Glossary

8.1 Evolution and diversity of marine fishes

3 min read•july 22, 2024

Marine fishes have a rich evolutionary history spanning over 500 million years. From early jawless agnathans to the diverse array of modern bony and cartilaginous fishes, their adaptations have allowed them to thrive in various aquatic environments.

Fish diversity is shaped by habitat complexity, evolutionary time, and . Specialized body forms, feeding strategies, and sensory systems have enabled fishes to occupy numerous ecological niches, from coral reefs to the deep sea, contributing to their remarkable biodiversity.

Evolutionary History and Diversity of Marine Fishes

Evolution of marine fishes

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  • Early vertebrates
    • Agnathans (jawless fishes)
      • Ostracoderms were armored, extinct early jawless fishes
      • Cyclostomes are extant jawless fishes including hagfishes and lampreys
  • Gnathostomes (jawed fishes)
    • Placoderms were the first jawed fishes, now extinct
    • are cartilaginous fishes
      • include sharks, rays, and skates
      • Holocephalans are chimaeras, also known as ratfishes
    • are bony fishes
      • Actinopterygii are ray-finned fishes
        • Chondrostei include sturgeons and paddlefishes
        • Neopterygii are divided into Holostei (gars and bowfins) and Teleostei (modern bony fishes)
      • Sarcopterygii are lobe-finned fishes
        • Coelacanths are ancient lobe-finned fishes once thought extinct
        • Lungfishes have both and lungs for respiration

Types of marine fishes

  • Jawless fishes ()
    • Lack jaws and paired fins
    • Have a skeleton composed of cartilage
    • Include hagfishes and lampreys
  • Cartilaginous fishes (Chondrichthyes)
    • Possess jaws and paired fins
    • Have a skeleton made of cartilage rather than bone
    • Include elasmobranchs (sharks, rays, skates) and holocephalans (chimaeras)
  • Bony fishes (Osteichthyes)
    • Have jaws, paired fins, and a skeleton made of bone
    • Divided into Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes)
      • Actinopterygii have a and thin rays supporting their fins
      • Sarcopterygii have fleshy, lobed fins and some species possess lungs

Adaptations and Ecological Niches

Adaptations for marine environments

  • Locomotion
    • Body shape and fin placement adapted for different swimming modes
      1. Cruising for efficient long-distance swimming
      2. Acceleration for quick bursts of speed
      3. Maneuverability for navigating complex habitats
      4. Stability for precise movements and hovering
    • Bony fishes use swim bladders for buoyancy control
  • Feeding
    • Jaw and tooth morphology adapted for various diets
      • Filter feeding on plankton (whale shark, basking shark)
      • Grazing on algae and seagrasses (parrotfishes, surgeonfishes)
      • Predation on other fishes and invertebrates (groupers, barracudas)
      • Parasitism on other fishes (cookiecutter sharks, lampreys)
    • Specializations include elongated jaws (needlefishes), protrusible mouths (triggerfishes), and pharyngeal jaws (moray eels)
  • Respiration
    • Gills extract oxygen from water for respiration
    • Some fishes use the swim bladder as an accessory respiratory organ
    • Lungfishes possess true lungs for breathing air
  • Sensory systems
    • Vision adapted for different light levels (deep-sea fishes) and colors (reef fishes)
    • detects water movement and pressure changes for navigation and prey detection
    • Chemoreception includes taste and smell for locating food and mates
    • allows detection of electrical fields (sharks, rays)
  • Reproduction
    • External fertilization strategies include broadcast spawning and egg scattering
    • Internal fertilization leads to live-bearing (sharks) and egg-laying (skates)
    • Parental care involves nest building (sticklebacks), egg guarding (damselfish), and mouthbrooding (cardinalfishes)

Factors in fish species diversity

  • Habitat diversity
    • Coral reefs, seagrass beds, mangroves, open ocean, and deep sea offer unique niches and resources
    • Each habitat supports a diverse array of fish species
  • Evolutionary time
    • Fishes have evolved over more than 500 million years
    • Long evolutionary history has allowed for and adaptation to various niches
  • Isolation and allopatric speciation
    • Geographic barriers such as ocean basins, currents, and temperature gradients isolate populations
    • Isolated populations diverge over time, leading to the formation of new species
  • Sexual selection
    • Mate choice based on morphological traits (coloration, fin shape) or behavioral traits (courtship displays)
    • Sexual selection can drive divergence and speciation
  • Adaptive radiation
    • Rapid speciation and diversification occur when new niches become available
    • Cichlid fishes in African Great Lakes (Victoria, Malawi, Tanganyika) are a prime example of adaptive radiation

Key Terms to Review (21)

Adaptive Radiation: Adaptive radiation is an evolutionary process in which organisms diversify rapidly into a wide variety of forms to adapt to different environments and ecological niches. This phenomenon often occurs when a group of species colonizes a new area or when environmental changes create new opportunities, leading to the emergence of distinct adaptations that enhance survival and reproduction in various habitats.
Agnatha: Agnatha refers to a superclass of jawless fish, which includes species such as lampreys and hagfish. These ancient marine organisms are characterized by their lack of jaws, paired fins, and skeletal structures made primarily of cartilage rather than bone. Their unique adaptations and evolutionary significance provide insight into the early development of vertebrates and the diversity of marine fishes.
Bioluminescence: Bioluminescence is the natural phenomenon where living organisms produce and emit light through biochemical reactions. This striking ability is often seen in various marine species, serving functions such as communication, predation, and camouflage, and plays a crucial role in the ecology of both shallow and deep-sea environments.
Camouflage: Camouflage is a form of adaptation where organisms blend into their environment to avoid detection by predators or prey. This ability is essential for survival, as it helps animals conceal themselves in their habitats, which can vary widely from intertidal zones to open waters. The effectiveness of camouflage can be influenced by factors such as body shape, color patterns, and behavioral strategies that enhance an organism's chances of survival in the wild.
Carl Linnaeus: Carl Linnaeus was an 18th-century Swedish botanist and zoologist, widely known as the father of modern taxonomy. He developed a systematic framework for naming and classifying organisms, which laid the groundwork for our understanding of biological diversity and the evolutionary relationships among different species.
Charles Darwin: Charles Darwin was a British naturalist known for developing the theory of evolution through natural selection. His groundbreaking work, 'On the Origin of Species,' published in 1859, laid the foundation for modern evolutionary biology and transformed our understanding of the diversity of life on Earth, including marine fishes.
Chondrichthyes: Chondrichthyes is a class of cartilaginous fish that includes sharks, rays, and skates. These marine animals are characterized by having a skeleton made of cartilage instead of bone, which gives them flexibility and adaptability in their aquatic environments. They also possess unique features such as dermal denticles and specialized sensory organs that enhance their ability to detect prey and navigate through the ocean.
Dunkleosteus: Dunkleosteus was a large prehistoric fish that lived during the late Devonian period, approximately 358 to 382 million years ago. Known for its massive size and formidable jaw structure, Dunkleosteus is one of the most well-known placoderm fishes and plays an important role in understanding the evolution and diversity of marine fishes during its time.
Elasmobranchs: Elasmobranchs are a subclass of cartilaginous fish that includes sharks, rays, and skates, characterized by their skeletons made of cartilage instead of bone. This unique feature allows for greater flexibility and buoyancy, which is essential for their survival in various marine environments. They are among the most ancient groups of fish, with a rich evolutionary history that contributes to the vast diversity observed within marine ecosystems today.
Electroreception: Electroreception is the biological ability of certain animals to detect electric fields generated by other living organisms or environmental sources. This sensory adaptation is especially prevalent in aquatic environments, where it plays a critical role in navigation, hunting, and communication among marine species. Animals like sharks, rays, and some bony fishes possess specialized electroreceptors that allow them to perceive these electrical signals, enhancing their ability to interact with their surroundings.
Gills: Gills are specialized respiratory organs found in many aquatic organisms, primarily fish, that enable the extraction of oxygen from water. These structures are essential for the survival of most fish, as they allow for efficient gas exchange, facilitating their adaptation to various aquatic environments and contributing to their evolutionary success.
Habitat loss: Habitat loss refers to the decline or complete destruction of natural environments that support various species, leading to their decreased populations and potential extinction. It is often driven by human activities such as urban development, pollution, and climate change, significantly impacting biodiversity and ecosystem health.
Keystone Species: A keystone species is an organism that has a disproportionately large effect on its environment relative to its abundance. The presence or absence of a keystone species can significantly influence the structure and dynamics of an entire ecosystem, affecting many other species and their relationships within that environment.
Lateral line system: The lateral line system is a specialized sensory system found in many aquatic vertebrates, including fishes, that allows them to detect water movements and vibrations in their environment. This system consists of a series of mechanoreceptors known as neuromasts, which are located in canals along the sides of the body and head. By sensing changes in water pressure and movement, fishes can navigate, avoid predators, and locate prey, making it a crucial adaptation for survival in aquatic habitats.
Osteichthyes: Osteichthyes, commonly known as bony fishes, represent a diverse class of fish characterized by a skeleton primarily composed of bone tissue, as opposed to cartilage. This class includes the majority of fish species found in marine and freshwater environments today, showcasing a remarkable range of forms and adaptations that allow them to thrive in various habitats.
Overfishing: Overfishing refers to the depletion of fish stocks through excessive fishing activities, resulting in a decline in fish populations and disruption of marine ecosystems. This phenomenon poses significant threats to biodiversity, food security, and the overall health of marine environments.
Schooling behavior: Schooling behavior refers to the social behavior of fish when they aggregate in groups, often for purposes such as protection from predators, improved foraging efficiency, and enhanced reproductive success. This behavior is critical for the survival of many marine fish species and illustrates their complex interactions with both their environment and each other, showcasing evolutionary adaptations that have emerged over time.
Speciation: Speciation is the evolutionary process through which new biological species arise. It occurs when populations of a single species become genetically isolated from each other, leading to divergence over time due to different environmental pressures, mutations, or genetic drift. In marine environments, speciation plays a vital role in increasing biodiversity and adapting to various ecological niches.
Swim bladder: A swim bladder is a gas-filled organ found in many bony fishes that helps control buoyancy and maintain stability in the water column. By adjusting the volume of gas within this organ, fish can rise or sink without expending significant energy, allowing them to conserve energy while swimming and to occupy specific depths in their aquatic environment.
Tiktaalik: Tiktaalik is an extinct genus of lobe-finned fish that lived around 375 million years ago during the Devonian period, notable for its transitional features between fish and early tetrapods. This ancient creature showcases the evolutionary shift from aquatic to terrestrial life, representing a key stage in the development of land-dwelling vertebrates and offering insights into the evolutionary history of marine fishes.
Trophic levels: Trophic levels are the hierarchical positions in a food chain or food web that represent the flow of energy and nutrients through an ecosystem. Each level is defined by its role in the ecosystem, such as producers, primary consumers, secondary consumers, and so on, illustrating how energy is transferred from one level to the next.
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