Amphibians

Amphibians are a Class of Animals which are 'Cold-Blooded' Vertebrate typically living on land but breeding in water. Members of this Animal Class are Frogs, Toads, Salamanders, Newts and Caecilians or Blindworms. The 'Largest Genus' of Frogs is Pristimantis. There are more than 400 Species in this Genus, with more being described each year. Amphibians evolved in the Devonian Period and were top predators in the Carboniferous and Permian  Periods, but many lineages were wiped out during the Permian–Triassic extinction. One group, the Metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict Temnospondyls like Koolasuchus and the modern orders of Lissamphibia. There are 6,681 total number of Amphibian Species (amphibiaweb database) as of June 30, 2010. Amphibian diversity is highest in the tropics and Brazil has the most described Species at 811 Species. According to a study (iucnredlist.org/amphibians) nearly one-third, i.e. 32% of the World's Amphibian Species are threatened, representing 1,856 Species.

Amphibians are divided into 3 Subclasses, of which only Lissamphibia is Extant, other 2 are Extinct Subclasses:

1.  Subclass Labyrinthodontia - These included diverse Paleozoic and early Mesozoic group.

2.  Subclass Lepospondyli - These included small Paleozoic group.

3.  Subclass Lissamphibia - These include Frogs, Toads, Salamanders, Newts, etc.

All recent Amphibians are included in the subclass Lissamphibia, superorder 'Salientia', which is usually considered a Clade (which means that it is thought that they evolved from a common ancestor apart from other extinct groups), although it has also been suggested that Salamanders arose separately from a Temnospondyl-like ancestor. The Lissamphibia, superorder Salientia, are traditionally divided into three orders, but an extinct Salamander-like family, the Albanerpetontidae, is now considered part of the Lissamphibia, besides the superorder Salientia. Furthermore, Salientia includes all three recent orders plus a single Triassic Proto-Frog, 'Triadobatrachus'.

Subclass Lissamphibia

Family Albanerpetontidae - Jurassic to Miocene (extinct).

Superorder Salientia

i)  Genus Triadobatrachus - Triassic (extinct).

ii)  Order Anura (frogs and toads) : Jurassic to recent - 5,909 recent Species in 48 families.

iii)  Order Caudata or Urodela (salamanders, newts) : Jurassic to recent - 586 recent Species in 9 families.

iv)  Order Gymnophiona or Apoda (caecilians) : Jurassic to recent - 186 recent Species in 3 families.

Scientific Classification

Kingdom - Animalia.
Phylum - Chordata.
Subphylum - Vertebrata.
Superclass - Tetrapoda.
Class - Amphibia.

Life Cycle of Amphibians

The life cycle of most Amphibians begins in water when the Female lays eggs that are fertilized outside of her body. However there are a few Species, those of the genus Pristimantis lay eggs on land, which develop directly into miniatures of adults with no tadpole stage. These are the most widespread in Frogs of the New World tropics. In Africa the genus Arthroleptis also known as 'Squeakers' are all 'Direct Developers'. Most Species of the largest family of Salamanders, the 'Plethodontidae', are 'Direct Developers'. Some Caecilians are also 'Direct Developers'.

Larval Stage - The eggs hatch into Larvae, or 'Tadpoles', that breathe through external gills. The Larvae grow flat tails and feed on vegetation. Larval Stage lasts a few months.

Metamorphic Stage - The transition from Larva to Adult is known as 'Metamorphosis' and in some Amphibians such As Frogs, Toads, Salamanders, and Newts, the transformation is dramatic. Physical changes occur and external gills give way to lungs. The tadpoles also change from plant-eating animals to meat eaters. This is a brief period and is immediately followed by adult hood.

Adult Stage - When the Metamorphosis is complete the Amphibian reaches Adult stage. Amphibians usually reach full adulthood at three to four years of age. The adults return to the water to court and ultimately lay eggs.

History and Evolution of Amphibians

The first major groups of Amphibians developed in the Devonian Period (400 million years ago) from Fish similar to the modern Coelacanth and Lungfish  which had evolved multi-jointed leg-like fins that enabled them to crawl along the sea bottom. These Amphibians were as much as one to five meters in length. The first Amphibian was also the first tetrapod. An animal that lived about 365 million years ago, 'Acanthostega', is usually cited. Early Amphibians included creatures such as Diplocaulus, Ophiderpeton, Adelospondylus, Diplocaulus, and Pelodosotis. Amphibians never developed the ability to live their entire lives on land, having to return to water to lay their shell-less eggs. In the Carboniferous Period, the Amphibians moved up in the food chain and began to occupy the ecological position currently occupied by Crocodiles. These Amphibians were notable for eating the mega Insects on land and many types of Fishes in the water. During the Triassic Period, (230 million to 190 million years ago) the better land-adapted Proto-Crocodiles began to compete with Amphibians, leading to their reduction in size and importance in the biosphere.

Physical Features / Characteristics of Amphibians

• Ectothermic - Amphibians do not have a constant body temperature, their body temperature varies with the environment. Amphibians derive heat from outside their body. They are active in warm environments and tend to become sluggish on exposure to cold temperatures.

• Permeable & Delicate Skin - The skin of Amphibians is delicate as it lacks the protection that scales or hair provide other groups of animals. Amphibian skin is permeable to water and is characterized by having numerous mucous glands that help keep their skin from drying out. Many Amphibians can absorb oxygen directly into their bloodstream through their skin and are also able to expel carbon dioxide waste back into the air.

• Poison Glands - All Amphibians also have poison glands in their skin that produce a range of toxins depending on the Species, some toxins are merely unpalatable while others are lethal. These toxin protect certain Species against predators.

• Simple Body Structure - Most adult Amphibians have 4 limbs with 4 digits on the front limbs and 5 digits on the hind limbs (caecilians are the exception to this rule, having no limbs or digits). Most Frogs and toads have a large head, muscular hind limbs, and no tail. Salamanders are long-bodied and have a tail, some Species have a reduced number of digits or even lack hind limbs entirely. Amphibians lack a rib cage and have only short ribs. They also lack a diaphragm. In this way they have a simple body structure as compared to other Class of Animals.

• Musculo-Skeletal System - Amphibians have distinct walking legs, each with five or fewer toes. This represents an adaptation of land locomotion. The skull is flat, except for an expanded area that encases the small brain. Only nine vertebrae make up the Frog's backbone, or vertebral column. The Frog has no ribs while in other Amphibians ribs are underdeveloped.  The Frog does not have a tail. Only a spike-like bone, the 'Urostyle', remains as evidence that primitive Frogs probably had tails. The 'Urostyle', or 'Tail Pillar', is a downward extension of the vertebral column. The shoulders and front legs of the Frog are somewhat similar to man's shoulders and arms. The Frog has one 'Forearm' bone, the 'Radio-Ulna'. The hind legs of the Frog are highly specialized for leaping. The single shin bone is the 'Tibiofibula'. A third division of the Frog's leg consists of two elongated anklebones, or 'Tarsals'. These are the 'Astragalus' and the 'Calcaneus'. The 'Astragalus' corresponds to the human talus (anklebone).

• Bilateral Symmetry - Amphibians have Bilateral Symmetry i.e. the body is divided into equivalent right and left halves by only one plane.

• Respiratory System - Amphibians possess few internal septa, large alveoli and therefore a slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping. However, most Amphibians are able to exchange gasses with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularized skin must remain moist in order for the oxygen to diffuse at a sufficient rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic Amphibians in these situations can rely primarily on cutaneous respiration, as in the 'Titicaca 'Water Frog' or 'Hellbender Salamanders'. In air, where oxygen is more concentrated, some small Species can rely solely on 'Cutaneous Gas Exchange', most famously the 'Plethodontid Salamanders' which have neither lungs nor gills. Many aquatic Salamanders and all Tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.

• Circulatory System -  The Amphibian Heart has a divided Atrium but a Single Ventricle. The Right Atrium receives impure blood with little oxygen from the body proper, and The Left Atrium receives purified blood from the lungs that has just been oxygenated, but these two types of blood are mixed partially in the Single Ventricle. Mixed blood is then sent, in part, to the skin, where further oxygenation can occur. The Frog heart is the only organ contained within the coelom, which has its own protective covering. This is known as the 'Pericardium'.

• Anamniotic Eggs - The eggs of Amphibians have no hard shell and instead are encased in a 'Gelatinous Mass' to prevent the egg from drying out.

• Estivation - Some Amphibians become inactive when conditions are unfavourable for survival. This period of inactivity is called 'Estivation' when it occurs during hot, dry weather and 'Hibernation' when it occurs in response to cold temperatures. Activity resumes when favourable conditions return. Dormant state requires very little oxygen for breathing, which is achieved through cutaneous gas exchange i.e. 'Breathing via Skin'.

• Vicinity to Water Features - Amphibians are most often found near ponds, marshlands, swamps, and other areas where freshwater is available, because of their water dependant Anatomy and Life Cycle.

• Good Bio-Indicators - Most Amphibians are 'Good Bio-Indicators' which means that they show whether our Bio-Environment is working well or not. They are  'Good Bio-Indicators' as their skin is smooth and sometimes moist, making it quite permeable to gases and water. This permeability makes Amphibians particularly vulnerable to toxins in air and water such as herbicides, pesticides, and pollutants.

Weight - The 'Largest Amphibian' weighs up to 50-65 Kg. and the 'Smallest Amphibian' weighs few grams.

Size - The 'Largest Amphibian' can reach a length of 1.8 metres (5.9 ft) and the 'Smallest Amphibian' is about 9.6 to 9.8 millimetres (3/8 inches) long.

Geographical Range and Habitat

Almost all Amphibians live in a moist habitat which includes Lakes, Ponds, Creek etc. They are also found in Tropical Rainforests.

Diet - Amphibians are generally Carnivorous. In water, they consume a wide variety of Fish, Reptiles, Crustaceans and other Amphibians. On land, they feed on worms and insects like Spiders, Flies etc.

Predators - The predators of Amphibians include all classes of Vertebrates and some Arthropods. Aquatic eggs of Amphibians are mostly the prey of Fish and Aquatic Invertebrates. The Leech is the most common Invertebrate predator of the eggs. Some Salamanders feed on each others' eggs. Larval and adult Newts also feed on eggs of some Species of Anurans and Salamanders.

Reproduction

Most Amphibians breed in freshwater, however there are some Species that breed in moist places, like burrows in the ground and among leaf litter or in wild habitats. For the purpose of reproduction most Amphibians require fresh water. Amphibians reproduce sexually. Fertilization of eggs is either external or internal depending upon the Species. In Frogs - fertilization is external, in Salamanders it is either Internal or External while in Caecilians, fertilization is internal. Most Frogs and Toads, involves a Male holding a Female in a pose called 'Amplexus'. In 'Amplexus', the Male releases sperm over the Female's eggs as they are laid. In many Salamanders the Male deposits a packet of sperm called a 'Spermatophore' onto the ground. The Female then pulls it into her cloaca where fertilization occurs internally. Caecilians and Tailed Frogs use internal fertilization, the Male deposits sperm directly into the Female's cloaca via a reproductive organ. Some Amphibians lay their eggs and let them develop by themselves. Others, lay eggs and guard them. The mother is usually the one who guards the eggs. A salamander will guard her eggs by wrapping herself around them. Some Frogs carry their eggs embedded in their backs. Eggs laid in water form into large clumps, or they are scattered and deposited at different sites. Normally, the clumps are attached to sticks or vegetation in the water. This serves to maintain the position of the clutch. The terrestrial eggs are sometimes in strands connected by jelly between each of them. They can also be their eggs in piles and form nest around them.