Super Class : Pisces
Classification of Animal Chordates of Class 11
The Superclass Pisces (L. Piscis = fish) are the really jawed vertebrates. They have organs of respiration and locomotion which is related to a permanently aquatic life. The respiratory organs are the gills and the organs of locomotion are paired and impaired fins. All are poikilothermous.
It is divided into three classes :
Class I : Placodermi
Extinct fishes with primitive jaw
The name placodermi means “Armoured fish” or “Plate-skinned”.
Earliest jawed vertebrates.
Appeared in silurian, flourished in devonian and carboniferous and became extinct in permian.
Both paired and unpaired fins were present. Caudal fin heterocercal.
Autostylic jaw suspension; gill-slits are covered by operculum.
Examples : Climatius - Spiny Shark; Coccosteus; Dunklosteus.
Class II : Chondrichthyes - cartilaginous fishes.
- Body may be laterally compressed or dorsoventrally flattened. It consists of head, trunk and tail.
- Fins are supported by horny fin-rays.
- The pelvic fins bear claspers in the male which are posterior in location.
- There are generally two dorsal fins. The caudal fin is asymmetrical (heterocercal).
- Skin has unicellular epidermal mucous glands and dermal scales (placoid scales).
- Endoskeleton is entirely cartilaginous. Notochord is persistent but reduced.
- Mouth is ventral. Jaws well developed. The alimentary canal opens into the cloaca. Intestine has a scroll valve.
- Respiratory system includes 5-7 pairs of lamelliform (plate-like) gills. Gill-slits are without gill covers (opercula) except Chimaera.
- Swim bladder, which regulates buoyancy, is absent.
- Heart is two-chambered, having one auricle and one ventricle. Sinus venosus and conus arteriosus are present. Renal portal system is well developed. Red blood corpuscles are oval, biconvex and nucleated. They are poikilothermal animals.
- There are 10 pairs of cranial nerves.
- Olfactory sacs do not open into the mouth cavity by internal nares. Ear consists only of membranous labyrinth, bearing three semicircular ducts. Lateral line sense organs are well developed, which detect waves and currents in water.
- Nitrogenous waste matter is urea.
- Sexes are separate. The reproductive ducts discharge into the cloaca. Male usually has claspers which are used for copulation. Fertilization is internal. Most forms are ovoviviparous or oviparous. Some are viviparous. Life history is simple.
- Most chondrichthyes are marine. All are predaceous. The largest shark Rhinodon measures about 21 metres.
Scoliodon – Dog fish
Sphyrna – Hammer-headed shark
Pristis – Jaw fish
Torpedo – Electric ray
Trygon – Sting ray
Chimaera – Rat or Rabbit Fish or king of herrings.
Fig. Female Indian dogfish shark (Scoliodon sorrakowah) in lateral view
Bottom dwelling, carnivorous fish. It stuns or kills the prey and enemies with electric shock from its electric organs, which are modified muscles. The skin is scale less. Large openings called spiracles are present behind the eyes and are used for taking in water for breathing purpose.
Fig. Hydrolagus = Chimaera Fig. Electric ray Torpedo
Chimaera/Rabbit fish/Rat fish or king of herrings
Represents both the characters of a shark and a bony fish. Like shark it has cartilaginous skeleton and a pair of pelvic claspers. Placoid scales are present in the young chimaera. Like bony fish, it possess operculum on each side. It has distinct anus and urinogenital aperture. Cloaca is absent.
Class III : Osteichthyes - Bony fishes
Body is often spindle-shaped and stream lined. It facilitates movement through water.
Fins are supported by cartilaginous or bony fin-rays. Pectoral and pelvic fins act as balancers and brakes during swimming. The fins lack claspers in both the sexes. There is generally a single dorsal fin. Caudal fin is homocercal. The tail helps to propel the fish by its lateral movements.
Skin has unicellular mucous glands and dermal scales.
Endoskeleton is partly or wholly bony, replaced by distinct vertebrae.
Mouth is terminal or subterminal. Alimentary canal opens out by anus. Intestine generally lacks a scroll valve.
Respiratory system includes 4 pairs of gills. Gill-slits covered by gill covers (opercula).
Gas filled swim bladder is often present which acts as a buoyancy regulator. In some bony fishes, the swim bladder is used as a lung for breathing air.
Heart is 2-chambered, having one auricle and one ventricle. Sinus venosus and Conus arteriosus are present. They are poikilothermal animals.
There are 10 pairs of cranial nerves.
Olfactory sacs are dorsal. They communicate with the mouth cavity in lung fish only. Ear consists only of membranous labyrinth having three semicircular ducts. Lateral line sense organs are well developed.
Nitrogenous waste matter is mostly ammonia.
Sexes are generally indistinguishable externally. Fertilization is generally external. Most forms are oviparous. Some are ovoviviparous or even viviparous. Some fishes show parental care of eggs.
The bony fishes occur in fresh, brackish, salt, warm and cold water. Many deep sea fishes are luminescent. Some fishes can change colours, and some can leave water and crawl on land. Most fish used as food are bony fishes. The common food fishes of India are:
(a) Freshwater Species (b) Marine Species
(i) Labeo rohita (Rohu) (i) Harpodon (Bombay Duck)
(ii) Labeo calbasu (Calbasu) (ii) Anguilla (Eel)
(iii) Catla catla (Catla) (iii) Sardinella (Salmon)
(iv) Cyprinus carpio (Carp) (iv) Hilsa (Hilsa)
Some air-breathing fishes, use their paired fins to move about on land, gave rise to the first land vertebrates. A living fossil of this group is Latimeria, a lobe-finned fish.
Examples : Anguilla - Eel fish, Labeo - Rohu, Hippocampus - sea horse, Anabas - climbing perch, Gambusia - mosquito fish. Neoceratodus - Australian lung fish, Protopterus - African lung fish, Lepidosiren - American lung fish, Solea - flat fish.
Labeo : Labeo rohita and Labeo calbasu are the common fresh water edible carps. Body is covered by large overlapping scales. Head is scaleless with subterminal mouth and a pair of short barbels. The fish is herbivorous but in the juvenile stages it is carnivorous.
Fig. Labeo rohita : External features in side view
Cat Fish : Indian Cat Fish (Clarius batrachus) lives in ponds. It is carnivorous. The head bears sensory threads called barbels. The body is smooth and without scales. Other fresh water cat fishes of India are Mystis seengala ( = singharee), Rita rita and Wttogonta attu (= Mullee).
Fig. Catfish or Chital : Notopterus chitala
Climbing Perch : Head bears a number of small spines in front of eyes and along the opercular margins. Dorsal and vental fins have stiff rays in the anterior region and soft rays in the posterior region. The fish can come out of water and creep on land by means of operculum and pectoral fins in search of small animals. Here it is often picked up by birds and taken to their nests (hence the belief of climbing). For remaining out of water, the fish gulps in air and stores it in accessory respiratory chambers.
Fig. Climbing perch : Anabas testudineus
Flying Fish : Flying fish is known so because it can leap into air by powerful tail and remain air borne for long distance due to gliding by means of large pectoral fins.
Fig. Flying fish Exocoetus
Remora : The anterior part of dorsal fin is modified into sucker for fixing to the under surface of shark. Remora feeds on the left-over of Shark’s prey. The relationship is that of commensalism.
Cosmoid : Absent in living fishes; consists of four distinct layers - outermost ganoine, followed by cosmine, spongy bone and innermost compact bone.
Placoid : Backwardly directed spine arising from a basal plate.
Ganoid or rhomboid scales are thick, diamond or rhomboid plates.
Cycloid : Thin, flexible, circular in outline, marked with concentric lines of growth which can be used for determining age.
Ctenoid : Bear numerous small comb-like spines.
Air bladder is associated with respiration, hydrostasis, sound production and audition.
Parental care :
Nest building : e.g., Gasterosteus, bow fin, Protopterus, Lepidosiren.
Shelter in mouth : e.g., Males of cat fish, Arius.
Brood pouch : e.g., Hippocampus
Mermaids purses : Oviparous shark lay fertilised eggs inside protective horny egg capsule.
Viviparity : e.g., Scoliodon.
POINTS TO REMEMBER
Hilsa is the only Indian fish that migrates from the seas to the river for breeding.
Largest fish : The whale shark, upto 18m long, weighs over 40 tonnes. Smallest fish - Pygmy goby, upto 11 mm long, weight 4-5 mg. Fastest fish - Sail fish, about 109 km/h.
Devonian Period : “Age of fishes”.
Most primitive fish (fossil): Climatius.
Most poisonous fish: Stone fish.
Anabas (climbing perch) : It has accessory respiratory organs for breathing atmospheric air which enable the fish to take long overland excursions in search of earthworms.
Catadromous fishes live in fresh water and go down to sea for breeding e.g., Anguilla.
Anadromous fishes live in sea water and move to the rivers for breeding e.g., Hilsa, Salmo (Salmon).
Pomfret is the most widely eaten fish in India.
They are the first cold-blooded vertebrates from evolutionary point of view which came to the land.
The skin is scaleless smooth or rough having glands which keep it moist.
Paired fins are absent. Unpaired fins may be present. Two pairs of limbs are used for locomotion.
Urinary bladder is absent in fishes.
Some elasmobranchs retain urine in blood to maintain hypertonicity.
Electric organs are modified muscles.
Ampulla of Lorenzini are thermoreceptors.
Bioluminescent fishes : Anamalops, Porichthyes etc.;
Sound producing fishes: Mola, Balistes.
Poisonous glands are found in - Sting ray, Eagle ray, Chimaera, Diodon, Tetrodon.
Fish byproducts are - Fish meals and fertilizers; fish flour, fish proteins, fish oil, shagreen, fish glue, isinglass, leather, artificial pearls etc.
Crossopterygians gave rise to amphibia.
Parental care is well developed in Hippocampus.
Isinglass is a gelatinous product obtained from the air bladders of certain fishes such as carps, salmons, cat fishes etc. It is used for making cements, jellies and for clarification of wines and beers. The finest quality of isinglass is obtained from Russia.
Gambusia fish was introduced into several tropical regions to control malaria.
There are about 43,000 species of animals with backbones and over half of these are fishes.
Bombay Duck is a bony fish.
Seabass (Diploprion) is hermaphrodite (bisexual fish).
Stenohaline Fishes : They have only a narrow range of salinity tolerance and hence remain restricted to either fresh or salt water.
Euryhaline Fishes : They migrate between fresh water, estuarine and salt waters and thus have a wide salinity tolerance.
Dipnoi - means double breathers as they respire through gills as well as lungs. Dipnoans are lung fishes e.g., Protopterus, Lepidosiren, Neoceratodus etc.
Dipnoans have incompletely divided three-chambered heart.
Weberian Ossicles : These are named after the name of their discoverer (Weber, 1820. In certain fishes (e.g., Labeo), a chain of four small bones (Claustrum, Scaphium, Intercalarium and tripus) connects the air bladder and internal ear on either side. They are considered helpful in the process of hearing.
Rohu does not have stomach.
Sternothermal fishes can tolerate a narrow range of variation in temperature e.g., Trouts.
Eurythermal fishes can tolerate a wide range of variation in temperature, e.g., Mahseer (Tor Tor).