In this article we will discuss about the classification of osteichthyes.
The class Osteichthyes includes a large assemblage of true bony fishes. There are well over 30,000 to 40,000 living species, both freshwater and marine. Some of the freshwater forms are the carp, perch, bass, trout, catfish, sucker, etc., while the marine fishes are the tarpon, meckerel, tuna, sailfish, barracula, flying fish, etc.
The classification of class Osteichthyes described here has been largely followed after A.S. Romer (1966) which has also been followed by most authors including Storer and Usinger. However, several new schemes of classification, such as Greenwood, et.al., (1966), Lander and Liem (1983), Nelson (1984) and Pough, et.al, (1989) are also known. Two subclasses are recognised- Sarcopteygii and Actionopterygii.
Only important orders of each subclass have been described here:
Subclass I. Sarcopterygii:
(Gr., sarcos = fleshy; pterygium = fin)
1. Paired fins leg-like or lobed, with a fleshy, bony central axis covered by scales.
2. Dorsal fins two, caudal fin heterocercal with an epichordal lobe.
3. Olfactory sacs usually connected to mouth cavity by internal nostrils or choanae.
4. Popularly called fleshy or lobe finned, or air breathing fish.
Sarcopterygii has been divided into two superorders or orders- Crossopterygii and Dipnoi.
Order 1. Crossopterygii:
(Gr., crossoi = a fringe; pteryx = fin)
1. Paired fins lobed covered with scales. Pectoral fins are supported by a jointed median axis bearing radials. Caudal fin three-lobed.
2. Scales covered by a layer of cosmine.
3. Premaxillae, maxillae and squamosal bones present.
4. Internal nares present or absent.
5. Spiracles present.
6. Air-bladder vestigial.
Latimeria, Coelacanthus (Extinct).
Order 2. Dipnoi:
(Gr., di = double; pnoe = breathing)
1. Cycloid scales covering the skin.
2. Single gill-slit on either side present and covered by operculum.
3. Paired fins lobed.
4. Tail fin symmetrical (diphycercal), with no trace of separate dorsal fins.
5. Internal nares present opening into mouth; spiracles absent.
6. Air-bladder single or paired, lung-like.
7. Vertebrae cartilaginous arches, notochord remaining as an unconstricted rod.
8. Premaxillae and maxillae absent.
9. Lower jaw (palato-quadrate) firmly fused to neurocranium (skull autostylic).
10. Stomach absent, intestine ciliated. No hepatic caeca. Spiral valve present.
11. Auricle partly divided into two, ventricle completely divided by a ridge.
12. Ventral aorta shortened into a spirally twisted muscular bulbus cordis provided with valves.
13. Pulmonary and systemic circulation separate.
14. Cleavage holoblastic.
Lung fishes. Only three living genera- Neoceratodus, Protopterus and Lepidosiren.
Subclass II. Actinopterygii:
(Gr., actis – ray; pteryx = fin)
1. Includes all ray-finned fishes.
2. Paired fins thin, broad, without fleshy basal lobes and supported by dermal fin-rays.
3. Radials of paired fins not arranged bi-radially.
4. One dorsal fin, may be divided.
5. Caudal fin without epichordal lobe.
6. Tail generally homocercal, in a few heterocereal or semi-heterocercal.
7. Scales either ganoid or reduced to thin horny structures or completely absent in some.
8. Gill-slits covered by operculum. Spiracles absent.
9. Olfactory sacs not connected to mouth cavity. Internal nares absent.
10. Squamosal bone absent.
Actinopterygii has been divided into three infraclasses or superorders- Chondrostei, Holostei and Teleostei.
Superorder A. Chondrostei:
1. Primitive ray-finned fish or cartilaginous ganoids.
2. Tail fin heterocercal.
3. Scales usually ganoid
4. Mouth opening large.
Order 1. Polypteriformes:
1. Typical rhomboid ganoid scales.
2. Dorsal fin of eight or more finlets.
3. Pectoral fins with a small prominent scale covered fleshy lobe.
4. Ossified skeleton.
5. Spiracles present.
6. Air-bladder bilobed opening into the intestine ventrally.
Order 2. Acipenseriformes:
1. Body covered with five rows of bony scutes.
2. Snout elongated, having barbles on the ventral surface.
3. Caudal fin heterocercal.
4. Skeleton largely cartilaginous. Endocranium cartilaginous.
5. Jaws without teeth.
Acipenser (Sturgeon), Polydon (Paddle-fish).
Superorder B. Holostei:
(Gr., holos = entire; osteon – bone)
1. Intermediate ray-finned fish, transitional between Chondrostei and Teleostei.
2. Ganoid or cycloid scales.
3. Tail fin heterocercal.
4. Mouth opening small.
Order 1. Amiiformes:
1. Thin, overlapping cycloid scales.
2. Casual fin abbreviate heterocercal.
3. Long dorsal fin.
4. Pectoral radials are attached to the scapulocoracoid cartilage.
5. Vertebral centra non-opisthocoelous.
6. Premaxillary not protractile, firmly articulated with the cranium.
7. Snout normal rounded.
8. Spiracles and clavicles absent.
9. Presence of a single swim-bladder.
Order 2. Semionotiformes or Lepidosteiformes:
1. Scales rhomboidal ganoid and in oblique rows.
2. Body elongated.
3. Nasal opening at the end of much elongated snout.
4. Caudal fin abbreviate heterocercal.
5. Vertebrae completely ossified opisthocoelous.
6. Air-bladder cellular.
Lepidosteus or Lepisosteus (Garpike).
Superorder C. Teleostei:
(Gr., teleos = complete; ostgon = bone)
1. Scales cycloid ctenoid or absent.
2. Mouth terminal, small.
3. Tail fin mostly heterocercal.
4. Endoskeleton more or less bony.
5. Single external gill-slit on each side of the head covered over by operculum.
6. Cloaca and claspers absent.
7. Air-bladder usually present.
8. Spiracle is lost.
9. Spiral valve in the intestine absent.
10. Conus arteriosus greatly reduced. There is an enlarged bulbus arteriosus.
11. Advanced or modem ray-firmed fishes.
Order 1. Clupeiformes:
1. Scales cycloid and well developed.
2. Head and operculum not scaled.
3. Single dorsal and a small ventral fin without spines. Ventral fin may be absent. Pelvic fins abdominal.
4. Caudal fin homocercal.
5. Air-bladder communicate with the pharynx.
6. Vertebral centra completely ossified.
7. No auditory vesicles.
8. Weberian apparatus absent.
Clupea (Herringer), Salmo (Atlantic salmon), Sardinops (Pacific sardine), Esox (Pike), Notopterus (Chital fish), Elops, Gadusia, Ilisha, etc.
Order 2. Scopeliformes:
1. Deep sea fishes having phosphorescent organs.
2. Dorsal and anal fins without spines. An adipose fin present.
3. Mouth wide and provided with numerous small teeth.
4. Air-bladder absent.
Harpodon (Mumbai duck).
Order 3. Cypriniformes or Ostariophysi:
1. Fins either without spines or dorsal, anal and pectoral have a spine each.
2. Ventral (pelvic) fins abdominal.
3. Air-bladder connected with the pharynx by a duct.
4. A peculiar Webarian apparatus, connecting the internal ear with the air-bladder, present.
Representatives of this order are grouped in two divisions- Cyprini, Siluri.
Division I. Cyprini:
1. Body covered with scales or naked. Never covered with bony plates.
2. Third and fourth vertebra not fused with each other.
Labeo (Rohu), Cirrhina, Botia, Nemachilus, Cyprinus (Carp) Puntius, Tor, Esomus, Oxygaster, etc.
Division II. Siluri:
1. Body naked, not covered by scales.
2. Maxillary bone reduced supporting the barbules.
3. Second, third, fourth and sometimes the fifth vertebrae are generally fused.
Arius, Heteropneustes or Sacobranchus (Singhi) Clarius (Magur), Wallago (Lachi), Mystus (Tengra), Ompok, Ailia, Silonia, Bagarius, etc.
Order 4. Anguiliformes:
1. Body elongated eel-like or snake-like.
2. Scales vestigial or absent.
3. Dorsal and anal fins long and confluent behind.
4. Pelvic fins, if present, abdominal.
5. Fins devoid of spines.
6. Air-bladder with duct.
Anguilla (Freshwater eel), Muraena (Moray).
Order 5. Beloniformes (Synentognathi):
1. Body elongated covered with cycloid scales.
2. Fins without spines.
3. Pectoral fins large and high on body.
4. Ventral (pelvic) fins abdominal.
5. Some of them are capable of jumping into the air and glide with the help of enlarged pectoral fins.
Belone or Xenentodon (Garfish), Hemirhamphus (Half beak), Exocoetus and Cypselurus (Flying fishes).
Order 6. Syngnathiformes (Solenichthyes):
1. Body, covered with protective layer of scales or bony rings.
2. Snout tubular with suctorial mouth.
3. Pectoral fins small, pelvics absent and a single dorsal fin present.
4. Fin-rays of dorsal, pectoral and pelvic fins not branched.
5. Tail prehensile in sea horse, not in pipe fish.
6. Air-bladder closed.
7. Males possess brood pouch for the development of the young.
Hippocampus (Sea horse), Fistularia (Flute fish), Syngnathus (Pipe fish).
Order 7. Ophiocephaliformes or Channiformes:
1. Body covered with cycloid scales.
2. Head depressed, covered with plate-like- scales.
3. Fins without spines.
4. Air-bladder very long and without duct.
5. Accessory respiratory organs present.
Ophiocephalus or Channa (Snake head fish).
Order 8. Symbranchiformes:
1. Body elongated eel-like or snake-like devoid of scales.
2. Dorsal, caudal and anal fins continuous. Pectoral fins absent.
3. Fins without spines
4. Gill-slits single or join to form a transverse ventral slit.
5. Air-bladder absent.
Amphipnous, Symbranchus (Eels).
Order 9. Mastacembeliformes:
1. Freshwater eel-like fishes.
2. Dorsal, caudal and anal fins confluent. Sometimes a small fin separate.
3. Some free spines present in front of dorsal fin. Anal fin with three spines.
4. Ventral (pelvic) fins absent, but pectoral fins present.
5. Nostrils on tubular tentacles at the end of snout.
6. Buccal cavity enlarged for air breathing.
Order 10. Perciformes or (Percomorphi):
1. Two dorsal fins, ventral (pelvic) fins thoracic with not more than 6 rays.
2. Fins usually with spines.
3. Weberian apparatus absent.
4. Air bladder without duct.
Anabas (Climbing perch), Perca (Yellow perch), Lates (Bhetki).
Order 11. Scorpaeniformes:
1. Enlarged heads and pectoral fins.
2. Projecting spines from gill-covering.
Pterois (Scorpion fish).
Order 12. Pleuronectiformes:
1. Bottom dwellers.
2. Body flat, lying on one side, adapted for bottom living.
3. Head asymmetrical.
4. Both eyes present on the upper or dorsal side and close to each other.
5. Dorsal and anal fins fringing the body and along the caudal encircle the body. Fins usually without spines.
6. Air-bladder absent in adults.
Synaptura, Pleuronectes, Solea (Flat fishes).
Order 13. Echeneiformes (Discocephali):
1. Body covered with cycloid scales.
2. First dorsal fin modified into a flat oval adhesive disc or sucker on head for attachment. It possesses 12-28 transverse ridges which are modified spines.
3. No spines in second dorsal and anal fins.
4. Air-bladder absent.
Echeneis or Remora (Sucker fish).
Order 14. Tetradontiform (Plectognathi):
1. Body usually globular.
2. Body scales modified into spines. Scutes or bony plates cover the body.
3. Strong jaws with a sharp beak.
4. Gill-slits small like a hole on either side of fish in front of pectorals.
5. Ventral fines thoracic or subthoracic.
6. Air-bladder present or absent.
7. Some inflate by swallowing water.
Diodon (Porcupine fish), Terodon (Globe fish), Ostracion (Coffer fish or Trunk fish).
Order 15. Lophiiformes (Pediculati):
1. First ray of spinous dorsal fin placed on the head is transformed into a fishing organ consisting of a rod (illicium) and a lure called esca.
2. Mouth large with long pointed teeth.
3. Body with minute scales or scaleless.
4. Pelvic fins present or absent. Pectorals well developed.
5. Air-bladder absent.
6. Luminescent organs present.
Lophius and Antennarius (Angular fishes).