Fish – Definition, Examples, Characteristics

What Is a Fish - Characteristics

Fish are aquatic vertebrates that typically have gills, fins, and a streamlined body. They inhabit diverse environments, from freshwater rivers and lakes to salty oceans and deep seas.

Shared Characteristics with Other Vertebrates

Fish, like all vertebrates, possess a backbone or spinal column. This characteristic places them within the subphylum Vertebrata. Other shared features include a brain protected by a skull, a complex nervous system, and bilateral symmetry.

Fish Characteristics

Fish possess several distinct features that distinguish them from other vertebrates:

  • Gills: For extracting oxygen from water.
  • Fins: For propulsion, steering, and balance.
  • Scales: Covering the body for protection and reducing water resistance.
  • Lateral Line System: A sensory organ that detects water movements and vibrations.
  • Swim Bladder: An internal gas-filled organ that helps maintain buoyancy (in most bony fish).

Examples of Fish and Misidentified Non-Fish

Invertebrates, mammals, birds, reptiles, and amphibians are not fish. Also, just because an animal has the word “fish” in its name, it doesn’t mean it’s a fish:

  • Examples of Fish:
    • Salmon (Salmo salar): A well-known migratory species that lives in both fresh and saltwater.
    • Goldfish (Carassius auratus): A popular freshwater aquarium fish.
    • Great White Shark (Carcharodon carcharias): A large predatory marine fish.
    • Manta Rays (genus Mobula): Large triangular-shaped fish with the largest brain to body ratio of all fish.
  • Non-Fish Examples Often Mistaken for Fish:
    • Dolphin: A marine mammal often confused with fish due to its aquatic lifestyle.
    • Sea snake: A reptile that spends part of its time in the water.
    • Jellyfish: An invertebrate with a gelatinous body and tentacles.
    • Starfish: An echinoderm with radial symmetry, distinct from the bilateral symmetry of fish.

Adaptations for Living in an Aquatic Environment

Fish have a range of adaptations that allow them to thrive in aquatic environments. These adaptations help them to move efficiently, breathe, find food, avoid predators, and reproduce. Here are some key adaptations:

1. Body Shape and Streamlining

  • Streamlined Body: Most fish have a streamlined body shape that reduces water resistance and helps them to swim efficiently.
  • Lateral Compression: Many fish have bodies that are laterally compressed. This helps them maneuver easily through water.

2. Locomotion and Fins

  • Fins: Fish have various types of fins (dorsal, pectoral, pelvic, anal, and caudal fins) that aid in propulsion, steering, and stability. The caudal (tail) fin is particularly important for propulsion.
  • Fin Placement: The placement and size of fins varies, adapting different species to specific environments, such as swift currents or stagnant waters.

3. Respiration

  • Gills: Fish extract oxygen from water using gills, which are highly efficient at extracting dissolved oxygen. Gills have a large surface area and thin membranes to facilitate gas exchange. (The lungfish also has lungs and it can breathe air.)
  • Counter-Current Exchange: The blood flow in gills runs opposite to the flow of water, maximizing oxygen uptake.

4. Buoyancy Control

  • Swim Bladder: Many bony fish have a swim bladder. A swim bladder is an internal gas-filled organ that helps maintain buoyancy. By adjusting the gas volume in the swim bladder, fish control their buoyancy and remain at different water depths without expending much energy.
  • Oil-Rich Livers: In cartilaginous fish like sharks, the liver is rich in oil. The oil helps with buoyancy.

5. Sensory Systems

  • Lateral Line System: A series of sensory organs along the sides of the fish that detect vibrations and water movements, aiding in navigation and predator avoidance.
  • Electroreception: Some fish detect electrical fields generated by other organisms, which is useful for hunting and navigation in murky waters.
  • Well-Developed Eyes: Many fish have well-developed eyes adapted to their specific environment, whether it is the bright, clear waters near the surface or the dark, murky depths.

6. Feeding Adaptations

  • Mouth Position and Shape: The shape and position of a fish’s mouth depend on its feeding habits. For example, bottom feeders have downward-facing mouths, while predators have forward-facing mouths.
  • Teeth and Jaw Structure: Different fish have various types of teeth and jaw structures suited to their diet, from the sharp teeth of predators to the grinding plates of herbivores.

7. Camouflage and Defense

  • Coloration: Many fish have coloration that helps them blend into their environment, whether through counter-shading (darker on top, lighter on the bottom) or mimicry.
  • Scales and Armor: Scales provide protection from predators and parasites. Some fish have bony plates or spines for added defense.

8. Reproductive Adaptations

  • Diverse Reproductive Strategies: Fish exhibit a wide range of reproductive strategies, from laying thousands of eggs to live-bearing young. These strategies help ensure the survival of their offspring in various environments.
  • Nesting and Brooding: Some fish build nests or exhibit parental care to protect their eggs and young from predators.

9. Temperature Regulation

  • Ectothermy: Most fish are ectothermic, meaning their body temperature is the same as the surrounding water temperature. This adaptation helps them survive in various thermal environments.
  • Regional Endothermy: Some fish, like certain species of tuna and sharks, maintain higher temperatures in specific body regions (muscles, brain). This enhances their performance in cold waters.

10. Hydrodynamics

  • Mucus Secretion: Many fish secrete mucus over their scales, reducing friction and helping them move more smoothly through water. It also offers protection against pathogens.

Evolutionary History of Fish

Fish are among the earliest vertebrates, with origins tracing back to over 500 million years ago during the Cambrian period. Early jawless fish, such as ostracoderms, gave rise to more complex forms. The development of jaws in fish like placoderms marked a significant evolutionary milestone, leading to the diversification of the group into the vast array of species seen today.

Fish Taxonomy

Fish are classified into three major groups:

  1. Agnatha (Jawless Fish): Includes hagfish and lampreys.
  2. Chondrichthyes (Cartilaginous Fish): Includes sharks, rays, and skates.
  3. Osteichthyes (Bony Fish): The largest group, including both ray-finned (Actinopterygii) and lobe-finned fish (Sarcopterygii).

Detailed Taxonomy of Fish

  • Agnatha:
    • Class Myxini: Hagfish
    • Class Petromyzontida: Lampreys
  • Chondrichthyes:
    • Subclass Elasmobranchii: Sharks and rays
    • Subclass Holocephali: Chimaeras (ratfish)
  • Osteichthyes:
    • Subclass Actinopterygii: Ray-finned fish
      • Order Perciformes: Perches and allies
      • Order Cypriniformes: Carps and minnows
      • Order Siluriformes: Catfish
    • Subclass Sarcopterygii: Lobe-finned fish
      • Order Coelacanthiformes: Coelacanths
      • Order Ceratodontiformes: Lungfish

Approximately 40% of fish species live in freshwater, while the remaining 60% inhabit marine environments.

Typical Anatomy of a Fish

Fish External Anatomy
(Lukas3, CC Attribution-Share Alike 3.0 Unported)

Most fish have streamlined bodies and scales. Their gills, fins, and swim bladder are distinctive external features.

  • Body Structure: Streamlined for efficient swimming.
  • Gills: Located on the sides of the head for breathing.
  • Fins: Dorsal, pectoral, pelvic, anal, and caudal fins for movement and stability.
  • Scales: Ganoid, cycloid, or ctenoid scales providing protection.
  • Swim Bladder: Present in many bony fish for buoyancy control.

Key Physiological Aspects

(Sharon High School, CC Attribution-Share Alike 3.0 Unported)

Like humans, fish have a heart, digestive system, kidneys, and several of the same senses. However, they have gills instead of lungs, often lay eggs, and sometimes sense electrical fields.

  • Circulation: Fish have a closed circulatory system with a heart that pumps blood in a single circuit.
  • Respiration: Gills extract oxygen from water and expel carbon dioxide.
  • Digestion: Begins in the mouth, continues through the esophagus, stomach, intestines, and ends at the anus.
  • Excretion: Kidneys filter waste from the blood, excreting ammonia or urea.
  • Reproduction: Can be oviparous (egg-laying), ovoviviparous (egg-hatching inside the mother), or viviparous (live-bearing).
  • Senses: Includes vision, smell, taste, touch, hearing, and the lateral line system for detecting water movements.

Fish Cognition

Fish exhibit a range of cognitive abilities, including problem-solving, memory, and learning. Studies indicate that fish navigate complex environments, recognize individuals, and exhibit social behaviors. Manta rays are among the fish that pass the mirror test and may be self-aware.

Electrogenesis in Fish

Some fish, like electric eels and electric rays, possess specialized cells called electrocytes that generate electric fields. These electric fields are used for navigation, communication, and defense. Also, some fish sense electric fields.

Thermoregulation in Fish

Most fish are ectothermic (cold-blooded), relying on external sources to regulate their body temperature. However, some species, like the opah (Lampris guttatus), are endothermic (warm-bloodedness). Other endothermic or partially endothermic fish include billfish, tuna, the great white shark, and thresher sharks. An endothermic fish maintains elevated body temperatures through metabolic heat production.

Conservation Status

Many fish species are threatened or endangered due to overfishing, habitat destruction, pollution, and climate change. Conservation efforts are crucial to protect these species and their ecosystems.

Economic and Ecological Importance

Fish play vital roles in ecosystems as predators, prey, and contributors to nutrient cycles. Economically, they are essential for commercial and recreational fisheries, providing food and livelihoods for millions of people worldwide.

Behavior and Social Structure

Fish exhibit a wide range of behaviors, from solitary hunters like the great white shark to complex social structures seen in schooling fish like herring. Some fish, like cichlids, display parental care and protect their young from predators.

Practical Applications and Studies

Fish are important in various scientific research fields, including genetics, physiology, and environmental science. Zebrafish (Danio rerio) are a model organism in developmental biology and genetics research due to their transparent embryos and rapid development.

Interesting Fish Facts

Some fish display unusual “unfishlike” behaviors:

  • Lungfish: Breathes air and survives out of water for extended periods.
  • Clownfish: Exhibits sequential hermaphroditism, changing sex during its lifetime.
  • Mudskippers: Amphibious fish that walk on land using their pectoral fins.
  • Goblin Shark: Possesses a protrusible jaw that extends to catch prey.
  • Parrotfish: Produce a mucus cocoon at night to protect themselves from predators.


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