up on unsuspecting prey. Specialized pigment cells called chromatophores enable this process by expanding or contracting⁸ to alter the fish's appearance.

The batfish is a fascinating example of aquatic camouflage, which uses a clever tactic when faced with danger. Notably, the batfish plays dead! The batfish deters predators by imitating a dead leaf floating on the water’s surface. It's natural coloration and shape further enhance this unique strategy, helping it avoid becoming a meal for larger predators like other fish.

2. Some fish can “taste” with their fins

You might be surprised to learn that catfish and certain species of fish from the carp and shark families possess the extraordinary ability to "taste" their surroundings using taste buds on their fins⁹.

While these fish swim, their specialized chemoreceptor cells detect specific chemical stimuli in the water, providing valuable information about potential food sources and nearby predators.

The distribution of taste buds across a fish's body varies depending on habitat, diet, and feeding style. For instance, some catfish species boast over 100,000 taste buds, mainly concentrated on their barbels and fins. This adaptation helps bottom-dwelling fish uncover hidden food among sediment and debris on riverbeds or ocean floors. 

Moreover, fish can also differentiate between tastes like sweet, sour, salty, and bitter, much like humans. Other fish, like sharks, also have taste buds on their fins, allowing them to sample the water for traces of prey or danger. 

3. Fish can live in all types of water

Fish habitats range from vast oceans to serene lakes, winding rivers, river basins, and small ponds. While fish breathe the same air everywhere, each ecosystem presents unique challenges and ecological conditions that fish have evolved to overcome.

Saltwater fish like the vibrant clownfish and formidable sharks navigate saltwater easily in the oceans. On the other hand, freshwater fish such as the elusive trout and voracious piranha find their homes in tranquil lakes and rivers. 

Some fish, like salmon, exhibit extraordinary osmoregulatory capabilities, allowing them to inhabit multiple types of water throughout their lives. Salmon, a well-known anadromous fish, hatch in freshwater streams. As they mature, they migrate to the open ocean. These fish live there for most of their lives before returning to their natal freshwater habitats to spawn. 

Similarly, certain eel species undergo the opposite migration, transitioning from freshwater to saltwater environments as they mature. 

4. Fish can communicate through sound

Fish have developed a fascinating ability to communicate with one another. Most fish talk to their fellow aquatic inhabitants using various vocalizations⁶. Many fish species, such as groupers, damselfish, and sharks, produce grunts, clicks, and drumming noises for multiple purposes.

These sounds warn other fish of potential danger and express emotions like aggression or even help them engage in courtship. While some fish sleep, their companions can warn them of incoming threats. Some fish create noise by grinding their teeth together. Others rapidly contract muscles around their swim bladder, generating vibrations that resonate through the water.

In specific scenarios, these sounds are essential for survival. Besides warning of predators, fish talk to each other to maintain territorial boundaries or coordinate social interactions within their group.

5. The sailfish is the ocean’s fastest fish

The sailfish holds the title of the ocean's fastest fish. Capable of reaching speeds up to 68 miles per hour (110 km/h), these agile predators outpace even the quickest marine creatures. Their exceptional speed comes from a combination of streamlined bodies, long slender bills, and highly efficient muscular systems.

In addition to these features, the sailfish has a unique flexible backbone that enables it to generate incredible force and achieve rapid bursts of acceleration. When chasing after schools of smaller fish like sardines or anchovies, sailfish use their speed and agility to herd and corner their prey.

Their extraordinary speed is a powerful defense mechanism, allowing sailfish to evade predators such as sharks. Sailfish can even leap out of the water. Today, their impressive speed has made sailfish a sought-after target for sport fishermen.

6. Salmon migrate for thousands of miles

Salmon undertake an arduous journey from their oceanic feeding grounds to the freshwater environments where they were born³. This migratory phenomenon spans thousands of miles and is driven by the salmon's innate desire to reproduce in their natal streams. 

To navigate the vast ocean, these fish rely on Earth's magnetic field and their keen sense of smell. They can pinpoint their birthplace with astonishing precision and determination.

Upon reaching their freshwater spawning grounds, salmon display awe-inspiring endurance and tenacity. They battle powerful currents, propelling themselves upstream. Like acrobats, they often leap over obstacles like waterfalls and rapids to reach their destination. During the journey, salmon also undergo striking changes in coloration and body shape, showcasing the extraordinary nature of their migration.

Salmon expend tremendous energy during the migration. Many individuals cease to feed entirely, focusing solely on the task. Imagine running a marathon without stopping for food or rest! The culmination of this incredible journey is the creation of a new generation of salmon. Females dig nests in gravel stream beds to lay their eggs, and males fertilize them.

7. Male deep-sea anglerfish follow a bizarre mating ritual 

In the eerie and cold water of the deep ocean, where sunlight barely reaches, the deep-sea anglerfish has evolved an astonishing and peculiar mating ritual². 

One of the most fascinating aspects of this process is the parasitic bond between the male and female anglerfish. The much smaller male relies on his highly developed sense of smell to find a suitable mate. Upon locating a female, he attaches himself to her body using his sharp teeth, secreting an enzyme that dissolves their tissues at the point of contact, ultimately fusing their skin and blood vessels.

This remarkable phenomenon is known as sexual parasitism. As the male anglerfish becomes a permanent part of the female's body, he loses his eyes, internal organs, and reproductive organs, making him completely reliant on her for sustenance and oxygen. 

In return, the male provides the female with a continuous supply of sperm for reproduction. This mutual arrangement enables the female anglerfish to produce offspring without external fertilization, significantly increasing her reproductive success in an environment where encounters with potential mates are rare and unpredictable.

8. Not all fish are cold-blooded

Since all fish breathe air through their gills, you might assume they are all cold-blooded, relying on their environment to regulate their body temperature. 

However, in 2015, a remarkable discovery by the NOAA Southwest Fisheries Science Center challenged this belief for one of our list's more recent interesting facts. The center revealed the opah, or moonfish, as the first known fully warm-blooded fish⁷. It is the only fish that can maintain a higher body temperature than its surroundings.

Compared to cold-blooded fish, the opah generates and preserves warmth through constant movement, continuously flapping its pectoral muscles and creating heat via muscular exertion. Specialized blood vessels in their gills retain heat and distribute it throughout the body. This warm-blooded adaptation enables the opah to thrive in colder, deeper waters, enhancing muscle performance, swimming speed, and reaction times. 

Consequently, the opah can hunt and evade predators more effectively than other fish species struggling to maintain energy levels in frigid conditions.

While the opah is a fascinating example of a fully warm-blooded fish, some species, such as particular tuna and sharks, exhibit partial warm-bloodedness. These fish employ a process called regional endothermy, which allows them to warm specific parts of their bodies, providing a similar advantage in colder environments. 

9. Pufferfish eat toxic algae 

Pufferfish, found chiefly in tropical and subtropical ocean waters, have a unique self-preservation method: they consume toxic algae. This unusual diet includes algae containing tetrodotoxin (TTX)⁵, accumulating in the pufferfish's liver, skin, and reproductive organs. As a result, their flesh becomes highly poisonous, deterring predators and posing risks to humans.

This toxic diet makes pufferfish one of the most venomous vertebrates on Earth. Some pufferfish species, especially the Takifugu genus, seek out these poisonous algae. 

Intriguingly, pufferfish can control the toxin levels in their bodies by adjusting their diet. Although they primarily feast on invertebrates like crabs, snails, and sea urchins, they sometimes consume seagrass and algae when their preferred foods are scarce.

The pufferfish's peculiar appearance and toxic diet have long fascinated humans, with some cultures even considering them a delicacy.

Read more: Pufferfish facts.

10. Some fish can walk, and some fish can fly 

In the diverse world of fish, some species have developed remarkable adaptations to navigate land and air. Mudskippers, for example, are small amphibious fish found mainly in tropical and subtropical regions. These fascinating creatures belong to the family Oxudercidae and possess the extraordinary ability to walk on land using their modified pectoral fins. This unique method of locomotion allows them to explore intertidal zones like mudflats, mangroves, and estuaries in search of food and shelter.

What makes mudskippers even more impressive is their capacity to survive out of water for short periods. Their specialized skin and mouth lining can absorb oxygen, allowing them to breathe in the open air.

In contrast, flying fish from the Exocoetidae family has evolved a different, yet equally astonishing, method of evading predators. These marine ray-finned bony fish, which belong to the order Beloniformes, glide gracefully above the water's surface. Their enlarged, wing-like pectoral fins enable them to leap out of the water and swim for up to 200 meters (660 feet) before returning to their aquatic habitat.

11. The Greenland shark can live for up to 400 years

The mysterious Greenland shark holds the record for being the longest-lived fish. This remarkable creature can live for up to 400 years, inhabiting the frigid depths of the North Atlantic and Arctic Oceans. 

As they prowl the ocean floor searching for prey, researchers use radiocarbon dating techniques on the eye lenses of captured sharks to estimate their ages. This method reveals a range of 272 to 512 years, with a median age of 392 years. This impressive longevity places the Greenland shark far ahead of other long-lived marine animals, such as the bowhead whale, the Galapagos tortoise, and some early fish.

Despite growing slowly at less than 1 cm per year, Greenland sharks (Somniosus microcephalus) eventually reach striking sizes. Some individuals measure over 20 feet in length. These sharks reach sexual maturity around the age of 150, which contributes to their extended lifespans. As apex predators, they feed on diverse marine animals, including fish, seals, and even small whales.

The Greenland shark's remarkable age and slow reproductive rate make it particularly vulnerable to human impacts, such as overfishing and habitat disturbance.

Related: Types of Sharks 

12. Electric fish generate electric fields

Electric fish, like electric eels, can generate electric fields for various purposes, such as navigation, communication, and hunting prey. Living in environments where visibility is often limited, these fantastic creatures depend on this unique skill to thrive. 

For instance, the electric eel uses its electric powers to find its way through dense vegetation and detect nearby objects, even in total darkness. This remarkable ability, called electrolocation, helps electric fish dodge obstacles and accurately pinpoint prey.

Not only are electric fish masters of navigation, but they also display incredible versatility in using electric fields for communication and hunting. By adjusting the strength and frequency of the electric discharges they produce, these fish can send various signals to their fellow electric fish. These signals can be simple warnings or even intricate mating calls.

Electric fish are also skilled hunters, using their powers to stun their prey. The electric eel, for instance, can unleash a shocking 860 volts of electricity—enough to immobilize its prey in just one zap. This powerful defense mechanism deters predators, ensuring electric fish maintain a strong position within their ecosystems.

Related: Animals that start with E.

13. Male seahorses “give birth” to their young

Seahorses of the genus Hippocampus are the only fish where males play a crucial role in pregnancy and giving birth¹.

This extraordinary process starts when female seahorses lay or deposit their eggs inside a specialized pouch on the male seahorse’s belly, the brood pouch, working as an incubation chamber. The number of eggs can vary from a few dozen to over a thousand, depending on the species.

The egg transfer from the female to the male is carefully coordinated. During courtship, the seahorse pair performs an elegant dance, intertwining their tails and swimming side by side. As they glide gracefully through the water, the female releases her eggs, which the male collects and fertilizes within his pouch. 

Throughout the gestation period, lasting from two to four weeks, the male seahorse nourishes and protects the developing baby seahorses. He ensures they receive the oxygen and nutrients needed to survive. Once the young seahorses, also called "frys," are fully developed, the male expels them from his pouch through a series of contractions, releasing them into the world.

14. Fish defend themselves in various ways

Most fish have evolved an array of fascinating self-defense mechanisms to protect themselves from potential predators. 

One such adaptation is the presence of spines, a feature found in various fish species like lionfish, pufferfish, and porcupinefish. These sharp, needle-like structures often carry venom and can inflict significant pain or injury upon any creature attempting to bite or touch them. Merely having these spines deters predators, signaling that these fish are not easy prey.

Other fish, such as flounders, cuttlefish, and leafy sea dragons, also use camouflage to protect themselves. By changing their coloration and even skin texture, they blend seamlessly with their surroundings, making them nearly invisible to both predators and prey.

Creative defense methods

Besides spines and camouflage, some fish use other creative methods of self-defense. For example, the hagfish, often called a "slime eel," releases copious amounts of slime when threatened, potentially clogging the gills of attacking predators and allowing the hagfish to escape unharmed. 

Furthermore, some fish, like the Lionfish found in the Indian Ocean, use venom to ward off predators and act as deadly predators. The Lionfish’s 18 dorsal fins contain venom glands, capable of releasing a potent neurotoxin when threatened. The venom can cause extreme pain, swelling, and even paralysis in human victims.

15. Some fish are remarkably intelligent

Recent research continues to reveal astonishing insights and new fish facts challenging perceptions of the cognitive abilities of fish¹⁰. Far from being simple-minded creatures, most fish display impressive mental skills, including problem-solving, long-term memory, and learning from experience.

For example, the African cichlid fish are clever swimmers that use tools like rocks to crack open the hard shells of their prey, showcasing problem-solving prowess. Meanwhile, the archerfish exhibit incredible accuracy in their decision-making skills. They can target their prey with a precise water jet by selecting the perfect trajectory.

Other fish can also learn from their environment and experiences. Goldfish, for instance, can be trained to associate specific sounds with food through classical conditioning, debunking the myth that they have poor memory. Studies have even found that fish like damselfish navigate their surroundings using landmarks, pointing to an advanced spatial intelligence level.

16. The whale shark is the biggest fish in the world

One of the most interesting fish facts is that fish come in all shapes and sizes. For example, the whale shark is the largest fish on Earth. This magnificent creature can reach an astounding 12 meters in length and weigh as much as 18.7 tonnes. 

Despite its colossal size, the whale shark poses no threat to humans. It is a herbivorous fish that primarily consumes plankton, small fish, and squid by opening its massive mouth and filtering water through its gills.

Interestingly, each whale shark exhibits a unique pattern of white spots and stripes on its dark blue-gray body, much like human fingerprints. Known for their highly migratory nature, whale sharks often travel thousands of kilometers searching for food.

It is also worth noting that sometimes people assume the blue whale is the biggest fish, but it is not technically a fish but rather a mammal. Read more in our compilation of whale facts.

17. Fish enjoy symbiotic relationships with other marine life

Fish thrive with other sea creatures thanks to cooperation and mutual support. Among these symbiotic partnerships, the bond between clownfish and sea anemones is an excellent example of mutualism. Both parties benefit from each other's presence, creating a harmonious balance in the underwater realm.

With their vibrant orange and white stripes, clownfish find shelter within the stinging tentacles of sea anemones. This deters predators that would otherwise be drawn to the clownfish's conspicuous coloration. In return for this protection, clownfish defend their anemone hosts from polyp-eating fish. Likewise, their constant movement between the tentacles helps oxygenate the anemone and keep it clean from debris.

Another noteworthy example of a symbiotic relationship in the marine world is the intricate dance between cleaner fish, such as wrasses, and their larger counterparts. These diligent fish establish designated cleaning stations, attracting various fish visitors, including fearsome predators like sharks. At these stations, these cleaners meticulously remove parasites, dead skin, and other debris from the larger fish.

In exchange for their grooming services, these sea cleaners receive a steady food supply and protection from potential predators. These predators refrain from attacking them due to their beneficial role. 

18. Fish are threatened by overfishing and pollution

Finally, in our list of facts about fish, sadly, fish populations are increasingly threatened. Overfishing has led to a shocking decline in global fish populations; nearly 90% of the world's fish stocks are now fully exploited or overexploited. This unsustainable practice threatens saltwater and freshwater fish species since many fish caught in these habitats are not restocked quickly⁴. 

Implementing sustainable fishing practices, such as catch limits and gear regulations, is crucial to prevent further decline and support the recovery of fish populations. Additionally, when consumers choose sustainably sourced seafood, they help ease the pressure on struggling fish stocks.

However, overfishing isn't the only threat to fish populations today. Pollution and habitat destruction also have a significant impact. 

Pollution sources like plastic waste, agricultural runoff, and oil spills can cause fish deaths, disrupt reproductive cycles, destroy coral reefs, and contaminate their habitats. For instance, pollution from industrial activities has severely affected the Gulf of Mexico, where many fish species live, leading to the infamous "dead zone."

Read more: Environmental impact of fishing.