Fish Biology, Anatomy and Physiology
What the exam tests
Fish biology, anatomy and physiology questions test external identification, organ function and adaptation to water. You should know fins, scales, lateral line, gills, swim bladder, two-chambered heart, kidney, digestive tract and otoliths, and relate structure to respiration, osmoregulation, buoyancy and sensory function.
Core rules
- Gills use countercurrent flow: water and blood move in opposite directions, improving oxygen uptake.
- The swim bladder adjusts buoyancy; it is not a lung in most teleosts.
- The heart has two main chambers, one atrium and one ventricle, and pumps deoxygenated blood to the gills.
- The kidney regulates water and ion balance; freshwater fish tend to gain water and excrete dilute urine.
- Otoliths are ear stones used in ageing; scales and fin rays also help identification.
Worked example
A fish is taken from water and placed on a bench. It gapes and dies quickly. Reason: gill lamellae collapse without water support, reducing the surface area for gas exchange, and the fish cannot obtain enough oxygen from air. The two-chambered heart does not change this; the limiting factor is gill function.
Traps
Do not say fish have a three-chambered heart. Do not call the swim bladder a lung in rohu or catla. Do not ignore countercurrent exchange. Do not confuse the lateral line with a fin.
Key facts
Fins include dorsal, caudal, anal, pelvic and pectoral. Cycloid and ctenoid scales occur in many teleosts. The operculum protects the gills. The liver and spleen are separate organs.