Agnatha, or jawless fish, are a unique group of vertebrates that lack jaws and paired fins, distinguishing them from other fish. They are represented today by two main families: the lampreys (Petromyzontidae) and the hagfish (Myxinidae). Lampreys are eel-like, parasitic or free-living vertebrates with a toothed, funnel-like mouth used for attaching to hosts, while hagfish are scavengers known for their slime-producing ability and primitive characteristics.
- Share the defining characteristics of Agnatha, including their lack of jaws and paired fins.
- Identify the two main modern groups of Agnatha: lampreys and hagfish.
- Describe the lifestyles of lampreys (parasitic or free-living) and hagfish (scavengers).
Lampreys and hagfish are the two main groups of modern Agnatha.
Lampreys: Living Relics of the Jawless Fish
Lampreys (Petromyzontidae) are eel-shaped, jawless vertebrates that use a circular, toothed mouth to attach to and parasitize other fish, although some species are non-parasitic. They possess a notochord instead of a vertebral column and have a simple, cartilaginous skeleton. Lampreys undergo a lengthy larval stage, called ammocoetes, during which they filter-feed before metamorphosing into adults.
- Describe the unique feeding mechanism of lampreys using their toothed, circular mouth.
- Explain the difference between parasitic and non-parasitic lamprey species.
- Outline the lamprey life cycle, emphasizing the larval (ammocoete) stage.
Lamprey larvae (ammocoetes) filter-feed during their extended larval stage.
Hagfish: Masters of Slime and Scavengers
Hagfish (Myxinidae) are jawless, eel-like scavengers famous for producing large quantities of slime as a defense mechanism. They lack vertebrae, possessing a notochord for support, and have rudimentary eyes. Hagfish feed primarily on dead or dying animals, burrowing into carcasses to consume them from the inside, and play a vital role in marine ecosystems by recycling nutrients.
- Highlight the defensive role of slime production in hagfish.
- Contrast hagfish feeding habits with those of lampreys.
- Describe the skeletal support system of hagfish.
Hagfish produce slime as a unique defense mechanism.
Hagfish are not true vertebrates; they are invertebrate chordates.
Conclusion
Agnatha, including lampreys and hagfish, represent some of the most primitive fish alive today, showcasing jawless designs that highlight early vertebrate evolution. Lampreys are known for their parasitic lifestyles and unique mouthparts, while hagfish are remarkable scavengers with extraordinary slime defenses.
- Agnatha include lampreys and hagfish, both jawless and eel-like.
- Lampreys use toothed mouths to parasitize hosts; hagfish produce defensive slime.
- Both groups provide key insights into vertebrate ancestry and evolution.
Agnatha are jawless, have eel-like bodies, and include lampreys and hagfish.
Hagfish act as scavengers, recycling nutrients in marine ecosystems.