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5 Shocking Reasons Why Scientists Say 'Fish' Don't Technically Exist Anymore

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The question "Why fish don't exist" is not a philosophical riddle, but a bombshell of modern evolutionary biology that redefines how we understand the entire tree of life. While the word 'fish' remains perfectly useful in a culinary or casual sense, as of late 2025, the scientific community—specifically those who practice modern systematics known as cladistics—has largely discarded the traditional classification of *Pisces* as a valid, natural group. This isn't about semantics; it’s about a profound shift in how we trace evolutionary lineage, revealing that the creatures we call 'fish' are actually a scattered, unnatural collection of species that excludes their own descendants: you and me. This deep dive into the world of ichthyology and taxonomy is fueled by both a fascinating scientific concept and the popular 2020 book, *Why Fish Don't Exist: A Story of Loss, Love, and the Hidden Order of Life* by science reporter Lulu Miller. The book uses the life of pioneering ichthyologist David Starr Jordan to explore how we impose order on a chaotic world, perfectly illustrating the scientific problem at the heart of the title. The core issue lies in the concept of paraphyly, which proves that the common-sense grouping of "fish" fails the most fundamental test of modern classification.

The Man Who Named the Un-Nameable: David Starr Jordan

The story of why 'fish' as a class is defunct is inextricably linked to the legacy of one of America's most prominent ichthyologists, David Starr Jordan (1851–1931). Jordan’s life work was the very act of naming and classifying the world's aquatic life, a monumental task that ironically laid the groundwork for the category's eventual demise.
  • Full Name: David Starr Jordan
  • Born: January 19, 1851, Gainesville, New York, U.S.
  • Died: September 19, 1931, Stanford, California, U.S.
  • Primary Field: Ichthyology (the study of fish) and Taxonomy
  • Key Scientific Contribution: Systematically classified and named over 1,000 species of fish.
  • Academic Roles: President of Indiana University (1885–1891), First President of Stanford University (1891–1913).
  • Notable Works: *A Classification of Fishes* (1923), *Fishes* (1907).
  • Controversial Legacy: He was a prominent advocate and leader in the American eugenics movement, a dark aspect of his history that complicates his scientific legacy.
Jordan was a tireless cataloger, believing that by naming and organizing every species, he could bring order to the chaos of the natural world. He traveled the globe, meticulously documenting fish specimens. Even after the 1906 San Francisco earthquake destroyed his life's work—jars of specimens with their identifying labels scattered and mixed—he painstakingly re-labeled them, demonstrating an almost desperate need to impose structure. This drive for classification, while foundational to the Linnaean system, is what modern cladistics ultimately dismantled, proving that imposing a neat, traditional category like "fish" obscures true evolutionary history.

1. The Scientific Fatal Flaw: Paraphyletic Grouping

The most critical and recent reason why scientists dismiss the class *Pisces* is that it is a paraphyletic group. This is the single most important concept in understanding the claim.

What is a Paraphyletic Group?

A paraphyletic group is a taxonomic classification that includes a common ancestor but *excludes* one or more of the ancestor's descendants. In the world of modern phylogeny (the study of evolutionary relationships), a group is only considered valid—or monophyletic—if it includes the common ancestor *and* all of its descendants. The traditional class 'fish' is defined as all gill-bearing, aquatic, craniate vertebrates that lack limbs with digits. This definition includes jawless fish (Agnatha), cartilaginous fish (Chondrichthyes), and bony fish (Osteichthyes). The problem? The bony fish group, specifically the Sarcopterygii (lobe-finned fish), contains the common ancestor of all tetrapods—the four-limbed vertebrates, which include amphibians, reptiles, birds, and mammals.

The Tetrapod Dilemma

To be a valid monophyletic group, the category "fish" would have to include all descendants of its common ancestor. Since the first creatures to crawl onto land—the ancestors of all life on earth with four limbs—evolved directly from a species of lobe-finned fish, they are technically descendants of that "fish" lineage. By excluding tetrapods from the category of 'fish,' the traditional classification is scientifically invalid. It's like having a family tree where you define the 'Smith' family as all the children, grandchildren, and great-grandchildren of the patriarch, but you arbitrarily decide to leave out all the cousins who moved to California. In a strict cladistic sense, if you call a salmon a 'fish,' you must also call a cow, a bird, and a human a 'fish' to maintain a valid monophyletic grouping. Since this is absurd, the category itself is rejected.

2. It’s a Polyphyletic Mess of Different Lineages

While the primary issue is paraphyly, the term 'fish' also lumps together creatures that are only distantly related, confusing the evolutionary picture. The entire group we call 'fish' is not defined by a single, shared, derived trait (a synapomorphy). Instead, they are defined by what they *lack* (limbs with digits) and what they *do* (live in water and have gills). The three major non-tetrapod vertebrate groups are:
  1. Jawless Fish (Agnatha): Lampreys and hagfish. These are the most primitive vertebrates, lacking jaws and paired fins.
  2. Cartilaginous Fish (Chondrichthyes): Sharks, rays, and skates. Their skeletons are made of cartilage, not bone.
  3. Bony Fish (Osteichthyes): The vast majority, including Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish).
A shark is more closely related to a human than it is to a lamprey. Furthermore, a lobe-finned fish is more closely related to a human than it is to a ray-finned fish (like a tuna or a goldfish). The only thing that unites them is their aquatic lifestyle, which is a superficial, not a genetic, connection. This makes the traditional term 'fish' an evolutionary mishmash that obscures the true relationships between these different vertebrates.

3. The Rise of Cladistics and the Fall of Traditional Taxonomy

The reason for this paradigm shift lies in the adoption of cladistics (also called phylogenetic systematics) by most modern biologists. The older Linnaean system of classification, which gave us the classes *Pisces* (fish) and *Reptilia* (reptiles), was based on shared physical features (phenetics). It grouped organisms by how they *looked* and *lived*. Cladistics, however, is based purely on evolutionary history. It uses genetic data and shared derived characteristics to create a cladogram (a branching tree diagram) that only recognizes monophyletic groups (clades). * Traditional View: Grouped all aquatic, finned, gilled creatures into *Pisces*. This was convenient but inaccurate. * Cladistic View: Splits the traditional 'fish' into several distinct, valid clades. The two largest groups of bony fish are now recognized as separate, valid clades: Actinopterygii and Sarcopterygii. The Sarcopterygii clade *must* include tetrapods to be complete. The term 'fish' is a relic of a pre-evolutionary understanding of life, a "grade" of organization rather than a genuine "clade" that reflects a single, unbroken evolutionary lineage. This is why many contemporary biology textbooks and scientific papers no longer use the class *Pisces* at all. They refer instead to specific, valid clades like Teleosts (modern ray-finned fish), Chondrichthyes, or the broader Gnathostomata (jawed vertebrates). The word 'fish' simply doesn't exist in the modern scientific dictionary of valid taxonomic groups.
5 Shocking Reasons Why Scientists Say 'Fish' Don't Technically Exist Anymore
why fish don't exist
why fish don't exist

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