The Main Types of Fish in Salt and Freshwater
It is hard to pass a river or a lake without glancing into the waters in case there are there are fish to be seen. Many people depend on fish as a source of food. Almost everyone is fascinated by the strangeness and beauty of creatures that live in an entirely different realm to our own, air-breathing, one.
When I was young graduate, I was lucky enough to study many kinds of aquatic life. I spent plenty of time in small boats in the rough Atlantic waters off the coast of Ireland monitoring plankton blooms. In my breaks, I caught mackerel without trying (they jump into your boat) and sea bass with a lot more effort.
In the tropics I have dived in some of the most wondrous places on the planet, from reefs to estuaries.
I hope the different types of fish on this page give an idea of why these creatures are so important and why they have always fascinated me
The 3 Main Kinds of Fish
For scientists there are three main types of fish:
- the bony fish,
- the cartilaginous fish
- the ancient jawless fish.
The bony fish are the largest group of fish and include most of the important types that are fished for food. They have a bony skeleton rather than a cartilage-based skeleton like the sharks and rays.
Included among this group are such common fish as eels, goldfish, salmon, anchovies and even sea horses.
The early ancestors of modern fish appeared around 500 million years ago. Often they were armored with bony plates to protect the head.
Most bony fish have since evolved to favor speed and agility (for catching prey or avoiding being caught).
Deep fish bony fish, like the Angler Fish, take a different approach, though. Their is little food available at great depths and energy conservation is the most important survival factor. They swim slowly or drift passively waiting for food to come to them.
A few species like the reef-dwelling Lion Fish, pictured below are poisonous. Poisonous creatures do not hide away or use camouflage of any kind. They rely on other fish to recognize that they too dangerous to attack.
Cartilaginous Fish (Sharks and Rays)
The sharks and rays (scientific name for this group: Actinopterygii) are an incredibly successful group of animals.
They do not have bones but have a skeleton of cartilage instead. There are only about 440 kinds of shark but they are often the top predators, where they are found.
Some very primitive kinds of fish- the Hagfish and Lampreys- still exist from the time before animals evolved jaws.
The Different Kinds of Bony Fish
Scientists break the bony fish down into four main groups. The scientific names are given in brackets.
- Sturgeons and Paddlefishes (Acipenseriformes)
- Reedfishes and Bichirs (Polypteriformes)
- Gars and Bowfins (Holostei)
- The Teleostei - this includes the majority of common fish.
There are examples of each group below.
If you are interested in the complex inter-relateness of fishes (including the huge numbers of extinct fish) this page has a wealth of information: geol.umd.edu/~tholtz/
Sturgeons and Paddlefishes
The Paddle fishes are an ancient group of fish. The American Paddlefish is sometimes called a 'Spoonbill' because of its distinctive mouth. It is a freshwater fish found mostly in the Mississippi and associated rivers and feeds by filtering zooplankton (tiny aquatic animals) from the water. This animal can be big at up to 65 kilograms.
The closely related Sturgeons- another ancient group- are sea, estuary and lake dwellers that range from the Arctic to the coasts of Europe and America. Some species are fished for their eggs (caviar).
Like the Paddlefish, Sturgeons have little bone in their bodies, Mostly they have a skeleton of cartilage. The bone that is present is in the form of protective plates just under the skin. Both groups lack scales.
Reedfishes and Bichirs
Birhcirs are unusual fish that are popular pets for aquarium owners. In the wild, they are mostly found in rivers in Africa. They have simple lungs and can cope with water where there is not much dissolved oxygen (such as slow flowing, hot rivers or warm ponds).
Reedfishes, sometimes called Ropefishes in the US, are long, thin, snake-like fish that, like the Bichirs, can breath air to some degree. They are also popular pets and are well known for jumping out of aquariums and trying to escape- a little like eels.
Gars and Bowfins
Bowfins are found in the Great Lakes and are another ancient order of fish. They are more like modern fish than most other primitive species, though. They rear their young and they have scales rather than bony plates. More here: wisc.edu/greatlakesfish/fbowfin
Gars have armored scales. They live in rivers and estuaries, for the most part, around eastern North America, Central America, and the islands of the Caribbean. They are savage predators with needle sharp teeth. In some ways they are like Pike.
Most fish living today (well over ninety percent) belong to this group.
One of the chief characteristics of the Teleostei is that the fish can move their jaws forward (as well as up and down). If you ever watch a goldfish feeding you will understand what a big advantage this is. The jaws of a Teleost are capable of very precise movement and this helps locate and secure tricky foods.
Because the Teleost are so important. I have picked out a few key groups.
- Cypriniformes, includes barbs, carp, goldfishes, loaches and minnows,
- Siluriformes includes the catfishes
- Salmoniformes, includes salmon and trout
- Pleuronectiformes, includes the flatfishes like flounders, sole, turbot, plaice and halibut.
- Perhaps the single most important group of Teleosti are the Perciformes (literally, fish that look like Perch) which includes almost forty percent of all fish species, including bass, cichlids, mackerel, perches and whiting.
Main kinds of Cartilaginous Fish
Sharks are a varied group of fish that evolved well before the Teleosti, described above. They are mostly predators, although, some, like the huge whale shark, are filter feeders.
The popular image of sharks as dangerous killers is at least partly true. White Tip sharks are deep ocean dwellers that will attack anyone they find in the water. Great White Sharks will occasionally attack surfers and swimmers close to shore.
Most sharks are not a threat, though. Some sharks like the carpet sharks, for example, are shy creatures who spend most of their time hiding on the sea bed.
Small and medium sized sharks like Dogfish or Bramble Sharks prefer squid and small fish to human beings!
Rays are close relatives of the sharks but they have significantly modified body shapes. Some are bat-like (the Batoids). Some are torpedo shaped (the Torpediniformes). This group includes the electric ray which can stun prey with an electric shock.
The sting rays are familiar to almost everyone as fish to be avoided. They live in a wide range of habitats including ocean and river. The river stingrays of Central America are members of the Potamotrygonidae family. They feed mainly on small fish and mollusks. The whip like tail has grooves that can deliver venom to anything that might threaten them and is potentially fatal to people.
The largest ray is the truly impressive Manta Ray. The biggest recorded specimen was 7.6 meters across and weighed in at 1,300 kilograms