Taxonomy of Finfish

2.1.2 Morphological charater use in ichthyotaxonomy

To separate a family taxon, genus taxon in addition to their use in identifying species taxon.

To separate closely related genera and species

Example -1

The order, Pleuronectiformes includes six families and all these families could be separated using the following morphological characters

1. Psettodidae – (i) Spiny rays present, (ii) eyes on left side or right side only, (iii) dorsal fin origin well posterior to eye.
2. Pleuronectidae – (i) Pelvic without spine, (ii) eyes only on right side, (iii) preopercle margin exposed, its hind margin free and visible.
3. Citharidae – (i) Eyes on one side (left or right), (ii) pelvic with one spine and five rays.
4. Bothidae – (i) Eyes only on left side, (ii) edge of preopercle margin free and visible, (iii) separate caudal fin.
5. Soleidae – (i) Pelvic without spine, (ii) preopercle margin exposed, its hind margin hidden by skin, (iii) eyes only on right side.
6. Cynoglossidae – (i) No free preopercular margin, (ii) eyes on left side, (iii) caudal fin

Example – 2

The genera in the family, Ariidae (marine catfishes) could be separated using the following morphological characters.

1. Osteogeneosus – one pair of stiff and semiosseous maxillary barbells (mental barbells absent)
2. Batrachocephalus – only 1 or 2 pairs of soft, minute rudimentary mental barbells (maxillary barbells absent)
3. Arius – three pairs of slender barbells (one pair maxillary and 2 pairs mandibular)

Example – 3  

• The presence of ventral scutes separate the closely related general such as Stolephorus, Thryssa and Thryssina.
• If scutes are present between pectoral and pelvic fins, the species of such type comes under the genus, Stolephorus.
• If the scutes are not present before the pectoral, such species are placed under the genus. Thryssina and when scutes are present before and behind pelvic, fin, such species are placed under the genus, Thryssa and Setipinna
• Species coming under the genus, Thryssa have the first pectoral fin ray normal, whereas in the species coming under the genus, Setipinna, the first ray is filamentous.

Example – 4  

• Some morphological characters may also change in accordance to length groups.
• In the genus, Atule of the family, Carangidae, the earlier authors while separating this genus with closely related genera, Alepes emphasized the extension of adipose eyelids.
• In the genus, Atule the adipose eyelid covers the entire eye except the central slit.
• But in larger length groups, the ventral part of the eye is also covered with adipose eyelids.
• Hence this character is applicable only in smaller specimens. In the genus, Alepes the adipose eyelid covers three fourth of the eye on all length groups