Aquatic animals and their adaptational characteristics


  • Any change in an organism that makes it slowly or rapidly better suited to the environment is called adaptation.
  • It can also be defined as the process of adjustment of an organism to its environment.
  • It increases the possibility of survival of an organism for a longer time as it helps the organism to get its food and be protected from enemies.
  • Organisms show adaptation in various ways. Some of them are:
  1. By losing and gaining the organs.
  2. By changing the colour of the body according to the habitat.
  3. By the modification of their organs.
  • Aquatic adaptation is the modifications that enable the animals to live in water.
  • Aquatic animals are mainly classified into two types. They are:

1. Primarily aquatic animals:

Those animals that never lived on land and breathe with gills. E.g., fish.

2. Secondarily aquatic animals:

  • Those animals which have later been compelled to lead aquatic life fall under this group.
  • E.g., turtles, crocodiles, polar bears, whales, dolphins, etc.
  • They are lung breathers but compelled later on in their life to lead aquatic life.

A) Primary aquatic adaptations

  • The animals show the following types of morphological, anatomical, physiological, and behavioral changes.

a) Body contour

  • The head, body, and tail are compressed so as to form a streamlined body.
  • It offers the least resistance during locomotion.

b) Locomotory organs

  • The limbs are modified into fins or paddles.
  • The aquatic animals move with the help of fins and also by lateral undulations of the body.
  • The fins of fish are of two types: the median fins (dorsal, ventral, and caudal fins) and paired fins (pectoral and pelvic fins).
  • The dorsal and ventral fins help in balancing, the caudal fin is the main propulsive organ and also serves as a rudder in changing the direction of movement.
  • The pectoral and pelvic fins help in propulsion and in changing direction.

c) Swim bladder

  • A swim bladder is a hollow organ filled with air or gas.
  • It is largely hydrostatic in function.
  • By controlling the gaseous content of the swim bladder, fishes either rise to the surface or move to the depths.
  • It also serves as an accessory respiratory organ.


Image result for swim bladder in fish


d) Respiration by gills

  • Gills and in some general body surface works as respiratory organs.
  • They utilize dissolved oxygen in the water.

e) Lateral line systems

  • All fish have a lateral line system extending all over their body.
  • It contains a neuromast organ that can detect pressure changes in the surrounding water.
  • The receptors are stimulated by the slow vibration of the body in relation to waves and current.

f) Skin and scales

  • The skin of fish is rich in mucus glands and in many cases the skin is protected by scales.
  • These protect the body from external injury and water current.

g) Presence of nictitating membrane

  • An eye-lid or nictitating membrane is present in the eye, which protects from dirty water and ensures vision in water.

h) Osmoregulation

  • The control of water content and the concentration of salts in the body of freshwater animals is known as osmoregulation.
  • This phenomenon is shown by aquatic animals.


Image result for primary aquatic adaptations


B) Secondary aquatic adaptations

a) Body contour

  • Like primary aquatic forms, the body contour has become streamlined. e.g., Cetacea, Sirenia, etc.
  • In these cases, all traces of external projections such as limbs, external ears, etc. disappear except for a pair of flippers which helps in balancing and steering.

b) Skeleton

  • Bones are light and spongy.
  • The interspaces are filled up with oil in the case of whales.

c) Locomotion

  • The elongated body and reduced limbs are present.
  • Whales develop great tail flukes with extremely powerful musculature.
  • Tail propulsion is found in whales, ichthyosaurs, and sirenians.
  • In these animals the hind limbs are absent and the tail is responsible for locomotion.
  • In marine turtles and sea-cows forelimbs are modified into paddles.
  • In aquatic amphibians, turtles, and crocodiles the limbs have webbed feet.

d) Shortening of neck

  • Bear short neck.
  • It helps the animals in balance and to swim in the water easily.
  • In most cases, the neck is entirely absent and the head is fused directly with the trunk.
  • In whales, cervical vertebrae fused to form a solid and compressed mass of bone.


  • The loss of armour, hair, and skin glands are its characters.
  • A layer of fat or subcutaneous tissue develops beneath the skin.
  • This serves for the retention of bodily heat.
  • A thick layer of fat (blubber) is present beneath the smooth skin of whales.

f) Respiration

  • All are lung breathers.
  • Lungs are large and highly elastic, thereby storing much air which is used during prolonged periods of diving.

g) Skull modification

  • The cranium is shortened in certain aquatic animals and the front part is produced into a pointed snout or rostrum.
  • The pointed snout cuts the water efficiently and makes it easy to swim.

h) Submergence

  • The capacity of submergence for swimming below the water surface is present in all these groups of animals.
  • For example, in whales the ribs are strongly arched, the lungs are massive, and the external nostrils communicate with the median “blow hole” which can be closed.

i) Lateral line system

  • Line receptors are found in amphibians while whales can echo-locate objects.


Image result for skull modification in secondary aquatic animals             Image result for lateralline system





Aquatic animals and their adaptational characteristics