Fish Classification: What Phylum Or Class Are They In?

There are about 28,100 species of fishes known to science.

They are divided into 4 classes, 59 orders, 490 families and 4,300 or so genera.

The classification system of fish is not stagnant and it will change when we have more knowledge, perhaps from DNA analyses. For ordinary purpose, however, the system I have used on these pages will be sufficient for many years. It is based on Fishes of The World (3rd Edition) by Joseph S. Nelson, 1994.

You can skip ahead to particular classes using these links, or otherwise read on:

• Classes Myxini & Cephalspidomorphi
• Class Chondrichthyes
• Class Osteichthyes

Welcome to this brief introduction to fish classification.

There are a few problems with the classification of fish. The first of which is the question “What is a fish?”.

We all use the word Fish regularly, but the term fish is not a natural group of animals in the way the terms Reptiles or Birds are.

When scientists arrange living things into groups for classification they try to make the classification system they use represent the actual relationships between those living things. The more categories that two organisms share the more closely they are related.

All living things are divided up into a series of classifications delineated as: Kingdom, Phylum, Class, Order, Family, Genus and species. You can also have Super and Sub attached to each group so that a Superclass can contain several Classes, each of which itself contains several Subclasses. Subclasses then contain Superorders which contain Orders, etc.

So you see we can end up with a lot of different terms, which is good because there are a lot of living things.

The term ‘Fish’ does not fit into any of these categories. This is because it is a general term, not a scientific one. We all know that a Trout or a Salmon is a fish. However some of the animals we commonly call fish are not much more closely related to Salmon and Trout than they are to Dogs and Cats.

The term fish is actually used to describe any animal that is part of the Subphylum Vertebrata, but is not a member of the Classes Amphibia, Reptilia, Aves (Birds) or Mammalia.

The Subphylum Vertebrata contains two Superclasses, Agnatha and Gnathostomata:

• Superclass Agnatha contains only two Classes of animals, both of which come under the general term of fish.
• Superclass Gnathostomata contains six Classes of animals, two of which are also included in the general term of fish.

Higher Classification of the Phylum Chordata

The animals we commonly refer to as fish then can be seen to be included in 4 classes of animals, that are as different from each other as Birds are from Reptiles. But because they all live in the sea, they get lumped together.

Below is a list of all the families of animals we call fish. If you want an entire checklist for the world, or to research a particular species, you need to visit www.fishbol.org.

When talking about and reading about species of fish when only a common name is given, you need to be careful.

Many people know that Bonito is a type of tuna. However in Fishbol 2004, we find that all the following 20 species are all called Bonito somewhere in the world:

Abramites eques, Auxis rochei rochei, Auxis thazard thazard, Caranx crysos, Cybiosarda elegans, Euthynnus affinis, Euthynnus alletteratus, Euthynnus lineatus, Gymnosarda unicolo, Katsuwonus pelamis, Megalaspis cordyla, Orcynopsis unicolor, Rachycentron canadum, Sarda australis, Sarda chiliensis chiliensis, Sarda chiliensis lineolata, Sarda orientalis, Sarda sarda, Selar boops, Thunnus alalunga.

This is not just a list of the different species called bonito in different parts of the world. In many countries more than one species is known as Bonito.

In fact, in England, Australia and Cuba the term Bonito applies to 3 different species. In Kenya, Namibia, Brazil and Germany it applies to 4 different species. In Colombia and Mexico, Bonito includes 5 species and in the USA the name Bonito is applied to 10 different species.

When you add to this the fact that the same species is also called different names in different countries, the problem of knowing exactly what species a common name refers to can get quite difficult. For Instance:- Katsuwonus pelamis is:

• Striped bonito, Ocean Bonito, Oceanic Bonito, Arctic Bonito and Bonito in the USA,
• Striped-bellied Bonito, Striped Bonito, Oceanic Bonito, Atlantic Bonito and Bonito in England,
• Bauchstreifiger Bonito in Germany,
• Bonitol, Bonito de altura, Bonito de veintre rayado and Bonito del sur in Spain,
• Bonito, Bonito de barriga listada, Bonito-de-barriga-riscada, Bonito de veintre rayado, Bonito-oceānico and Bonito rajado in Brazil,
• White Bonito in both Barbados and Suriname

The term ‘Bonito’ is also used in Cape Verde, Sri Lanka, Dominican Republic, French Polynesia, India, Namibia, New Zealand, Poland, Portugal, South Africa, Venezuela and more.

Katsuwonus pelamis has many more names than I have listed here. But I think this is enough to illustrate my point on the unreliability of common names – particularly for marine or other widely distributed species.

Class Myxini & Cephalaspidomorphi