Concise Notes on Fish Anatomy

FISH ANATOMY

BODY SHAPE

  • Related to lifestyle:
    • Fast swimmers: streamlined body (e.g., sharks, mackerels, marlins)
    • Laterally compressed: leisurely swimmers (e.g., snappers, wrasses)
    • Dorsoventrally flattened: (e.g., rays, skates, sea moths)
    • Elongated bodies: (e.g., eels)
    • Unique shapes: seahorses, trunkfishes, porcupine fish can inflate when threatened.

DIGESTIVE SYSTEM

  • Pathway of food intake: pharynx ➔ esophagus ➔ stomach
  • Stomach: J-shaped, food further digested by pyloric caeca (digestive enzymes)
  • Lack of stomach in some species: expanded intestine for digestion
  • Spiral valve increases intestine surface area, waste exits through cloaca.

CIRCULATORY SYSTEM

  • Heart: 2 chambers, located below gills
  • Blood flow:
    • Deoxygenated blood ➔ 1st heart chamber ➔ 2nd chamber ➔ gills (gas exchange)
    • Oxygenated blood ➔ body via arteries ➔ deoxygenated blood returns to heart via capillaries.

RESPIRATORY SYSTEM

  • Obtain oxygen from water through gills
  • Ventilation mechanism: mouth opens and closes, forcing water over gills
  • Cartilaginous fishes: modified gill slits (spiracles) for bottom-dwelling
  • Bony fishes have operculum covering gills.

GAS EXCHANGE

  • Oxygen diffuses from water to blood in gills, facilitated by countercurrent flow
  • Myoglobin in muscles stores oxygen for active swimmers.

REGULATION OF INTERNAL ENVIRONMENT

  • Marine bony fishes: less salty blood than seawater, osmoregulate
  • Cartilaginous fishes: increase blood solute concentration to reduce osmosis.

NERVOUS AND SENSORY SYSTEMS

  • Central nervous system coordinates body functions
  • Highly developed olfactory (smell) and visual senses, e.g., round lens in eyes.

HEARING

  • Fish perceive sound via inner ears and lateral line system for vibrations
  • Cartilaginous fishes detect weak electrical fields with ampullae of Lorenzini.

REPRODUCTIVE SYSTEM

  • Paired gonads located in the body cavity
  • Cartilaginous fishes: ducts open to cloaca, bony fish have separate openings
  • Types of reproduction: simultaneous hermaphroditism, sequential hermaphroditism (male to female change).

OSMOREGULATION

  • Cartilaginous fishes: retain urea; block loss of compounds through gills
  • Bony fishes: excrete salts via kidneys and gills, manage water balance through seawater consumption and urine.