Introduction: Adaptation to environment is one of the basic characteristics of the living organisms. Living organisms are plastic and posses the inherent properties to respond to a particular environment. It is a facet of evolution and involve structural diversities amongst living organisms that are heritable. Organisms exhibit numerous structural and functional adaptations that help them to survive as species and to overcome the tremendous competition in nature. All classes of vertebrates have their representatives leading to partial or total aquatic life. Water is a homogenous medium for animals. As a medium, it is heavy in concentration than air. Stable gaseous and osmotic concentration in a specific region. Temperature fluctuation is minimum for a particular region. Water bodies generally have very rich food resources. Characters of an Aquatic Animal: An aquatic animal should have the ability to swim to overcome the resistance of the surrounding medium. Therefore, it should have a streamlined body with an organ or ability to float. The animal should also have to over­come the problem of osmoregulation. There are two types of animals living in the present day water, which have undergone aquatic adaptation. According to their origin, they are primary and secondary aquatic animals. Adaptations to water habitat are of two types: Primary aquatic adaptations which includes primitive gill-breathing vertebrates (fishes); Those ani­mals, whose ancestors and themselves are living in the water from the very beginning of their evolution, are called primary aquatic animals. In other words, primary aquatic animals never had a terrestrial ancestry. They exhibit perfect aquatic adaptations. All fishes are primary aquatic animals. Secondary aquatic adaptations which are acquired as in reptiles, birds and mammals. Those ani­mals whose ancestors were lung breathing land animals, migrated to the water for some reason and ultimately got adapted to live in aquatic habitat, are called secondary aquatic animals. Some of them live partially while others live totally in the water. All aquatic reptiles, aves and mammals are represen­tatives of secondary aquatic animals. Amphibians are in a transitional form between primary and secondary aquatic life. Sensory adaptations like, electroreception for electrolocation and electro communication, olfaction (vomeronasal system), balance (spatial orientation, movement perception), vision (cornea curvature, retinal topography), and hearing (acoustics, ear anatomy) under the underwater sound reception mechanisms in various aquatic amniotes are well developed.

1. Aquatic Adaptations in Birds and Mammals
Dr. P.B.Reddy
M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM
PG DEPARTMENT OF ZOOLOGY
GOVERTNAMENT PG COLLEGE, RATLAM.M.P
reddysirr@gmail.com

3. Introduction:
Adaptation to environment is one of the basic characteristics of the living organisms.
Living organisms are plastic and posses the inherent properties to respond to a
particular environment.
It is a facet of evolution and involve structural diversities amongst living organisms that
are heritable. Organisms exhibit numerous structural and functional adaptations that
help them to survive as species and to overcome the tremendous competition in
nature.
 All classes of vertebrates have their representatives leading to partial or total
aquatic life.
 Water is a homogenous medium for animals.
As a medium, it is heavy in concentration than air.
 Stable gaseous and osmotic concentration in a specific region.
 Temperature fluctuation is minimum for a particular region.
 Water bodies generally have very rich food resources.
Characters of an Aquatic Animal:
 An aquatic animal should have the ability to swim to overcome the resistance of the
surrounding medium.
 Therefore, it should have a streamlined body with an organ or ability to float.
 The animal should also have to overcome the problem of osmoregulation.
 There are two types of animals living in the present day water, which have
undergone aquatic adaptation.

4. Adaptations to water habitat are of two types:
1. Primary aquatic adaptations which includes primitive gill-breathing
vertebrates (fishes); Those animals, whose ancestors and themselves are living
in the water from the very beginning of their evolution, are called primary
aquatic animals. In other words, primary aquatic animals never had a
terrestrial ancestry. They exhibit perfect aquatic adaptations. All fishes are
primary aquatic animals.
2. Secondary aquatic adaptations which are acquired as in reptiles, birds and
mammals. Those animals whose ancestors were lung breathing land animals,
migrated to the water for some reason and ultimately got adapted to live in
aquatic habitat, are called secondary aquatic animals. Some of them live
partially while others live totally in the water. All aquatic reptiles, aves and
mammals are representatives of secondary aquatic animals. Amphibians are
in a transitional form between primary and secondary aquatic life.
3. Sensory adaptations like, electroreception for electrolocation and electro
communication, olfaction (vomeronasal system), balance (spatial
orientation, movement perception), vision (cornea curvature, retinal
topography), and hearing (acoustics, ear anatomy) under the underwater
sound reception mechanisms in various aquatic amniotes are well
developed.

5. Adaptive Features of Primary Aquatic Animals:
A. External Modifications:
i. Body Contour: Streamlined body is the primary requisite for aquatic life.
ii. Fins: Caudal fin plays vital role in forward propulsion during swimming and also acts as a
rudder for navigation. Dorsal and anal fins help in stabilizing the body by preventing it from
yawing (turning around the vertical axis) and rolling (turning around the longitudinal axis),
during swimming. Pelvic and pectoral fins help to steer the body during locomotion.
B. Internal Modifications:
i. Modifications of Muscles for Locomotion:
ii. Modification of Bones for Muscular Attachment and Movement: Amphicoelous vertebrae
make the vertebral column in fishes rigid in the dorsoventral plane but flexible laterally and
help in the lateral undulation of the body.
iii. Modification for Respiration: Gills are the primary respiratory organ in fishes. These are
situated in the branchial chambers and guarded by operculum. Gill lamellae are richly supplied
with blood vessels and efficiently perform gas exchange from the water.
iv. Modification for Floating: Swim bladder, a hollow and large sac filled with gas, is present in
the abdominal cavity of most bony fishes.
v. Modifications for Tackling the Problem of Osmoregulation and other Aquatic Hazards: The
integument of most primary aquatic animals is rich in mucous gland and protected by scales.
Scale and mucous prevents the entry of external water harmful external parasites like bacteria,
fungus, parasitic protozoa, etc. from the fish body.
vi. Modification of Sense Organs: Sense organs have developed in fish body in accordance to
life in water: (e) Lateral Line Sense Organ:

6. Adaptive Features of Secondary Aquatic Animals:
A. Body Contour:
 Secondary aquatic animals have a more or less stream
lined body contour.
 Neck constriction disappears.
 Tail enlarges to take a shape like that of fish tail, e.g.,
aquatic Cetaceans, Sirenia and Pinnipedia.
 pinna or hair disappear from the body.
 Chest becomes cylindrical and modified to bring the
internal cavity higher up towards the back. This
ensures greater stability in floating and also increases
lung capacity.
B. Limbs:
 webbed feet are developed.
 Limbs are modified into paddles
 In the paddle, the entire limb skeleton is enclosed by
skin.
 Various bone joints of the limbs lose their mobility and
the entire structure acts as a single unit.
 In whales, forelimbs are modified into fish fin-like
structure, called flippers.
 Hind limbs of opossum and hippo are swimming organs,
while that in platypus acts as balancers.
In whales, dolphins and sirenians hind limbs are absent.

7. ii. Fins:
 Other than flippers some caudal or dorsal fin-like structures are present in whales.
 These structures are not supported by skeleton (fin rays) but strengthened by masses
of dense connective tissues.
 Dorsal fins may be small in size or sometimes tall and usually triangular in shape.
 Unlike fishes, caudal fins of aquatic mammals are horizontally flattened.
Caudal fin of whale is bilobed and known as fluke.
iii. Types of Locomotion:
In sea turtle, oar propulsion is present. In this type of propulsion, nearly equivalent
fore and hind limbs exert propulsive force.
 On the other hand in sirenians and cetaceans, the forward propulsive thrust comes
from the flattened tail or fluke. This type of propulsion is known as tail propulsion and
in such cases flippers and dorsal fins, if present, provide stability.
C. Modification of Endoskeletons:
i. Skull:
Cranium becomes shorter and wider. The skull at the front, tends to elongate and is
produced into snout or rostrum. Zygomatic and temporal arches become reduced to
vestiges.
ii. Neck:
In quick moving forms, loose neck hinders mobility. So neck is remarkably shortened. In
whales, the cervical vertebrae are fused to form a solid and compressed mass of bone.

8. iv. Girdles:
In pectoral girdle, scapula is well developed for muscular attachment. Pelvic girdle is either
reduced or completely lost.
v. Limb Bones:
Humerus and femur are comparatively shorter in length. Formation, of flippers or paddles
require broadening of digits. It is achieved by two ways. Firstly, number of phalangeal bones
increases (Hyperphalangy), e.g., pilot dolphin (Gobicephala).
Secondly, by the development of one or more additional rows of phalanges; i.e., extra digits over
normal five (Hyperdactyly), e.g., fossil aquatic reptile Ichthyosaurus platydactylus.
D. Modification of other Internal Organs:
i. Digestive System: Teeth may be sharp, simple and cone shaped (e.g., Dolphin) or may be
absent in one jaw (e.g., in upper jaw of sperm whale) or in both jaws (e.g., baleen whale).
Teeth, where present are numerous, e.g., in pilot whale it is over 100, in dolphin it is 200, etc. In
baleen whale, baleen plate develops as horny outgrowth from the epithelial lining of the palate
of mouth.
ii. Respiratory System: External nostrils are shifted towards the upper side, at the tip of the
head, e.g., turtles, crocodiles, beaver, dolphins, whales, etc. This adaptive feature allows the
animal to respire by exposing a little part of the body out of water.
External nostrils are absent in cormorants and pelicans. In whales, a sphincter muscle guards the
external nostril. The nostril remains closed while the animal roams under water. Enlarged chest
cavity houses the large lungs. Gaseous exchange in fully aquatic mammals takes place very
quickly in comparison to land mammals.

9. iii. Circulatory System:
 In cetaceans and sirenians the blood volume is almost double to that of their land
relatives. High haemoglobin content helps in carrying much more oxygen.
 The rate of heart beat decreases much in cetaceans, while submerged.
 Blood pressure is kept normal by contracting arterioles except in the brain and heart.
 A counter current blood circulation is present in the flipper of whale for
thermoregulation.
iv. Reproductive System and Reproduction:
 Testes are not disposed in the scrotum but situated in a pouch near inguinal region of
marine mammals.
 Aquatic mammals usually give birth to one precocious offspring at a time.
 Male marine turtles usually do not visit land in their lifetime, only female turtles come to
land for egg laying. Almost all marine snakes are viviparous.
v. Integument and its Glands:
 A thick subcutaneous layer of fat is present in whales, seals and penguins, known as
blubber. This layer is primarily concerned with thermoregulation. It also reduces specific
gravity of body providing buoyancy. Sweat and sebaceous glands are absent in aquatic
mammals.
 Mammary gland has a tendency to shift from its usual position, i.e., the lower abdomen.
In cetaceans a pair of mammae are situated in inguinal region. In sirenians paired mammae
are present posterior to paddle, while in coypu two pairs of mammary glands are situated
on the back. Milk in whales contain less water but rich in fat. Milk is stored in milk sinuses
and ejected out when necessary.

10. vi. Sense Organs:
External ears have a tendency towards elimination. Most of the whales are capable of
echo ranging and communicate between themselves with ultrasonic frequency up to
a great distance (about 160 km). Olfactory lobe of the brain is reduced, because
olfactory receptors are very less in number. Eyes are adapted for under water vision
and are piscine in nature.
Examples of secondary aquatic vertebrates:
Class Aves:
Grebe (Prodiceps), duck (Anser), swan (Cygnus), petrel (Fulmarus), albatross
(Diomedea), cormorant (Phalacro- corax), pelican (Pelicanus), gannet (Sula), jacana
(Hydrophasianus), gull (Larus), tern (Sterna) are amphibious and penguin
(Spheniscus), great auk (Hesperornis) are aquatic.
Class Mammalia:
Platypus (Ornitho- rhynchus), water opossum (Chironectes), water shrew (Neomys),
water rat (Hydromys), beaver (Castor), coypu (Myocaster), common otter (Lutra),
hippopotamus are amphibious and walrus (Odobenus), seal (Phoca, Pusa), sea lion
(Eumetopias), whales (Balenoptera, Megaptera), pilot whale (Gobicephala), dolphin
(Delphinus), Gangetic dolphin (Platanista), porpoise (Phocaena), sea cow (Dugong),
manatee (Manatus), etc. are aquatic.

11. Aquatic adaptations in Birds
Aquatic birds spend most of their time in water, either floating on the surface,
diving to catch fish, swimming gracefully, or simply wading through the water.
These birds have unique adaptations to help them survive in a variety of climates.
These adaptations include webbed feet, bills, and legs adapted to feed in the water,
and the ability to dive from the surface or the air to catch prey in water.
Adaptive Features of Secondary Aquatic Animals:
A. Body Contour:
 Secondary aquatic animals have a more or less stream lined body contour.
 Neck constriction disappears.
 Tail enlarges to take a shape like that of fish tail.
 Chest becomes cylindrical and modified to bring the internal cavity higher up
towards the back. This ensures greater stability in floating and also increases
lung capacity.
B. Limbs:
 webbed feet are developed.
 Limbs are modified into paddles
 In the paddle, the entire limb skeleton is enclosed by skin.
 Various bone joints of the limbs lose their mobility and the entire structure acts
as a single unit.

12. The nictitating membrane: It is a transparent or translucent third eyelid
present in birds. it protects their eyes from water .
Salt secreting nasal glands: Marine birds possess salt-secreting nasal
glands which produce hypertonic solutions of sodium chloride in
response to osmotic loads such as ingestion of sea water. ... The
presence of this gland must be considered a necessary adaptation to
marine life in animals whose kidney cannot excrete high salt
concentrations.
Oil glands: The preen gland, or uropygial gland, is an oil-producing
gland located near the base of the tail. It produces a diverse range of
biochemicals which are involved in chemical protection, water-proofing
and maintenance of plumage brightness.
Skeletal pneumaticity: The hollow part of a bird bone contain
extensions of the air sacs from the lungs. These air sacs help the bird to
get the oxygen it needs to fly quickly and easily.
Feathers: One of the major functions of feathers is to prevent water
from reaching the skin or weighing down the remiges and tail feathers
in flight.

13. Webbed feet: The webs push more water than just a bird foot with
spread-out toes would push. (It would be like trying to swim with
your fingers spread apart.) The webbed feet propel the bird through
the water. ... Webbed feet are useful on land as well as on water
because they allow birds to walk more easily on mud.
Beak or Bill: Large, long, and strong beaks. The shape of a bird's
beak is designed for eating particular types of food. The pouch on a
pelican's beak helps it take huge gulps of water to store the fish in it.
Herons and Cranes have long, strong beaks to catch fish.
Long legs: Long legs help keep their feathers high and dry when
wading into water in search of food. The benefits of wading birds'
long, thin, spread-out toes are threefold: Thin toes are easier to pick
up and put down when walking in water and squishy mud.

14. Aquatic Mammals:
There are several aquatic mammals. Aquatic mammals belong to several orders of Mammalia.
Mammals are primarily terrestrial animals. However, some of them have adopted an aquatic
mode of life. The aquatic mammals have evolved from terrestrial mammals. The fact that all of
them are not gill-breathers but breathe air through lungs, indicate their original terrestrial
mode of life.
All the aquatic mammals are really terrestrial lung-breathing forms which have reverted to an
aquatic life, and they have done so with remarkable success, the whales being the most
successful. They have reverted to water probably because of extreme competition on land for
food and shelter.
Depending on the degree for aquatic adaptation the aquatic mammals have been divided
into the following categories:
1. Amphibious Mammals:
These mammals do not live permanently in water. They live on land but go into water for food
and shelter.
They show only partial aquatic adaptations such as:
(i) Small external ears,
(ii) Webbed feet,
(iii) Flattened nails,
(iv) Subcutaneous fat.
The mammals of this category include the beaver (Castor), musk rat (Ondatra), nutria
(Myocaster), otter (Lutra), mink (Mustela) and many others. The amphibious mammals belong
to several orders of mammalia such as Carnivora, Rodentia, Artiodactyla, Marsupialia,
Monotremata, etc.

15. 2. Aquatic Mammals:
The mammals under this category spend most of the time in water and usually
come to land for reproduction. The typical examples are seals and
hippopotamus.
3. Marine Mammals:
These mammals never come to land and are perfectly at home in water. The
typical examples are whales.
Aquatic Adaptations:
The adaptations or specializations of truly aquatic mammals (Cetacea and
Sirenia) are divided into 3 major categories:
(i) Modifications of original structures,
(ii) Loss of structures, and
(iii) Development of new structures
A. Modifications of Original Structures:
1. Body Shape: Body shape is of prime importance. It is streamlined with
elongated head, indistinct neck and tapering which offers little resistance for
water.
2. Large Size and Weight: In aquatic mammals, the large size and body weight
help the aquatic mammals. Large size reduces skin friction and loss of heat, but
creates no problem for support in water due to buoyancy.

16. 3. Flippers: The forelimbs are transformed into skin-covered, un-jointed
paddles or flippers, having no separate indication of fingers. These paddles or
flippers can move as a whole only at the shoulder joint. The broad and
flattened paddles or flippers serve as balancers and provide stability during
swimming.
4. Hyperdactyly and Hyperphalangy: Presence of extra digits (hyperdactyly)
and extra phalanges (hyperphalangy) up to 14 or more in some forms, serve
to increase the surface area of flippers for greater utility for swimming in
water.
5. High and Valvular Nostrils: The nostrils are placed far back on the top of
head so that animal can breathe air without raising head much out of water.
The nostrils can also be closed by valves during diving under water.
6. Mammary Ducts: During lactation, ducts of mammary glands dilate to
form large reservoirs of milk, which is pumped directly into mouth of young by
the action of special compressor muscle. This arrangement facilitates suckling
of young under water.
7. Oblique Diaphragm: In aquatic mammals, oblique diaphragm makes the
thoracic cavity larger dorsal and barrel-shaped for providing more space to
lungs for expansion.

17. 8. Large Lungs: The large unlobulated and highly elastic lungs ensure taking
down maximum air Lore submergence. Like swim bladders of fishes, the dorsal
lungs also serve as hydrostatic organs in maintaining a horizontal posture
during swimming.
9. Intra-Narial Epiglottis: In aquatic mammals, elongated, tubular and intra-
narial epiglottis, when embraced by the soft palate, provides a continuous and
separate air-passage, thus, allowing breathing and feeding simultaneously.
10. Endoskeleton: In aquatic mammals, the cranium becomes small but wider
to accommodate the short and wide brain. The facial part of skull projects
forming elongated snout or rostrum. The zygomatic arches are reduced. Due to
reduced neck, the cervical vertebrae are fused into a solid bony mass.
Zygapophyses are reduced. Sacrum is also reduced. Ribs become arched
dorsally to increase thoracic cavity. Bones are light and spongy. In Cetacea,
bones are filled with oil.
11. Teeth: In toothed whales, teeth are monophyodont, homodont and
numerous, as many as 250. This helps in capturing or seizing prey, prevent its
escape and swallowing it without mastication. Usually, the mobility of jaws is
reduced as they have no function in mastication.

18. B. Loss of Structures:
1. There is a loss of hairs.
2. Skin surface usually remains smooth and glistening due to loss of hairs
except for a few sensory bristles on snout or lips in some cases.
3. Pinnae are also absent.
4. Presence of hairs and pinnae may obstruct or impede the ever flow of
water over body surface and interfere with the speed and elegance of
movement through water.
5. Nictitating membranes, eye cleansing glands, lacrimal glands and all
kinds of skin glands (sweat and sebaceous) are also absent because
they would have been useless under water.
6. Skin losses its muscles and nerves due to thickening and immobility.
7. Hind limbs are represented only by button-like knobs in the foetus but
disappear in the adult.
8. Pelvis is also rudimentary.
9. Finger nails are absent except for traces in foetus.
10. Scrotal sacs are also absent and testes remain inside abdomen.

19. C. Development of New Structures:
1. Tail Flukes: In aquatic mammals, some large, lateral or horizontal expansions of the skin
develop on tail. These expansions are called tail flukes. These are not supported by fin-ray. Their
up and down strokes not only propel the body through water but enable rapid return to the
surface for breathing after prolonged submersion.
2. Dorsal Fin: In most Cetacea develop an unpaired adipose dorsal fin without internal skeletal
support. It serves as a rudder or keel during swimming.
3. Blubber: The blubber is the thick subcutaneous layer of fat. Blubber acts as a heat insulator. It
not only retains the warmth of the body but also provides a ready reservoir of food and water
during emergency. The fat also reduces the specific gravity of the animal, thus, imparting
buoyancy. Blubber also provides an elastic covering to allow changes in body volume during
deep diving and also counteracts the hydrostatic pressure.
4. Baleen: In whalebone whales, teeth are absent. Instead, the upper jaw carries two transverse
rows of numerous triangular fringed horny plates of baleen or whalebone. These serve as an
effective sieve for straining plankton (mostly kril) which forms their chief food.
5. Foam: Foam is a fine emulsion of fat, mucus and gas. Each middle ear cavity sends an inner
pneumatic prolongation, which meets with the fellow on the other side below the skull. These
extensions contain foam. It probably serves to insulate sound and improves audition or hearing
under water.
6. Melon: The melon is a receptor present in front of nostrils It consists of a fatty mass traversed
by muscle fibres. It possibly serves to detect pressure changes in water.
7. Harderian Glands: In aquatic mammals, eyes under water remain protected by a special fatty
secretion of Harderian glands.