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Animal Husbandry and Fisheries Assistant · Chapter 6

Fish Biology

What to remember

  • A fish is a gill-breathing, fin-bearing, cold-blooded (poikilothermic) vertebrate with a two-chambered heart and single circulation.
  • Fish are grouped into Chondrichthyes (cartilage skeleton, no operculum) and Osteichthyes (bony skeleton, operculum, usually a swim bladder).
  • Feeding zone and food habit decide which carps are stocked together: Catla (surface), Rohu (column), Mrigal (bottom).

1. Classification and external features

Fishes belong to the superclass Pisces. Two main living classes are important.

FeatureChondrichthyes (sharks, rays)Osteichthyes (bony fishes)
SkeletonCartilageBone
Gill cover (operculum)Absent; 5 to 7 open gill slitsPresent; one opening each side
ScalesPlacoidCycloid or ctenoid (some ganoid)
Swim bladderAbsentUsually present
FertilizationInternalMostly external
ExamplesShark, ray, skateRohu, Catla, Tilapia, Seabass

The body has three parts: head (snout to the end of the gill cover), trunk (to the anus) and tail (anus to the tail fin). The body is streamlined and covered with mucus, which reduces friction and protects against germs.

Fins help in balance, steering and swimming.

  • Paired fins: pectoral and pelvic (pelvic fins correspond to hind limbs).
  • Unpaired (median) fins: dorsal, anal and caudal (tail) fin.
  • The caudal fin gives the main thrust.

Caudal fin types: protocercal (primitive, symmetrical, tail tip straight), heterocercal (upper lobe larger; vertebral column turns up; sharks), homocercal (externally symmetrical, vertebral column ends at the base; most bony fishes such as carps), diphycercal (symmetrical, column runs straight to the tip).

Scales are made in the dermis.

  • Placoid: tooth-like, in sharks.
  • Cosmoid: extinct fishes and lungfish.
  • Ganoid: diamond-shaped with enamel-like ganoine, in Polypterus and gar.
  • Cycloid: smooth edge, in carps.
  • Ctenoid: comb-like edge, in perch-like fishes such as Tilapia.
  • Catfishes such as Clarias have no scales.

Lateral line: a row of pores along each side of the body. It senses water movement, pressure and vibration. Scales with pores along it are counted as "lateral line scales" in identification.

2. Respiration and swim bladder

Fish take oxygen from water dissolved in it. Water enters the mouth, flows over the gills and leaves through the gill opening.

  • A bony fish has four pairs of gill arches. Each arch carries gill filaments, and each filament carries many thin lamellae where gas exchange happens.
  • Gill rakers on the inner side of the arch strain food. Plankton feeders have many long rakers.
  • Blood flows in the lamellae in the direction opposite to the water. This counter-current flow extracts a high share of oxygen.
  • The operculum pumps water over the gills.

Air-breathing fishes can live in oxygen-poor water or survive out of water for a time.

FishAir-breathing organ
Clarias (magur)Arborescent (tree-like) organ
Anabas (climbing perch)Labyrinth organ
Channa (murrel)Suprabranchial chamber
Heteropneustes (singhi)Long air sacs
LungfishModified swim bladder (lung)

Swim bladder is a gas-filled sac above the gut. Its main use is buoyancy (floating at a chosen depth). It also helps in hearing and sound production in some fishes. In a physostomous fish (carps, catfish) a duct joins it to the gut and the fish can gulp air. In a physoclistous fish (perch-like fishes) the duct is lost and gas is secreted by the gas gland. Sharks have no swim bladder; they depend on oil in the liver and on swimming.

3. Digestion and food habits

  • Carps have no true stomach and no jaw teeth. They have pharyngeal teeth in the throat that grind food against a horny pad.
  • Predators such as Channa and Wallago have a distinct stomach and sharp teeth.
  • The intestine is longer in herbivores and plankton feeders and shorter in carnivores. This is a standard clue in questions.
  • The liver makes bile; the pancreas is often diffuse in carps (hepatopancreas).

Feeding zones in a carp pond

SpeciesZoneMain food
CatlaSurfaceZooplankton
Silver carpSurfacePhytoplankton
RohuColumn (middle)Plankton, decaying plants
Grass carpColumn and marginalAquatic weeds, tender plants
MrigalBottomDetritus, decaying matter
Common carpBottomOmnivore: worms, insects, plants

Using all zones together (composite culture) uses the pond's natural food fully.

4. Circulation, excretion and osmoregulation

  • The heart is two-chambered: one atrium and one ventricle. Before the atrium lies a thin sinus venosus; after the ventricle is the bulbus arteriosus (in bony fishes) or conus arteriosus (in sharks).
  • Heart holds only deoxygenated blood. Blood goes heart → gills → body → heart. This is single circulation.
  • Red cells of fish have a nucleus.
  • The kidney is mesonephric and lies along the backbone. Most fishes excrete ammonia (ammonotelic). Sharks keep urea in blood to balance salts.
  • Freshwater fish gain water by osmosis, so they drink little and pass much dilute urine and take salts in through the gills. Marine bony fish lose water, so they drink seawater and pass little urine and excrete extra salt through gill chloride cells.
  • Euryhaline fishes tolerate wide salinity (Milkfish, Mullet). Stenohaline fishes tolerate a narrow range.
  • Anadromous fish go from sea to freshwater to breed (Hilsa). Catadromous fish go from freshwater to sea to breed (eel).

5. Nervous system, senses and endocrine glands

  • Brain has forebrain (smell), midbrain (vision), cerebellum (balance and movement) and medulla.
  • Eyes have a spherical lens and no eyelids. Nostrils are for smell only and do not open into the mouth.
  • The inner ear controls balance and hearing. Carps, catfishes and their relatives have Weberian ossicles linking the swim bladder with the ear, which improves hearing.
  • Barbels (whiskers) on catfish and some carps carry taste and touch cells.
  • The pituitary gland controls growth and breeding. Gonadotropin from the pituitary stimulates ovulation, which is why pituitary extract is used in induced breeding.

6. Reproduction, growth and common calculations

Most fishes are oviparous. Eggs are shed and fertilized in water. Guppy, Molly and many sharks are viviparous or ovoviviparous.

  • Fecundity is the number of ripe eggs in a female before spawning. It rises with body size.
  • Gonadosomatic index (GSI) = (gonad weight ÷ body weight) × 100. It shows the stage of maturity.
  • Condition factor K = 100 × W ÷ L³ (W in g, L in cm). A higher K means a plumper fish.
  • Length-weight relationship: W = a × L^b. When b is about 3 the growth is isometric.
  • Age is read from rings on scales and from ear stones (otoliths). A dark ring is laid in slow growth (winter).

Worked examples

  • 1. A fish weighs 80 g and is 20 cm long. K = 100 × 80 ÷ 8000 = 1.0.
  • 2. Gonad weighs 15 g in a 150 g fish. GSI = 15 ÷ 150 × 100 = 10 %.
  • 3. A 0.5 g sub-sample of ovary holds 1,000 eggs; the whole ovary weighs 100 g. Fecundity = 1,000 × 100 ÷ 0.5 = 200,000 eggs.

7. Important species in Andhra Pradesh and their traits

  • Indian major carps: Catla (Gibelion catla), Rohu (Labeo rohita), Mrigal (Cirrhinus mrigala). They breed in monsoon rivers but not in still ponds, so hatcheries use induced breeding.
  • Exotic carps: Silver carp, Grass carp, Common carp.
  • Air-breathing and hardy fishes: Magur, Singhi, Murrel, Climbing perch.
  • Brackish and marine fishes of interest: Asian seabass (Bhetki), Mullets, Milkfish, Pearlspot.
  • Tilapia is a prolific mouth-brooder. The male digs a nest and the female carries the eggs and young in her mouth.
  • Deltaic waters of the Godavari and Krishna and the Kolleru lake are known fishing grounds; always check the latest official release for current notifications.

Exam traps

  • 1. Sharks have placoid scales and no operculum; carps have cycloid scales and an operculum.
  • 2. Fish heart is two-chambered; amphibian heart is three-chambered.
  • 3. Fish heart carries only impure blood; it is not a double circulation.
  • 4. Anabas has the labyrinth organ; Clarias has the arborescent organ; Channa has the suprabranchial chamber.
  • 5. Catla is a surface feeder; Mrigal is a bottom feeder; Rohu is a column feeder.
  • 6. Anadromous (sea to river) is not the same as catadromous (river to sea).
  • 7. Heterocercal tail has unequal lobes; homocercal looks equal outside but the backbone ends at its base.
  • 8. Carps are stomachless; Channa has a stomach.

One-liners

  • 1. Fish are poikilothermic (cold-blooded).
  • 2. Lateral line senses water vibration and pressure.
  • 3. Gas exchange takes place in gill lamellae.
  • 4. Counter-current flow gives efficient oxygen uptake.
  • 5. Four pairs of gills are normal in bony fishes.
  • 6. Swim bladder gives buoyancy.
  • 7. Pharyngeal teeth help carps grind food.
  • 8. Weberian ossicles join swim bladder and ear in carps.
  • 9. Pituitary gonadotropin stimulates ovulation.
  • 10. Otoliths and scale rings give the age of fish.
  • 11. GSI = gonad weight ÷ body weight × 100.
  • 12. Condition factor K = 100 W ÷ L³.

Practice questions

  1. Fish are described as poikilothermic because

    1. they have warm blood
    2. they breathe air
    3. their body temperature changes with the surroundings
    4. they lay eggs
    Answer

    C. their body temperature changes with the surroundings

    Poikilotherms do not regulate body temperature internally.

  2. Statements: 1. Anabas has a labyrinth organ. 2. Clarias has an arborescent organ. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both pairings are correct.

  3. Which fin gives the main forward thrust?

    1. Pectoral fin
    2. Caudal fin
    3. Anal fin
    4. Pelvic fin
    Answer

    B. Caudal fin

    The tail (caudal) fin produces the main thrust.

  4. The gas gland is a feature of

    1. shark liver
    2. physostomous swim bladder
    3. lungfish lung
    4. physoclistous swim bladder
    Answer

    D. physoclistous swim bladder

    In physoclists the duct is closed and gas is secreted by the gas gland.

  5. Which fish feeds mainly at the surface of a pond on zooplankton?

    1. Catla
    2. Grass carp
    3. Common carp
    4. Mrigal
    Answer

    A. Catla

    Catla is the surface feeder on zooplankton.

  6. Which type of scale is found in sharks?

    1. Ctenoid
    2. Cycloid
    3. Ganoid
    4. Placoid
    Answer

    D. Placoid

    Sharks have tooth-like placoid scales.

  7. The heart in fish receives blood from the body and pumps it to the

    1. lungs
    2. gills
    3. brain
    4. kidney
    Answer

    B. gills

    Single circulation: heart pumps to the gills then to the body.

  8. A fish that migrates from the sea to freshwater to breed is called

    1. Catadromous
    2. Anadromous
    3. Euryhaline
    4. Stenohaline
    Answer

    B. Anadromous

    Anadromous fish, such as Hilsa, enter rivers to breed.

  9. Statements: 1. Catla is a surface feeder. 2. Mrigal is a bottom feeder. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements describe feeding zones correctly.

  10. Which fish group has Weberian ossicles?

    1. Carps and catfishes
    2. Sharks
    3. Seahorses
    4. Eels
    Answer

    A. Carps and catfishes

    Weberian ossicles link the swim bladder to the inner ear in carps and catfishes.

  11. Which fish carries eggs in the mouth until they hatch?

    1. Rohu
    2. Catla
    3. Hilsa
    4. Tilapia
    Answer

    D. Tilapia

    Tilapia is a mouth-brooder.

  12. A fish weighs 80 g and is 20 cm long. Its condition factor K = 100W/L³ is

    1. 1.0
    2. 0.5
    3. 2.0
    4. 8.0
    Answer

    A. 1.0

    K = 100 × 80 ÷ 8000 = 1.0.

  13. Which of these is a viviparous fish?

    1. Rohu
    2. Guppy
    3. Catla
    4. Mrigal
    Answer

    B. Guppy

    Guppy gives birth to live young.

  14. Which of these fishes has no stomach?

    1. Wallago
    2. Shark
    3. Rohu
    4. Channa
    Answer

    C. Rohu

    Carps are stomachless fishes.

  15. A marine bony fish keeps water balance by

    1. absorbing water through scales
    2. avoiding all drinking
    3. passing large dilute urine
    4. drinking seawater and excreting salt through gills
    Answer

    D. drinking seawater and excreting salt through gills

    It loses water to the sea and must drink and remove excess salt.

  16. Match the fish with its air-breathing organ: Heteropneustes - ?

    1. Long air sacs
    2. Labyrinth organ
    3. Arborescent organ
    4. Suprabranchial chamber
    Answer

    A. Long air sacs

    Singhi has long air sacs along the body.

  17. Statements: 1. Herbivorous fishes have a longer intestine than carnivores. 2. Catadromous fish breed in rivers. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Catadromous fish breed in the sea.

  18. Which fish is a plankton feeder on phytoplankton and lives at the surface?

    1. Common carp
    2. Silver carp
    3. Mrigal
    4. Grass carp
    Answer

    B. Silver carp

    Silver carp feeds on phytoplankton at the surface.

  19. A freshwater bony fish handles water balance by

    1. passing no urine
    2. drinking much water
    3. excreting salt through gills
    4. passing large amounts of dilute urine
    Answer

    D. passing large amounts of dilute urine

    It gains water by osmosis, so it excretes dilute urine.

  20. A 0.5 g ovary sub-sample has 1,000 eggs. If the ovary weighs 100 g, fecundity is

    1. 2,000
    2. 50,000
    3. 200,000
    4. 100,000
    Answer

    C. 200,000

    1,000 × 100 ÷ 0.5 = 200,000.

  21. Statements: 1. Homocercal tail has equal lobes outside. 2. Diphycercal tail is found in most bony fishes like carps. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Carps have the homocercal tail.

  22. A fish with an unequal-lobed tail and the backbone turning into the upper lobe has a

    1. Protocercal tail
    2. Homocercal tail
    3. Diphycercal tail
    4. Heterocercal tail
    Answer

    D. Heterocercal tail

    Sharks have the heterocercal type.

  23. Which air-breathing organ is found in Anabas?

    1. Arborescent organ
    2. Air sacs
    3. Labyrinth organ
    4. Suprabranchial chamber
    Answer

    C. Labyrinth organ

    Anabas (climbing perch) has the labyrinth organ.

  24. Statements: 1. Fish heart pumps only deoxygenated blood. 2. Fish have double circulation. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Fish have a two-chambered heart with single circulation.

  25. The direction of blood in gill lamellae relative to water is

    1. Absent
    2. Opposite (counter-current)
    3. Same (parallel)
    4. Perpendicular only
    Answer

    B. Opposite (counter-current)

    Opposite flow keeps a gradient and gives high oxygen uptake.

  26. Which class of fishes has a cartilage skeleton?

    1. Chondrichthyes
    2. Osteichthyes
    3. Both equally
    4. Neither
    Answer

    A. Chondrichthyes

    Sharks and rays are cartilaginous.

  27. Gonadosomatic index (GSI) is calculated as

    1. body weight ÷ gonad weight × 100
    2. gonad weight ÷ body weight × 100
    3. body weight × gonad weight
    4. gonad length ÷ body length
    Answer

    B. gonad weight ÷ body weight × 100

    GSI shows the gonad share of body weight in percent.

  28. Which part of a fish carries gas-exchange lamellae?

    1. Gill rakers
    2. Swim bladder
    3. Operculum
    4. Gill filaments
    Answer

    D. Gill filaments

    Lamellae lie on the gill filaments.

  29. Statements: 1. Carps have a well-developed stomach. 2. Carps have pharyngeal teeth. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Carps are stomachless.

  30. Statements: 1. Freshwater fish drink large quantities of water. 2. Marine bony fish excrete salt through gills. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Freshwater fish drink little; marine fish excrete salt through gills.

  31. Which carp is a bottom feeder in composite culture?

    1. Rohu
    2. Catla
    3. Mrigal
    4. Silver carp
    Answer

    C. Mrigal

    Mrigal feeds on detritus at the bottom.

  32. Statements: 1. Sharks have an operculum. 2. Bony fishes have four pairs of gills. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Sharks have open gill slits and no operculum.

  33. Which fish has a suprabranchial chamber?

    1. Clarias
    2. Catla
    3. Channa
    4. Anabas
    Answer

    C. Channa

    Murrel (Channa) has this chamber above the gills.

  34. A fish of 10 cm length weighs 20 g. Its K = 100W/L³ is

    1. 0.2
    2. 0.02
    3. 20
    4. 2.0
    Answer

    D. 2.0

    K = 100 × 20 ÷ 1000 = 2.0.

  35. A fish weighs 15 g in gonad and 150 g in body. Its GSI is

    1. 1 %
    2. 100 %
    3. 10 %
    4. 15 %
    Answer

    C. 10 %

    GSI = 15 ÷ 150 × 100 = 10 %.

  36. Which fin type is unpaired?

    1. Pectoral fin
    2. Pelvic fin
    3. Dorsal fin
    4. Both pectoral and pelvic
    Answer

    C. Dorsal fin

    Dorsal, anal and caudal fins are median unpaired fins.

  37. The sense organ that detects water movement along the side of a fish is the

    1. Barbel
    2. Gill raker
    3. Operculum
    4. Lateral line
    Answer

    D. Lateral line

    The lateral line senses vibration and pressure.

  38. Which organ do fishes use for breathing under water?

    1. Skin folds only
    2. Nostrils
    3. Lungs
    4. Gills
    Answer

    D. Gills

    Gills with lamellae take dissolved oxygen from water.

  39. Which fish is a suitable example of a euryhaline fish?

    1. Milkfish
    2. Catla
    3. Rohu
    4. Mrigal
    Answer

    A. Milkfish

    Milkfish tolerates both fresh and saline water.

  40. How many chambers does the fish heart have?

    1. Two
    2. Five
    3. Four
    4. Three
    Answer

    A. Two

    A fish heart has one atrium and one ventricle.

  41. Statements: 1. Fish red cells have a nucleus. 2. The lateral line detects vibration. Which is correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are true.

  42. In which fish is the arborescent organ the air-breathing organ?

    1. Anabas
    2. Labyrinth fish only
    3. Channa
    4. Clarias
    Answer

    D. Clarias

    Clarias breathes air with a tree-like arborescent organ.

  43. Which structure gives a bony fish its buoyancy?

    1. Lateral line
    2. Swim bladder
    3. Barbel
    4. Gill raker
    Answer

    B. Swim bladder

    The gas-filled swim bladder adjusts buoyancy.

  44. Age of a fish can be estimated from

    1. Scale rings and otoliths
    2. Fin colour
    3. Barbel length
    4. Number of fins
    Answer

    A. Scale rings and otoliths

    Rings in scales and ear stones show growth periods.

  45. Carps grind food with

    1. Canine teeth
    2. Pharyngeal teeth
    3. Jaw teeth
    4. Tongue plates
    Answer

    B. Pharyngeal teeth

    Carps have no stomach or jaw teeth; they use pharyngeal teeth.

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