What Category is a Fish Considered? An In-Depth Biological Look

November 23, 2025 •

4 min read

With approximately 50 percent of all living vertebrates being fish, their biological classification is a complex and fascinating topic. While many people simply refer to them as 'fish,' this aquatic life-form actually represents a diverse group of animals with varying characteristics, and is not a single taxonomic class in modern phylogeny.

Quick Summary

Fish are aquatic vertebrate animals that fall into multiple biological classes, not a single one like mammals or birds. Modern classification separates fish into jawless, cartilaginous, and bony fishes, a grouping that is considered paraphyletic since it excludes tetrapods which evolved from fish. Fish are cold-blooded, breathe with gills, and have fins, distinguishing them from other marine life like mammals.

Key Points

Animal Kingdom: All fish are part of the Animalia kingdom, as they are living organisms that eat, breathe, and sleep.
Vertebrate Subphylum: Fish are classified as vertebrates, meaning they possess a backbone or spinal column, though some primitive species have a cartilaginous notochord instead.
Multiple Classes: The traditional single class of 'Pisces' is scientifically outdated; fish are now divided into several classes, including jawless, cartilaginous, and bony fishes.
Paraphyletic Group: As a group, 'fish' are considered paraphyletic because they include a common ancestor but exclude some descendants, namely tetrapods (four-limbed vertebrates).
Bony vs. Cartilaginous: Bony fish (Osteichthyes) have skeletons made of bone, while cartilaginous fish (Chondrichthyes), such as sharks, have skeletons made of cartilage.
Ecological Significance: Accurate classification is vital for managing fisheries, conservation efforts, and understanding the health of complex aquatic food webs.

Understanding the Fish Category: More Complex Than It Seems

At the highest level of biological organization, a fish is considered an animal. This places it in the Animalia kingdom, a broad category that includes everything from insects to humans. From there, its classification becomes more specific, leading to the subphylum Vertebrata, which means it possesses a backbone or spinal column. However, the traditional class 'Pisces' is no longer recognized in modern phylogenetics, as it would include the ancestors of land vertebrates, making it an unnatural grouping. Instead, the term 'fish' is now used to describe a broad collection of aquatic vertebrates, which are separated into several distinct, scientifically recognized classes.

The Major Classes of Fish

Modern ichthyology, the scientific study of fish, recognizes three main classes of extant (living) fish:

Myxini (Hagfish): The most primitive vertebrates alive today, hagfish are jawless fish that lack a true vertebral column, instead retaining a notochord throughout their lives. They also have no scales or paired fins and are known for producing large amounts of protective slime.
Chondrichthyes (Cartilaginous Fish): This class includes well-known species like sharks, rays, and chimaeras. As their name suggests, their skeletons are made of cartilage, not bone. They possess jaws, paired fins, and lack a swim bladder, relying on their livers and constant swimming for buoyancy.
Osteichthyes (Bony Fish): This is the largest and most diverse group of vertebrates, with a skeleton made primarily of bone. This superclass includes the vast majority of fish species people are familiar with. It is further divided into two subclasses:
- Actinopterygii (Ray-finned fishes): Possess fins supported by thin, flexible, bony rays. This subclass includes common species such as tuna, salmon, and cod.
- Sarcopterygii (Lobe-finned fishes): Characterized by fleshy, paired fins supported by internal bony structures. This group includes coelacanths and lungfish, the latter being the closest living relatives to tetrapods (four-limbed vertebrates).

Comparison: Fish vs. Marine Mammals

While both fish and marine mammals live in the water, they are fundamentally different biological categories. Understanding these distinctions helps clarify why fish are in their own grouping and not considered mammals.


Feature	Fish	Marine Mammals (e.g., whales, dolphins)
Body Temperature	Cold-blooded (ectothermic); body temperature adjusts to the surrounding water, with some exceptions.	Warm-blooded (endothermic); they maintain a constant body temperature.
Respiration	Breathe underwater by extracting oxygen with gills.	Breathe air through lungs; must surface regularly to breathe.
Skeletal System	Skeletons are made of cartilage (sharks, rays) or bone (tuna, salmon).	Skeletons are made of bone.
Reproduction	The majority lay eggs (oviparous), though some give live birth (ovoviviparous).	Give live birth and nurse their young with milk.
Tail Movement	Move their tails from side to side to propel themselves.	Move their tails up and down to propel themselves.
Body Covering	Typically covered in scales, though some are scaleless.	Have hair or fur, at least at some stage of life.

The Evolutionary Link and Paraphyletic Nature

Fishes were the first vertebrates to evolve, and their classification is crucial for understanding the evolutionary history of all vertebrates. The reason a modern biologist doesn't use the single, old-fashioned class 'Pisces' is because of cladistics, a system of classification based on evolutionary relationships. All tetrapods, including humans, evolved from a group of lobe-finned fish. This means that the group 'fish' is paraphyletic—it includes an ancestor (the lobe-finned fish) but excludes some of its descendants (the tetrapods). Therefore, scientifically, a land vertebrate like a human is a type of bony fish in a cladistic sense, which highlights the flawed nature of the outdated grouping.

The Significance of a Proper Fish Classification

Beyond academic interest, the accurate classification of fish is vital for real-world applications. It is fundamental to fisheries management, allowing scientists to monitor populations, regulate fishing quotas, and protect endangered species. In aquaculture, it helps in breeding and managing commercial species. From a broader ecological perspective, knowing the roles of different fish categories provides insight into complex marine food webs and the overall health of aquatic ecosystems.

Conclusion

A fish is an animal and, more specifically, an aquatic vertebrate. However, the term refers not to a single category but to a diverse and complex collection of life-forms that have evolved along different paths. From the ancient, jawless hagfish to the sprawling diversity of modern bony fish and the highly adapted cartilaginous sharks, the world of fish encompasses multiple biological classes. Understanding these precise categories, rather than lumping them under a single, outdated term, provides a more accurate view of their evolutionary history and ecological significance. For a deeper understanding of classification, consult authoritative biological and ichthyological resources like those at the University of Hawaiʻi at Mānoa.

Frequently Asked Questions

Yes, a fish is a type of animal. It belongs to the kingdom Animalia and, more specifically, to the subphylum Vertebrata, which includes all animals with a backbone.

Most fish are cold-blooded, or ectothermic, meaning their body temperature fluctuates with their environment. However, a few exceptions, like certain species of tuna and sharks, are warm-blooded (endothermic).

A key difference is that fish are cold-blooded and breathe with gills underwater, while marine mammals are warm-blooded, breathe air with lungs, and give live birth.

The traditional class is no longer used because it is a paraphyletic group. It excludes tetrapods, which evolved from an ancient group of fish, making it an inaccurate reflection of evolutionary relationships.

Most fish breathe by extracting oxygen from the water using specialized organs called gills, which are located behind their head.

Sharks are fish. They belong to the class Chondrichthyes, the cartilaginous fish, and have a skeleton made of cartilage instead of bone.

While most fish cannot breathe air, some species, like lungfish, have specialized lung-like organs that allow them to survive out of water for extended periods.