Zoology Notes on Aquatic Adaptation :- 1. Introduction to Aquatic Adaptation 2. Adaptive Features of Primary Aquatic Animals 3. Secondary Aquatic Animals.

Introduction to Aquatic Adaptation:

Changes of the body organization to exploit water as habitat are known as aqua­tic adaptation. All classes of vertebrates have their representatives leading to partial or total aquatic life.

Characters of Water as Habitat:

Water is a homogenous medium for animals. Compa­ratively, 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.

Primary Aquatic Animals:

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 Animals:

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.

Adaptive Features of Primary Aquatic Animals:

A. External Modifications for Efficient Loco­motion in Water:

i. Body Contour:


Streamlined body is the primary requisite for aquatic life. There are no protuberances on the body except fins. The anterior part (entrance) of the fish’s body has the least cross sectional area the middle portion (run) has the highest cross sectional area, while the posterior part is again with shorter area. The great backward tapering reduces the drag due to turbulence (Fig. 4.16).

Head, body and tail are laterally compressed. The edges of the jaws and gill covers of fishes fit precisely with the sub- conical head. Eyes are deeply placed on either side of the head. The mucous secretion from the epidermis has protective function against infection to the skin and lessens the drag due to friction.

Steamlining of the Body of Fish

ii. Fins:

There are two types of fins — paired and unpaired. Unpaired fins include caudal, dorsal and anal fins, while pectoral and pelvic fins are paired. 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. The strong lateral movements of tail fin produce turning in horizontal plane or pitch­ing. Paired fins counter balance this pitching. Pectoral fins also act as a brake.

B. Internal Modifications:

i. Modifications of Muscles for Loco­motion:

The locomotion in fishes is per­formed by the lateral undulation of the flexi­ble body. Bundles of, ‘>>’ shaped, muscles, called myotomes, are arranged on lateral side of the body in alternate fashion. These bundles are separated by connective tissues known as myocomata.


Alternate contraction of myotomes produces a series of curves along the body. These curves originate from the head region, pass along the body and ends in the tail. The thrust on the static water against this lateral undulation generates forward movement of the fish.

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. In relation to gill breathing, necessary modi­fications have developed in fishes, such as venous heart, afferent and efferent branchial systems, etc.

iv. Modification for Floating:

Swim bladder, a hollow and large sac filled with gas, is present in the abdominal cavity of most bony fishes. This structure is homolo­gous with the lungs of terrestrial vertebrates but serves as hydrostatic organ in fishes and helps in floating the fish at certain depth of the water body. A detail of the swim bladder is given in volume 2 (under 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 pro­tect the fishes in two ways – firstly these prevents external water from penetrating through the skin and secondly continuous mucous secretion washes out the 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:

(a) Eyes:

In all fishes eyes are very large. Lens is spherical with a graded refractive index, which is much higher in the centre. Lens is attached to a retractor lentis muscle in the teleostean fishes. The muscle has originated from the falciform process and presumed to be nutritive. Rete mirabili are present in the choroid layer, which produce a high oxygen tension for retina. Sclera is pro­vided with cartilage to resist pressure of water.

(b) Fate of Ear:

Weberian ossicle is a chain of modified vertebrae, which is consi­dered as internal ear in some fresh water fishes (Cyprinus, and Gobio can hear well) and have connection with swim bladder. Fishes have only internal ears.

(c) Chemical Senses:

Sense of taste and smell are well developed in fishes. They have receptors in the lining of the buccal cavity, on the barbells and all over the body. Nostrils have no connection with mouth and serve as chief receptor for chemo-sensation, i.e., per­ceive smell and presence of other chemical substances in the water.

(d) Thermo-receptor:

In the elasmobranches, numerous pores on the dorsal and ventral sides of the head lead to a sense organ, known as ampulla of Lorenzini. This structure is known to be a thermo-receptor but also responds to the changes in salinity.

(e) Lateral Line Sense Organ:

Two shal­low canals on either side of the body, embed­ded in dermis and extending from head to tail, are called lateral line sense organs. This is the most vital sense organ in fishes. Group of sensory cells, called neuromast, are sunk in the canal.

Hair of neuromast cells are embedded in a gelatinous cup — the capula and are sensory in nature. Lateral line sense organs are associated with distant touch and it records the direction, velocity and perhaps the size of the moving object of the ambient water. The neuromast cells of the lateral line are connected with the Xth cranial nerve.

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.

Any protuberances, like 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. Modification for Locomotion:

i. Limbs:

In aquatic birds, Pinnipedia and platypus, webbed feet are developed (Fig. 4.17c). Limbs are modified into paddles in some secondary aquatic animals. 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 (Fig. 4.17a).

Adaptations of the Appendicular Skeleton

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 aquatic verte­brates two methods of propulsion can be seen. 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.

iii. Ribs:

Ribs become highly arched dorsally and move upward from their point of attachment on the centrum. The articulation of ribs is loose. This type of modification makes the chest cavity spacious and shift towards the upper part of the body. This is due to the accommodation of large lungs, which enables them to spend long time under water, after single inspiration.

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 (Gobi-cephala).

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 platy- dactylus.

D. Modifiiation of other Internal Organs:

i. Digestive System:

Remarkable modi­fication in the buccal cavity can be seen in secondary aquatic animals. 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.

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.

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.

Some examples of secondary aquatic verte­brates:

Class Reptilia:

Swamp and river turtles (Emys, Trionyx), Alligator, gharial (Gavialis), Crocodylus, marine iguana (Amblyrhynchus), water snake (Natrix) are amphibious and leathery turtle (Desmochelys), green turtle (Chelonia), sea snakes (Hydrophis) are fully aquatic.

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.