Fish Biology, Taxonomy and Identification
External morphology and fin formula
Count fins and rays; record unbranched rays with Roman numerals and branched rays with Arabic numerals. D. iii/8 means three unbranched dorsal rays followed by eight branched rays. Pectoral, pelvic, anal and caudal formulas follow the same logic; the number after the slash is branched rays. Scale type: cycloid in carps, ctenoid in perches, placoid in sharks; lateral-line counts run along the row.
Taxonomy and regional species
True fish are chordates, usually class Actinopterygii. Rohu (Labeo rohita), catla (Catla catla) and mrigal (Cirrhinus mrigala) are cyprinids; hilsa (Tenualosa ilisha) is a clupeid; bagda (Penaeus monodon) is a crustacean, not a fish. Identify by body shape, barbels, lips, dorsal-fin position, caudal fin and scale type.
Worked example
A specimen has cycloid scales, two pairs of barbels, a terminal mouth and D. iii/8. The cycloid scales and barbels place it among cyprinids, but the fin formula alone cannot separate rohu from mrigal; check lip shape, dorsal-fin length and body depth. Reasoning: use a suite of characters, not one formula.
Traps
- Counting all dorsal rays as branched.
- Using local names as scientific identification.
- Confusing golda, a prawn, with a fish.
- Forgetting that lateral-line count differs from scale-row count.