The Zoological Nomenclature of Fish: Understanding Their Classification

When discussing fish in zoological terms, it is important to note that "fish" is not a taxonomic term. This means there is no single family of organisms recognized as being one. Instead, fish refer to a diverse group of water-dwelling animals that include bony fish, hagfish, cartilaginous fish, and a number of other groups, some of which are only loosely related. For example, lungfish are more closely related to tetrapods (four-legged animals) than they are to lampreys. Despite all being classified as 'fish', they do not all belong to a single taxon called 'fish'.

The term 'fish' is considered a polyphyletic term, meaning that it includes organisms that are not necessarily biologically related. Not all vertebrates are classified as fish, and thus the term has no taxonomic meaning. However, the overwhelming majority of known fish do belong to a single clade called Actinopterygii. In fact, about half of all living vertebrates are in this clade.

Understanding Vertebrates and Actinopterygii

Vertebrates are animals with a backbone, and they represent a crown taxon, meaning a group that includes all descendants of a common ancestor. While not all vertebrates are considered fish, fish are well-defined within this larger classification. The clade Actinopterygii, also known as ray-finned fish, comprises about half of all existing vertebrate species. These fish are characterized by having bones in their fins, which makes them distinct from other groups of fish like lobe-finned fish.

Reclassifying Non-Actinopterygii Fishes

Given that fish is a broad term that includes diverse and only loosely related species, it can be challenging to classify every organism under the umbrella of 'fish'. For instance, animals like lampreys and hagfish, while often categorized with fish, do not belong to the Actinopterygii clade. Therefore, if one were to create a fish clade, the choice would be between making a broader category including all existing fish or specifically naming the Actinopterygii clade as the encompassing group.

If you were to form a fish clade, it would make sense to choose Actinopterygii as the primary taxon. Any species not fitting within this clade could then be reclassified into other appropriate categories. For example, hagfish could be placed in the group Agnatha, which includes cyclostomes, while lampreys could be classified in their own group, Myxini.

Conclusion

In summary, while the term "fish" is widely used, it has no strict taxonomic meaning. The overwhelming majority of known fish belong to the Actinopterygii clade, making it a fitting choice for a fish clade. By understanding the complex classification and reclassification of fish, we can better appreciate the diverse and complex world of aquatic life.