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Fish – Everything You Need To Know

Welcome to our fish category page! Read further to learn about the exciting adaptations that set these creatures apart from other animals.

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Introducing Fish

Synanceia is a genus of fish of the family. Synanceiidae, the stonefishes, whose members are venomous, dangerous.
Synanceia is a genus of fish of the family Synanceiidae, the stonefishes, whose members are venomous and dangerous. Image by johnanderson via Depositphotos

Fish are the most diverse group of vertebrates on Earth. There are over 33,000 known species. They range from tiny species like Paedocypris, fully grown at only 7.9 mm, to massive whale sharks. They inhabit various aquatic environments, from the deepest parts of the ocean to the streams up on mountains. I have spent my adult life studying fish and the environment in which they live. When I free-dive in the kelp forest in Cape Town, I am mesmerized by them. Fish are among my favorite animals. 

What is the biggest fish in the world?

Whale Shark
The Whale Shark is the largest fish in the world. By criso via DepositPhotos

One of the most popular fish to dive with and the biggest fish in the world is the whale shark. Whale sharks aren’t whales, as the name suggests; they are indeed sharks. They belong to the class chondrichthyan, which includes sharks, skates, rays, and chimaeras. Reaching over 12 meters, they are gentle giants of the marine world. They are quite popular to dive with because of their gentle, non-threatening demeanor. Whale sharks are found in tropical and subtropical oceans around the world. 

Why do fish swim in schools?

Fish swim in schools for several reasons. They swim in schools primarily for protection from predators. Predators may get ‘confused’ by the movement of many fish as a single larger. The sheer size of the school might deter potential predators. Fish also swim in a school for increased hydrodynamic efficiency. Similar to cyclists, who use each other to reduce wind resistance, scientists believe that fish use each other to reduce their energy expenditure. Additionally, schooling increases the chances of finding a mate. Schooling brings a large number of individuals of the same species together. 


The zebra shark (Stegostoma fasciatum) is a species of carpet shark and the sole member of the family Stegostomatidae.
The zebra shark (Stegostoma fasciatum) is a species of carpet shark and the sole member of the family Stegostomatidae. By Theo Kruse / Burgers’ Zoo – Luipaardhaai – Burgers’ Zoo, CC BY-SA 3.0, via Wikimedia Commons

Fish are divided into three main classes: Agnatha (jawless fish), Chondrichthyes (cartilaginous fish), and Osteichthyes (bony fish). Sharks, a suborder in the Chondrichthyes class are among the most popular animals in the world. They are both feared and admired. Famous movies such as Jaws, The Reef, and Open Water are partly to blame for the fear humans have for sharks. In reality, sharks have much more to fear from humans than we do from them. Overfishing, habitat destruction and the demand for shark fins have led to a decline in shark populations worldwide.

Sharks are apex predators and ecosystem engineers. Their top-down control on species help maintains the diversity and health of many oceanic ecosystems, especially our beloved coral reefs. Sharks are one of my favorite animals to see on a dive. I maintain a safe and respectful distance and feel completely comfortable with them in the water. We can help change the stigma surrounding sharks by fostering a healthy respect and appreciation for them. 

Want to learn more about sharks? Read more here.

Do sharks have scales?

In short, yes, sharks do have scales. However, they are different from the scales of bony fish. Shark scales are called dermal denticles or placoid scales. They are specialized tooth-like scales that cover the skin of sharks and other elasmobranchs. The rough texture of shark scales make it difficult for parasites and other organisms to attach. It also provides a protective layer for sharks.


Coho spawning in the salmon river. Bureau of Land Management Oregon and Washington, Public domain, via Wikimedia Commons

Salmon are among the most interesting species. They are known for their remarkable life cycles. They are anadromous, meaning they are born in freshwater, migrate to the ocean for their adult lives and then return to freshwater to spawn. Salmon are very interesting to me, because they are able to return to the exact location they were born in to spawn. The amount of time salmon spend in their birthplace varies among species, however most of these fish return to the streams in which they were born.

Scientists believe that salmon use the earth’s magnetic field and their acute sense of smell to find their way back to their birthplace. The beautiful circle of life, after spawning salmon die in their birthplace. Their bodies provide essential nutrients to the river environment.

Characteristics that define a fish

A multitude of characteristics make these animals unique. Most of these characteristics are present in all species, however there are some exceptions.

VertebrateAll fish are vertebrates. They have a backbone or spinal column.
AquaticAll fish live in water. An exception might be mudskippers 
GillsFish absorb oxygen through their gills
FinsTheir fins aid in movement, stability and steering through the water.
Cold-bloodedMost fish are ectothermic, or cold-blooded 
ScalesMany fish have scales covering their body. Scales protect them and aid movement through the water.
Swim bladderMany fish have a swim bladder to help control buoyancy.

How do fish sleep?

They do not have eyelids and thus lack the ability to close their eyes to sleep. Their ‘sleep’ is more accurately described as rest, or a period of reduced alertness. During these periods, their metabolism slows down. However, during these periods of rest, they are still somewhat aware of their surroundings.

How do fish reproduce?

The modes of reproduction among fish is broadly categorized into three groups: oviparous (egg-laying), viviparous (live-bearing), with a third category, ovoviviparous (a combination of the first two).

  1. Oviparous: The majority of fish species are oviparous. They lay eggs in the water, which are then fertilized externally. 
  2. Viviparous: Viviparous fish give birth to live young. This is less common among fish but is found in several species, including some sharks and rays. 
  3. Ovoviviparous: Eggs are fertilized internally and remain inside the female’s body but the embryo’s primary nutrition comes from the egg yolk, rather than from the mother. 

How do fish form schools?

Schooling is an innate behavior for many fish species. Vision plays an important role in the formation and maintenance of schools. They use their eyes to align themselves with their neighbors, matching speed and direction. They also use their lateral line system to detect water currents and pressure changes. This helps them sense the movement of nearby fish, even in murky water or complete darkness. 

These fascinating animals have inspired a wide range of innovations. They are so well adapted to live in their aquatic worlds that humans have tried to mimic their design—for example, some swimwear is designed to simulate fish scales to improve speed. I hope you learned something about fish from this article. Please click on the links added throughout to learn more!

Newest Category: Fish

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