Fish are fascinating aquatic vertebrates with diverse adaptations. From primitive jawless species to advanced jawed fishes, they've evolved unique features for life underwater. Their respiratory, circulatory, and osmoregulatory systems are finely tuned for aquatic environments.

Sharks and rays, with cartilaginous skeletons, have keen senses for hunting. Bony fishes, including ray-finned and lobe-finned species, show remarkable diversity. Their adaptations, like swim bladders and specialized fins, have allowed them to thrive in various aquatic habitats.

Jawless and Jawed Fishes

Jawless vs jawed fishes

• Jawless fishes (Agnatha) lack jaws and paired fins, are primitive vertebrates with a cartilaginous skeleton, and the notochord persists throughout life without being replaced by a vertebral column (hagfish and lampreys)
• Jawless fishes lack scales and represent the earliest vertebrates in evolutionary history
• Jawed fishes (Gnathostomata) possess jaws and paired fins, are more advanced than jawless fishes, have a bony or cartilaginous skeleton, and the notochord is replaced by a vertebral column (sharks, rays, and bony fishes)
• Most jawed fish species have scales and the evolution of jaws allowed for diversification of feeding strategies and increased success compared to jawless fishes

Sharks, Rays, and Bony Fishes

Characteristics of sharks and rays

• Sharks and rays (Chondrichthyes) have a cartilaginous skeleton and placoid scales that are tooth-like in structure
• Sensory systems of sharks and rays include:
  • Ampullae of Lorenzini that detect electrical fields produced by prey
  • Lateral line system that detects water movement and pressure changes for navigation and hunting
  • Keen sense of smell for tracking prey over long distances
• Reproductive strategies of sharks and rays involve internal fertilization and either:
  • Oviparity where eggs are laid and develop externally (skates and some shark species)
  • Viviparity where live birth occurs after internal development (most sharks)
  • Ovoviviparity where eggs hatch internally and live birth occurs (some sharks and rays)
• Sharks and rays have low fecundity, slow maturation, and long gestation periods compared to bony fishes

Adaptations of bony fishes

• Bony fishes (Osteichthyes) have a skeleton composed of bone rather than cartilage
• Many bony fishes possess a swim bladder, a gas-filled organ that aids in buoyancy control
  • Some bottom-dwelling species lack a swim bladder (flatfishes and some eels)
  • In some species, the swim bladder functions as an accessory respiratory organ (lungfishes and gouramis)
• Ray-finned fishes (Actinopterygii) have thin, bony rays supporting their fins and are a highly diverse group that includes most common fish species (salmon, tuna, and perch)
• Lobe-finned fishes (Sarcopterygii) have fleshy, muscular fins supported by bones and include:
  • Coelacanths, ancient fish once thought to be extinct until rediscovered in 1938
  • Lungfishes, which have gills and a highly vascularized swim bladder that functions as a lung
• Lobe-finned fishes are evolutionarily significant as they gave rise to tetrapods, the first four-legged vertebrates to transition onto land

Fish Physiology and Adaptations

Respiratory and circulatory systems

• Fish use gills for gas exchange, which are highly efficient due to countercurrent gas exchange
• Gills are composed of numerous filaments with lamellae, increasing surface area for oxygen absorption
• Blood flows through capillaries in the lamellae in the opposite direction of water flow, maximizing oxygen uptake

Osmoregulation and excretion

• Fish maintain osmotic balance through osmoregulation, which differs between freshwater and marine species
• Freshwater fish actively uptake ions through their gills and excrete dilute urine to counteract water influx
• Marine fish drink seawater and excrete concentrated urine to counteract water loss

Thermoregulation

• Most fish are ectothermic, relying on environmental temperature to regulate their body temperature
• Some species, like tuna and certain sharks, exhibit regional endothermy to maintain higher body temperatures in specific organs

External features and locomotion

• Fish possess fins for propulsion, stability, and maneuvering in water
• Scales cover the body of most fish species, providing protection and reducing drag during swimming