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1. Agnatha: This is the most primitive class of fish. They have no jaws, and undeveloped backbones. They are also known as Jawless Fish. Only two groups are still surviving in this class - hagfish and lampreys.
2. Chondrichthyes: This class of fish has jaws, and is characterized by their cartilaginous skeletons, and the lack of bones in their body. This class is divided into three types - sharks, skates, and rays.
3. Osteichthyes: These too, are jawed fish, and have partly or wholly bony skeletons and no cartilage. This class is subdivided into two subclasses - 1. Sarcopterygii - which are the lobe-finned fishes, and 2. Actinopterygii - which are the ray-finned fishes. A few of the ray-finned fishes left their water abode and ventured onto the land, thus becoming the ancestors for all terrestrial vertebrates.

1. Agnatha (Jawless Fish)

These jawless creatures are very primitive in nature. Not only do they lack jaws, most of the species in the two surviving groups also do not have a defined stomach, fins, or the ability to regulate their own body temperature (i.e. they are ectothermic). There is not too much information available about their mating or reproductive habits, but fertilization is believed to be external. While hagfish feed mainly on dead animals, lampreys mainly feed off other fish.

Hagfish

Lampreys

2. Chondrichthyes (Cartilaginous Fish)

This class of fish, as the name says, possess a cartilaginous skeleton. The young are born with a notochord, which is gradually replaced by cartilage. They have scales, gills, and paired fins. Although they have a skeleton, chondrichthyes do lack a ribcage, which means if taken out of water these creatures will die because their body weight will crush their internal organs. They also lack bone marrow. A heterocercal (where the top lobe is larger in size than the bottom lobe) tail in sharks, helps them in locomotion.

Sharks

Rays

Skates

1. Herbivores
2. Carnivores
3. Omnivores
4. Detritivores

Oarfish

Flying Fish

Milkfish

Filefish

1. Freshwater Fish
2. Marine Fish
3. Migrating Fish

1. Freshwater Fish

Freshwater fish are adapted to survive in very low levels of salinity. They require less than 0.05% level of salinity to survive. These fish have to do some extra work as compared to their saline water counterparts, because they have to utilize the very low levels of salt found in freshwater optimally, to maintain their internal body salt balance. Their main physical adaptations are scales on their body, and very well-developed kidneys. The scales help reduce water diffusion through their bodies, while the kidneys help to retain salts as much as possible. Quite a few of the freshwater species are now endangered due to water pollution caused by humans.

Puffer Fish

Piranha

Cat Fish

Swordtail Fish

Archer Fish

Cichlid

Butterfly Fish

Discus Fish

2. Marine Fish

Marine fish are typically adapted to water which has an average pH level of 8.2. Usually, these fish are characterized by brilliant colors and many different physical adaptations, which makes it easier for them to protect themselves from their predators. Marine fish are classified into three types, based on their feeding habits:

1. Herbivorous
2. Carnivorous
3. Omnivorous

Cardinal Fish

Surgeon Fish

Tang Fish

Damsel Fish

Hawk Fish

Grouper

Squirrel Fish

Nurse Shark

1. Catadromous: These are the fish which are born in marine/saline water, but migrate to freshwater sources to spend the major part of their lives.
2. Anadromous: These fish follow the exact opposite pattern - they are born in freshwater, but migrate to salt water bodies to spend most part of their adult lives. They return to freshwater bodies when they want to reproduce.

Tuna

Salmon

Dolphin

Sail Fish

Char

Trout

Capelin Fish

Bullshark

• Firstly, fish is one of the richest sources of omega-3 fatty acid which helps in lowering blood pressure, as well as maintaining a normal heart rate.
• Fish have a low content of fats and calories, while being a rich source of proteins.
• Fish has fatty acids which aid in faster growth of hair, contribute to making hair stronger and shinier, and also aid in the production of collagen which helps in reducing wrinkles and thus delaying the appearance of old age.
• Fish are an amazing source of minerals and vitamins. They contain calcium, which helps in maintaining healthy teeth and bones, help in growth and metabolism, and also help in keeping the eyes healthy.

Snapper Fish

Pacific Saury

Sable Fish (Barracod)

Bass

Dory

Cray Fish

Pompano

Wolf Fish

Walleye

Sturgeon

Barracuda

Conger Eel

Mullet

Scallop

Dog Fish

Monk Fish

1. They have excellent camouflage ability and can change their color to match their surroundings.
2. Seahorses are one of only two classes of fish that can swim vertically, the other being shrimpfish/razorfish.
3. Seahorses have eyes that can move independently of each other. So while one eye looks back, the other can look in the front or sideways!
4. In seahorses, it is the male who gets pregnant! The female deposits her eggs into the pouch of the male, where he fertilizes them, and carries them till they are ready to hatch. The male seahorse provides prolactin to the eggs, while the pouch regulates the temperature and salinity of water and also provides oxygen to the eggs.