Fish Biology, Anatomy and Physiology
Fish Biology, Anatomy and Physiology
This domain tests the structure and function of fish: external features, scales, fins, gills, circulation, digestion, and osmoregulation. Questions often ask you to link a structure to its function.
Core rules
- Scale types: cycloid and ctenoid in most bony fishes; placoid in sharks and rays; ganoid in gars.
- Fins: dorsal, caudal, anal, paired pectoral and pelvic. Caudal types include homocercal, heterocercal, and diphycercal.
- Gills: water enters through the mouth, passes over filaments and lamellae, and exits under the operculum. Countercurrent flow between water and blood maximises oxygen uptake.
- Circulation: a two-chambered heart (one atrium, one ventricle) pumps blood in a single circuit: heart → gills → body → heart.
- Swim bladder controls buoyancy in most teleosts; it is not a lung. Kidneys regulate water and salts. The lateral line detects vibration and pressure.
Worked example
A specimen has cycloid scales, two pairs of barbels, a single long dorsal fin, and a pharyngeal tooth pad. These features point to common carp (Cyprinus carpio), not to a marine sciaenid. Reason: barbels and cycloid scales are typical of cyprinids, while the long dorsal fin and pharyngeal teeth separate carp from catfish, which have naked skin and adipose fins.
Traps
Do not say fish have a four-chambered heart; do not state that gills actively pump oxygen into blood-uptake is by diffusion; do not use total length when a question asks for standard length; and do not assume the swim bladder stores oxygen for breathing.