AMPHIBIANS (TOADS AND FROGS)

 Objectives       

This  blog post provides readers with the following objectives. The reader will be able to:

o     Describe the external features of toad or frog. 

o     Discuss the adaptation of toad or frog to its habitat.

Amphibians (Toads and Frogs)

Phylum: Chordata
Sub-phylum: Vertebrata
Class: Amphibia

Order: Anura

Habitat: They live in ponds, marshes, rain forests, and other wet places.

They spend part of their life in water and part on land. They are poikilothermic (cold-blooded) vertebrates, meaning; their body temperature depends on the temperature of their environment.

Structure of a Toad

The body of toad consists of only head and trunk. It has short, round neckless body with dry skin and no tails. 

It has large flat head with wide mouth and sticky tongue, prominent bulging eyes and a pair of nostrils above the mouth.

Each eye has movable lower eyelid. The lower lid has a transparent fold of skin called nictitating membrane. This moves over the eye to clean, moisten and protect it.

Behind the eyes are circular ear drums or tympanum. The eardrums pick up sound vibrations in air and in water, which enable the toad to hear.

Two pairs of limbs arise from the trunk. The forelimbs are shot and stout with four digits which are not webbed. The forelimbs absorbs the shocks of landing after hopping. The hind limbs are longer and more muscular with five webbed digits. They push and lift the toad off the ground when hopping. The webbing offers a large surface area to push against water when swimming.

At the posterior end of the body is the cloaca, for discharging feces, sperms or eggs.

Nutrition in Toads

Toads are a carnivores. The food is mainly worms, slugs, spider, beetles and other insects. They have elongated, sticky tongue hinged at the front end of the lower jaw. They flick the tongue out to catch an insect resting on a leaf or even in flight and redraw it back into the mouth. They swallow the prey whole without much chewing. 

Locomotion in Toads

The feet have adaptations for movement both on land and in water. 

a.  On land, Toads move by hopping or leaping and crawling. The long, muscular folded hind limbs are straightened to push and lift the toad off the ground when hopping. The forelimbs absorb the shocks of landing after hopping. The toads usually move by crawling with the limbs moving diagonally.  

b. In water, toads swim by using the powerful hind limbs. The webbed feet increase the surface area that pushes against the water. The forelimbs are used in steering.

Excretion in Toads

Ammonia, excess salts and water are excreted by the kidney in urine through the cloaca. Carbon dioxide is excreted from the body through the lungs, buccal cavity or skin by diffusion.

 

Respiration in Toads

Gaseous exchange occurs through the skin (cutaneous respiration), lining of the mouth (buccal respiration) and lungs (pulmonary respiration)

□   Cutaneous Respiration: In water toads obtain oxygen by absorbing it through the skin. The skin is highly vascularized; a network of blood capillaries and remains moist to allow diffusion of oxygen at high rate. Carbon dioxide diffuses out from the blood capillaries in the reverse direction.

□   Buccal Respiration: This is employed when they are at rest on land. The mouth is closed and the floor of the buccal cavity is lowered. Toads takes in large volume of air into the buccal cavity through the nostrils. Gaseous exchange occurs through the mucus lining of the buccal cavity.

□   Pulmonary Respiration: This respiration occurs when toads are active and requires a lot of oxygen. The mouth is closed and the floor of the buccal cavity is lowered. Toads draws air in through there nostrils and pumps it into the lungs by movements of the mouth floor. Gaseous exchange takes place in the lungs. The floor of the buccal cavity is then raised, which force carbon dioxide out through the nostril.

Reproduction in Toads

Egg: During the breeding season, the male croak loudly to attract a female. The male mounts on the back of a female, gripping her under the arms with his forelimbs. In this way, he is carried around in the water by the female, sometimes for many days. The female lays eggs, the male releases sperms to fertilize them. Fertilization is external. The eggs are surrounded by a transparent gelatinous layer or a string of jelly. The eggs are hatch into free-living larvae called tadpole

Functions of the jelly

□       It protects the eggs from mechanical injury.

□       It protects the eggs from drying up.

□       It protects the egg from bacterial infection.

□       The jelly is very slippery and distasteful, protecting the eggs from predators.

□       It enables the eggs to float on the water surface.

□   It prevents overcrowding of eggs / allows room for development / better circulation of air       

Tadpole

It has oval body, cartilaginous skeleton, gills and large flattened tails with fins for swimming. Tadpole develops gill pouches that cover the gills. It has a relatively long, spiral shaped intestine. Tadpole undergoes some changes during development into adult by a process called metamorphosis. At metamorphosis, the spiral‐shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. It develops a large jaw, and its gills and gill sac disappear. Eyes and legs grow quickly, and a tongue is formed. The tail is reabsorbed by apoptosis.

These images are credited to D G Mackean www.biology-resources.com

Differences between Toad and Frog

Toad	Frog
Slightly webbed hind limbs	More webbed hind limbs
Spawn forms strings	Spawn forms clumps
Spend more time on land	Spend more time in water
Warty skin	Smooth skin
Presence of poison gland	Absence of poison gland
Skin has less mucous glands; relatively drier skin	Skin has more mucous glands; slimy skin

Differences between Tadpole and Tilapia

Tilapia	Tadpole
Possession of paired fin	Possession of only caudal fins
Tail relatively short	Tail relatively long
Fins have rays	Fins have no rays
Skin covered with scales	Skin has no scales
Lateral line present	No lateral line
Teeth are preset	Teeth are absent

Structural similarities between Tadpole and Tilapia

o   Stream-line shape

o   Presence of opercula

o   Possession of mouth

o   Possession of eye and nostril

o   Possession of tail 

o   Presence of gills

Adaptation of Toad to Its Environment

¨      The mucous glands secrete sticky and distasteful or toxic to cover the skin.

¨      The prominent eyes give toad a wide angle of vision.

¨      The webbed hind limbs allow the toad to swim in water easily.

¨      The nictitating membrane cleans, moisten and protect the eye when the toad is on land.

¨      The eardrums pick up sound vibrations in air and in water, which enable the toad to hear.

¨      The vascularized and moist skin allows gaseous exchange in water.

¨      Presence of lungs for respiration on land.

¨      Streamlined body for easy movement in water.

¨      The presence of powerful hind limbs for leaping or hopping.

¨      Croaking noise is made by males to attract the females for mating.

¨      The long sticky tongue for catching insects.

¨      The color change of the skin gives toad the ability to camouflage itself against predators.

 

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