chordates 1

Overview of Chordates and Fishes

Definition of a Chordate

  • Domain: Eukarya

  • Kingdom: Animalia

  • Phylum: Chordata

  • Four Key Traits (may be present at all or part of the life cycle):

    1. Dorsal Nerve Cord

    • Forms on the dorsal side and is hollow.

    • Eventually develops into the brain and spinal cord.

    1. Pharyngeal Gill Slits

    • Openings in the pharynx, function varies:

      • Form gills in fishes and amphibians.

      • In other chordates, gill slits may disappear prior to adulthood but contribute to jaw and neck cartilage.

    1. Notochord

    • A thin flexible rod located ventral to the dorsal nerve cord.

    • Provides a site for muscle attachment.

    • In vertebrates, it eventually forms vertebrae (the backbone).

    1. Post-Anal Tail

    • A muscular tail that serves multiple functions including movement, balance, communication, and mate attraction.

    • Typically lost during embryonic development in humans.

Classification of Chordates

Types of Chordates

  • Not all chordates are vertebrates.

    • Examples of invertebrate chordates include Lancelets and Tunicates.

  • Vertebrates: All others on the chordate tree, characterized by the presence of backbones (vertebrae) which provide muscle and skeletal support and protect the dorsal nerve cord.

Types of Vertebrates

Jawless Fishes

  1. Hagfish

    • Structure: Cartilaginous skeleton.

    • Movement: Lacks paired fins; moves by flexing body back and forth.

  2. Lamprey

    • Specified methods of eating for lampreys (not detailed).

Jawed Fishes

  • All vertebrates other than lampreys have jaws.

  • Evolutionary Significance of Jaws:

    • Jaws are considered an important evolutionary advancement, allowing for a wider diet and more efficient feeding mechanisms.

  • Paired Fins: Represents an important evolutionary trait, leading to locomotor improvements.

Types of Jawed Fishes:

  • Cartilaginous Fishes (e.g., Sharks & Rays)

    • Skeletal structure made of cartilage which is lighter yet provides less support than bone.

    • Predatory behaviors; possess highly developed senses.

  • Ray-finned Fishes

    • Skeleton composed of bone with calcium deposits.

    • Contains an operculum, a protective covering for gills.

    • Most bony fishes possess swim bladders, likely evolved from primitive lungs.

  • Lobe-finned Fishes

    • Structural adaptation of lobed fins that are precursors to limbs.

    • Common in brackish waters during the Devonian period, many species now extinct.

    • Lungfish (salamanderfish)

    • Ability to breathe air and survive on land for limited periods.

    • Adapted to shallow areas free from larger predators.

Breathing Mechanisms in Fishes

  • Water Pumping Techniques:

    • Some fish force water over gills by opening and closing their mouths.

    • Sharks maintain a continuous flow of water over their gills by swimming perpetually.

  • Gill Structure:

    • Gills are adapted to maximize gas exchange surface area with numerous filaments made up of disc-like lamellae.

Quiz Questions

  • Which of the following is NOT a trait of all chordates?
    A. Vertebrae (backbones)
    B. Notochord
    C. Dorsal Nerve Cord
    D. Pharyngeal gill slits

  • Which traits do ALL FISHES have in common?
    A. Cartilage skeleton
    B. Jaws
    C. Bony skeleton
    D. Vertebrae

Evolutionary Tree Connections

  • As the evolutionary tree progresses, skeletons become more mineralized (more bony structures appear).

  • Evolution of significant traits such as jaws, paired fins, bony skeletons, swim bladders, and legs show increasing complexity and adaptations in aquatic environments.

  • Advanced Traits of Later Vertebrates:

    • Amniotic egg, milk production, and four limbs are among the adaptations that further evolved from the lobe-finned ancestors of modern amphibians, reptiles, birds, and mammals.

Important Notes

  • 99% of all fishes are ray-finned fishes.

  • The evolution of features such as jaws and paired fins represents critical developments in vertebrate evolutionary history. E