Fish Groups

The oceans on our planet are such amazing and fascinating places with so many species of fish, among other sea dwelling creatures.

What is a fish?

To ask ‘what is a fish’ is not an easily answered question as the fish group is such a diverse one. Most fish are scaled, aquatic vertebrates which means they have a skeleton which includes a cranium or skull and a functional backbone. Some species of fish are not vertebrates such as the primitive-looking Hagfish which have a simple cartilaginous skull and a row of springy connective tissues instead of a backbone.

All fish live and survive in water whether it be sea water or freshwater. However, there are a few exceptions such as the Common Eels and Mudskippers who are able to move about on land. Some species of fish such as the Flying Fish, take to the air and are able to glide over 200 metres, skimming the waves like fast flying sea birds. There are other species of fish that remain encased in mud for many months or even years such as the Lungfish.

Fish are most often referred to as cold-blooded creatures, however, most fish are ‘poikilothermic’ which means they are able to maintain a steady body temperature in their changing environmental water temperature. Therefore, fish that live in icy waters are generally cold blooded and fish that live in warmer waters are warm blooded. Some species of fish are unable to maintain a constant body temperature and therefore have to raise their body temperature by conserving metabolic heat. As these fish lose their heat rapidly through their gills, they are adapted to fast swimming such as Mackerels and some species of shark.

Most fish have gills which are respiratory organs used to exchange gases (oxygen and carbon dioxide). Oxygen is taken into the fishes bloodstream and carbon dioxide is released into the water as a waste product. Some species of fish are able to supplement their oxygen intake with respiratory organs which are more similar to lungs than gills.

There are 5 major groups of fish, however, each class of fish is separated into smaller taxa such as orders, families, genera and species. The major groups also contain various subgroups and suborders.

The 5 major groups are as follows:

Ray-finned Bony Fish (Class: Actinopterygii)

Trout Fish This is the largest class of living fish. These fish come in a variety of different appearances. Their paired and median fins vary in size, shape and colour and are generally supported by spines or soft rays. This class of fish includes over 24,000 scientifically described species and includes around 12 super orders (total taxonomy).

Around 60% of these fish are marine dwellers. There are two subclasses: Chondrostei which includes reed fishes, sturgeons and paddlefishes and Neopterygii which includes salmon, trout, gulpers, piranhas, gars and bowfins. Species size can range from the tiny Paedocypris (Paedocypris micromegethes) at 8 millimetres to the huge Ocean Sunfish (Mola mola) which can weigh around 5,100 pounds and the long bodied Oarfish (Regalecidae family) which can measure 36 feet in length.

The external anatomy of a Trout fish (Salmonidae family). For a larger view, click on the image. Diagram opens in a new window, close when finished.

Cartilaginous Fish (Class: Chondrichthyes)

Shark This class of fish includes two subclasses: Elasmobranchii – sharks, rays and skates and Holocephali – the Chimaeras (Ghost Sharks or Rabbitfish). There are over 1000 species living in the Earths waters today. Sharks and their relatives have a skeleton made from cartilage (young sharks have a notochord which is gradually replaced by cartilage), articulating jaws, two chambered hearts, paired fins and most have 5 sets of gills.

Their skin is very tough and covered with dermal teeth called ‘placoid scales’ (except subclass Holocephali). The dermal teeth grow in one direction, making the skin feel smooth in one direction and very rough if rubbed in the other direction. The dermal teeth covering has two functions, to provide protection and to make the fish more streamlined for easier swimming and gliding through the water. As these fish do not have bone, their red blood cells are produced in their spleen and also in an organ called ‘Leydig’s Organ’ which is only present in cartilaginous fish. Chondrichthyes also lack ribs and if larger species were to leave the water, their own body weight would unfortunately crush their internal organs long before they would suffocate.

The external anatomy of a Shark (Superorder Selachimorpha). For a larger view, click on the image. Diagram opens in a new window, close when finished.

You can learn more about sharks in our Marine section HERE!

Lobe-finned Bony Fish (Class: Sarcopterygii)

This class of fish includes the Lungfish (Subclass Dipnoi) and Coelacanths (Order Coelacanthiformes). They are generally heavy bodied fish who have a bony skeleton and stout, fleshy pectoral and pelvic fins. These fins evolved into legs on the first tetrapod land vertebrates, the amphibians. Many early sarcopts have a symmetrical tail. Lungfish use their stubby fins to walk on land when they are searching for new waters if their water hole has depleted. They use their lungs to breathe air to get sufficient oxygen. The coelacanths, which are related to lungfishes and tetrapods, were believed to have been extinct since the end of the Cretaceous period, until the first Latimeria specimen was found off the east coast of South Africa. There are only two species that have survived in our waters today, the Coelacanths – Comorese coelacanth (Latimeria Chalumnae) and Indonesian coelacanth (Latimeria Menadoensis).

Coelacanths are opportunistic feeders, hunting cuttlefish, squid, snipe eels, small sharks and other fish found in their deep reef and volcanic slope habitats. Coelacanths are also known to swim head down, backwards or belly up to locate their prey.

The external anatomy of a Coelacanth. For a larger view, click on the image. Diagram opens in a new window, close when finished.

Lampreys (Class: Cephalaspidomorphi)

This class of fish contains five subgroups: Osteostraci, Galeaspida, Pituriaspida, Anaspida and Hyperoartia. These jawless fish lack the paired fins but have eyes and a backbone. As larvae, Lamprey (Family Petromyzontidae) are filter feeders and feed upon organisms in the water. As adults, they feed upon chucks of flesh from larger vertebrates using a sucker-like mouth which is lined with bony teeth.

Osteostracans, anaspids and lampreys share a single dorsal ‘nostril’, now known as a ‘nasohypophysial opening’. Most members of this class of fish are extinct.

Hagfish (Class: Myxini)

Hagfish (Family Myxinidae) are sometimes referred to as ‘Slime Eels’, however, they are not eels at all. These are primitive looking fish with no backbone. Instead, they have a flexible rod called a ‘notochord’ which enables them to tie themselves in knots. Because they are not vertebrates, Hagfish are not classed exactly as fish but as ‘fish-like chordates’.

Hagfish have very unusual feeding habits and slime-producing capabilities which has led them to be dubbed the most disgusting of all sea creatures. An adult Hagfish can produce enough slime to turn a 20 litre bucket of water to slime in minutes. Hagfish range in length from 18 centimetres, the Myxine Kuoi and the Myxine Pequenoi to the largest recorded species, Eptatretus Goliath, at 127 centimetres. Most Hagfish average around 46 centimetres in length. Hagfish have four hearts and two brains. Their eyes are simple and not compound, they have no true fins. They have barbels (like whiskers) around their mouths which are sensitive to touch and chemicals. Hagfish are typically found in large clusters on or near the bottom of the sea.