Q19.Which of the following is NOT a characteristic feature of bony fishes?
(A) Presence of paired fins
(B) Laterally compressed body
(C) Presence of homocercal tail
(D) Presence of placoid scales
Placoid scales are not found in bony fishes, making (D) the correct answer.
Bony fishes (Osteichthyes) possess bony skeletons, swim bladders, cycloid/ctenoid scales, and homocercal tails, distinguishing them from cartilaginous fishes (Chondrichthyes) that have placoid scales.
Option Analysis
(A) Presence of Paired Fins
Bony fishes typically have paired pectoral and pelvic fins for stability, steering, and maneuverability, supported by bony rays (lepidotrichia).
(B) Laterally Compressed Body
Many bony fishes exhibit lateral compression (e.g., perch, butterflyfish) for agile swimming in coral reefs or open water, though fusiform (torpedo-shaped) bodies occur in fast swimmers like tuna.
(C) Presence of Homocercal Tail
Homocercal tails are standard in bony fishes, featuring symmetrical upper/lower lobes with the vertebral column terminating at the tail base, optimizing thrust and stability.
(D) Presence of Placoid Scales
Placoid scales (denticles) are exclusive to cartilaginous fishes (sharks, rays), resembling tiny teeth with enameloid crowns and pulp cavities; bony fishes have cycloid (smooth) or ctenoid (comblike) scales.
The characteristic feature of bony fishes excludes placoid scales, which belong to cartilaginous fishes. Bony fishes (Osteichthyes >30,000 species) feature bony endoskeletons, thin cycloid/ctenoid scales, opercula, and homocercal tails for buoyancy and propulsion.
Bony vs. Cartilaginous Fish Traits
Placoid scales provide hydrodynamic roughness in sharks but are absent in bony fish, which rely on mucous-covered cycloid scales for streamlined movement.
Exam Relevance
Which is NOT a characteristic feature of bony fishes distinguishes Osteichthyes (bony skeleton, swim bladder) from Chondrichthyes (cartilage, placoid scales, spiracles). Key for NEET zoology: bony fish scales flex without dentine.
