Overview of Fish and Their Classes

Overview of Fish

• Fish are the most numerous and widespread vertebrates.
• There are approximately 22,000 species of fish.
• Distribution: 59% saltwater, 40% freshwater, 1% inhabit both environments.

Classes of Fish

• Three primary classes:
  • Class Agnatha (jawless fish)
  • Class Chondrichthyes (cartilaginous fish)
  • Class Osteichthyes (bony fish)

Class Agnatha

• Characteristics:
  • Slimy skin, unpaired fins, and eel-like bodies.
  • Possess a notochord and a cartilaginous skeleton.
• Major species:
  • Lampreys and hagfish, totaling approximately 70 species.

Lampreys

• Feed parasitically using a sucker-like mouth with hard teeth to extract blood from prey.
• Damage commercial fisheries, especially in the Great Lakes.
• Larval stage lives in streams for several years before metamorphosing into adults.

Hagfish

• Dwell on the ocean floor and are scavengers.
• Mouth has fleshy tentacles and rows of tooth-like plates for tearing flesh.

Class Chondrichthyes

• Composed of sharks, skates, and rays.
• Bones are made of cartilage.
• Advantages over Agnatha:
  • Possess biting jaws and paired fins, allowing for better stabilization.

Sharks

• Are carnivorous and prey on fish and bottom-dwelling animals.
• Features:
  • Streamlined bodies and paired fins enhance maneuverability and stability.
  • Paired gills facilitate efficient gas exchange.
  • Equipped with sharp, triangular teeth that can be continually replaced.
  • Lateral line system detects movement and vibrations in the water.

Reproduction in Sharks

• Males and females are generally separate, meeting only for mating.
• Internal fertilization occurs, producing 2 to 60 fully formed, independent young.

Skates and Rays

• Similar to sharks but have flattened bodies and enlarged pectoral fins.
• Often bury their mouths in the sediment and take in water through spiracles.

Class Osteichthyes

• Known as bony fish and represent the most numerous fish species.
• Examples include trout, salmon, cod, perch, sole, eel, and halibut.
• Three sub-classes:
  • Ray-finned fish
  • Lungfish
  • Lobe-finned fish

Ray-Finned Fish

• Exhibit a wide diversity, found in nearly all aquatic habitats.
• Their gills are protected by an operculum, a bony plate.
• Possess a two-chambered heart and flat, flexible scales covering their bodies.

Reproductive Process

• Fertilization occurs externally:
  • Females lay numerous eggs, and males deposit milt, a sperm-laden fluid, over them.