What Is a Fish?

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What is a fish?

The concept of "fish" certainly is more steeped in tradition than backed by scientists, despite the fact that countless ichthyologists (i.e., scientists who study fish) have written innumerable pages on the subject. The reality that fishes in the broadest sense have long played important roles in the promotion of industry and commerce, geographic exploration, politics, art, religion, and myth mandates that the definition of fish can vary according to human perspective and sometimes despite science. For example, from a chef's point of view, fishes come in two basic varieties—shellfish and finfish. Scientists eschew such groupings of distantly related creatures. However, lest they be hoisted with their own petards, ichthyologists might tread gently on the many concepts of fish, for they must acknowledge science's inability to form an absolute taxonomic definition of "fish" based on biological characteristics that are shared by all fishes and yet not shared with any "nonfish."

Defining characteristics

Widespread views of the particular characteristics that define fishes, of course, are biased by general familiarity with extant (i.e., living) species and, in particular, with the widespread and well-known bony fishes. Thus, the notion of a fish as an aquatic ectothermic vertebrate possessing gills, paired and unpaired fins, and scales usually suffices as a casual definition of fish. Reasonable as this definition may seem, some of these characteristics are shared with other groups of animals that are not considered fishes, while others of them are not common to all fishes. For example, although most fish live in water, some fishes, such as the walking catfish (Clarias batrachus) or African lungfish (Protopterus species) can spend considerable periods out of water. Furthermore, other fishes may spend much briefer, yet highly significant periods out of water, which allow them to feed (e.g., mudskippers, Periophthalmus

spp., and the arowanas, Osteoglossum spp.) or flee from predators (e.g., flyingfishes, Exocoetidae).

Similarly, whereas most fishes cannot control their body temperature other than through behavioral mechanisms involving migrations or local movements to and from waters of varying warmth, some lamnids (Lamnidae) and tunas (Thunnus spp.) and the swordfish (Xiphias gladius) can maintain body temperatures that are several degrees higher than the water that surrounds them for significant periods. Certainly, most fishes possess a well-developed vertebral column; however, hagfishes (Myxinidae) lack well-defined vertebrae, and there is disagreement among scientists regarding whether this characteristic exists because the ancestors of these fishes were similar or, antithetically, because vertebrae were "lost" from this lineage through evolutionary modification. In fact, so different are hagfishes from other fishes that Aristotle considered them members of another, illegitimate taxonomic group—worms. Unlike worms, fishes are chordates (phylum Chordata), and they possess skeletal components that form a cranium (i.e., a brain case). This characteristic (as well as many others) distinguishes them from some fishlike chordates, such

as the lancelets (Amphioxiformes), but, of course, amphibians, reptiles, birds, and mammals also have a cranium.

Gills cannot be used as an unequivocal characteristic defining fishes, because some amphibians have and use gills for at least a portion of their lives. Furthermore, whereas most fishes obtain oxygen from water through conventional gills, some fishes significantly supplement gill respiration by acquiring oxygen from the water or atmosphere via modified portions of the gills (e.g., the walking catfish) or skin (e.g., the European eel, Anguilla anguilla) or specialized tissues in the mouth (e.g., the North American mudsucker goby, Gillichthys mirabilis), gut (e.g., plecostomuses, Plecostomus species), swim bladder (e.g., the bowfin, Amia calva), or lungs (e.g., the Australian lungfish, Neoceratodus forsteri). Complicating matters still further, some fishes are obligate air breathers and must have access to the atmosphere or they will drown (e.g., the electric eel, Electrophorus electricus and the South American lungfish, Lepidosiren paradoxa).

At first glance, fins seem to define fishes. Several unrelated groups of nonfishes (e.g., lancelets, sea snakes, and some amphibians) possess finlike modifications associated with their tails that facilitate locomotion in water. Furthermore, although some fishes, such as hagfishes and lampreys (Petromyzontidae), lack paired fins, the paired appendages of amphibians, reptiles, birds, and mammals are considered homologous to the paired fins of fishes. Likewise, the scales that cover many common bony fishes are not a universally acceptable distinguishing feature, because numerous unrelated groups of fishes lack scales, for example, the hagfishes, the lampreys, and the North American freshwater catfishes (Ictaluridae). Moreover, those fishes that possess scales may be more or less covered by one of several basic scale types, for example, the placoid scales of sharks, the ganoid scales of gars, and the bony ridge scales of salmon and basses. These differences in the scales of fishes point to the fact that some other aquatic chordates, such as sea snakes, also have scales, even though the outer coverings of reptiles, birds, and mammals are heavily keratinized, whereas those of fishes are not.

Superclass Pisces as a polyphyletic group

Given that no one characteristic distinguishes all fishes from all other organisms, even the most committed ichthyologist must admit that the superclass Pisces (an assemblage that includes

all fishes) represents an unnatural or polyphyletic group. In fact, given our scientific understanding of fishes as of 2002, the only measure allowing them to stand together as a natural or monophyletic group requires the inclusion of all other craniates (i.e., amphibians, reptiles, birds, and mammals). Most biologists probably would agree that the consideration of all craniates as fishes would be of little scientific value and would betray the longstanding and widespread conception of a fish. In light of this situation, uncompromising cladists returning from a fishing trip for salmon are condemned to telling others of having been "salmoning" rather than "fishing."

General definition of fish

Despite the seemingly hopeless conundrum of defining "fish" scientifically, many scientists and non-scientists probably would agree that a general definition for this loose group of animals can be established. For these reasonable folks, a fish can be defined as an ectothermic chordate that lives primarily in water and possesses a cranium, gills that are useful virtually throughout life, and appendages (if present) in the form of fins. Those not willing to endorse this definition might rest easy by considering "fish" as the raison d'être for ichthyologists.