Jawless Fish Notes

Jawless Fish

Learning Intentions

  • Today, we are learning about SuperClass Agnatha and its members.
  • So we can appreciate the ecological roles and diversity of its members.

Success Criteria

  • I can list and describe the general features and characteristics seen in members of the SuperClass Agnatha.
  • I can relate Agnatha form to its functions.

Jawless Fish Classification

  • Kingdom Animalia
    • Phylum Chordata
      • Subphylum Urochordata (Tunicates)
      • Subphylum Cephalochordata (Lancelets)
      • Subphylum Vertebrata
        • Class Cyclostomata (Jawless Fish)
        • Class Chondrichthyes (Cartilaginous Fish)
        • Class Osteichthyes (Bony Fish)
        • Class Mammalia
        • Class Aves
        • Class Reptilia
        • Class Ascidiacea
        • Class Larvacea or Appendicularia
        • Class Thaliacea
        • Class Branchiostoma
        • Class Epigonichthyes
        • Class Amphibia

Characteristics of Jawless Fish

  • Superclass Agnatha, Class Cyclostomata
  • First appeared around 540 million years ago.
  • Considered the first fish species to evolve from ancient worms.
  • Composed of over 100 species:
    • Hagfish: over 70 species
    • Lampreys: over 30 species
  • Ectothermic (cold-blooded)
  • Cartilaginous skeleton
  • Heart contains 2 chambers
  • Have 7 or more paired gills
  • No fins
  • No scales

Teeth

  • Lampreys:
    • Rows of sharp teeth in a circular pattern.
    • Used for attaching onto prey like a parasite.
    • Grinding down the animal’s flesh.
  • Hagfish:
    • Two pairs of horny, comb-shaped teeth on a cartilaginous plate.
    • Moves in and out of its body.
    • Used to grasp food and draw it toward the pharynx.
    • Rows of sharp teeth to break down food.
  • Lack jaws, which limits possible food sources.

Metabolism and Digestion

  • Agnathans are ectothermic:
    • Do not regulate their own body temperature.
    • Metabolism is slow in cold water.
    • Do not have to eat very much.
  • No distinct stomach:
    • Long gut that flows throughout its body.

Body Covering

  • Covered in skin, not scales.
  • Hagfish:
    • Copious slime glands.
    • Defense mechanism.
    • Slime can clog up enemy fishes' gills, causing them to die.

Eyesight

  • Lampreys:
    • Light-sensitive pineal eye.
  • Hagfish:
    • Simple eyespots, not lensed eyes.
    • Almost blind.
    • Keen sense of smell and touch.

Lampreys

  • Toothed, funnel-like sucking mouth.
  • Resemble eels:
    • Scaleless, elongated bodies.
  • Range from 5 to 39.5 inches in length.
  • Lack paired fins.
  • Large eyes.
  • One nostril atop the head.
  • Seven gill pores on each side of the head.
  • Oldest divergences of the vertebrate lineage:
    • Lived over 400 million years ago.
  • Parasites:
    • Attach to bony fish and suck their blood and other bodily fluids.
  • Some non-carnivorous lampreys scrape algae from rocks.
  • Young lampreys are filter feeders.
  • Live in saltwater and freshwater habitats:
    • All types breed in freshwater.
  • Many species have countershading:
    • Dark on top, light on bottom.
    • Blend in with the environment from above or below.

Hagfish

  • Myxini (hagfish) are eel-shaped slime-producing marine animals.
  • Also called slime eels.
  • Only known living animals that have a cartilaginous skull but not a vertebral column.
  • Current living hagfish are the same type that lived 300 million years ago and are modern fossils.
  • Typically about 19.7 inches in length.
  • Generally a dull pink color and look quite worm-like.
  • Colors depend on the species, ranging from pink to blue-grey, and black or white spots may be present.
  • Eyes are simple eyespots, not lensed eyes.
    • Almost blind; keen sense of smell and touch.
  • No true fins.
  • Six or eight barbels around the mouth and a single nostril.
  • Scavengers:
    • feed mostly on dead organisms such as fish, worms, mollusks, and crustaceans.
  • Live only in salt water.

Reproduction - Lampreys

  • Fertilization and development is external.
  • No known parental care.
  • Only able to reproduce once.
  • After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them.
  • Reproduce in freshwater riverbeds:
    • Work in pairs to build a nest.
    • Bury their eggs about an inch beneath the sediment.
  • Hatchlings go through four years of larval development before becoming adults.

Reproduction - Hagfish

  • Mode of fertilization is not known.
  • Development is probably external.
  • No known parental care.
  • Not much is known about the hagfish reproductive process.
  • Believed that hagfish only have 30 eggs over a lifetime.
  • Hermaphroditic:
    • Have both male and female parts, but only one set of sex organs is active at a time.

Issues Related to Jawless Fish

  • Sea lampreys have become a major pest in the North American Great Lakes.
  • Highly invasive.
  • US and Canadian governments are working hard to stop them from becoming more of a pest and growing out of control.
  • Reached the great lakes via canals during the early 20th century.
  • Considered an invasive species.
  • No natural enemies in the lakes.
  • Prey on many species of commercial value, such as lake trout.

Lampreys and Leeches

  • Lampreys and leeches are NOT the same.
  • Leeches are from the phylum Annelida, the same phylum as Earthworms.
  • Leeches are invertebrates and don't possess the bone structure vertebrates have, thus making them vastly different from lampreys.
  • Lampreys:
    • Have gills and fins just like a fish.
  • Leeches:
    • Do not have gills and fins. Are invertebrates.