Amphibia

Amphibia is a diverse class of vertebrates that include frogs, toads, salamanders, newts, and caecilians. They are fascinating creatures that represent a vital evolutionary step between aquatic and terrestrial life forms. Amphibians are ectothermic (cold-blooded) and are characterized by their unique ability to live in both aquatic and terrestrial environments, though some species are more specialized for one over the other.

1.0Introduction

• The study of amphibians is known as batrachology, whereas the study of both reptiles and amphibians is referred to as herpetology.
• Amphibians are unique animals capable of thriving in both aquatic and terrestrial environments.
• The term "amphibian" originates from the Greek word "amphibious," meaning "to live a double life."
• There are over 8,000 species of amphibians worldwide, with the majority inhabiting freshwater ecosystems. Approximately 90% of these species are frogs.
• Most amphibians begin their life in water as larvae and undergo metamorphosis, transitioning to a primarily land-based lifestyle as they mature.

2.0Importance of Amphibians

• Amphibians play a vital role in ecosystems, serving as both predators and prey.
• They help control agricultural pests by feeding on insects and reducing mosquito populations, benefiting human health.
• As a crucial part of the food chain, amphibians provide sustenance for a variety of animals, including snakes and birds of prey.
• In some regions, frogs are harvested for human consumption and recreation. Many people also enjoy the symphony of frog calls on warm summer nights, a hallmark of their presence.
• Amphibians possess remarkable tissue regenerative abilities, capable of regrowing limbs and repairing cardiac, brain, spinal, and retinal tissues.
• • These abilities hold significant potential for medical advancements, offering insights into restoring sight, mobility, and treating neurological disorders.
  • Research has also uncovered amphibian-derived polypeptides with medical applications, such as regulating insulin and blood pressure.
• Amphibians contribute to nutrient cycling between aquatic and terrestrial ecosystems, circulating essential elements like phosphorus, carbon, and nitrogen, which enhance ecosystem health and resilience.
• Frog jumping competitions and keeping frogs as pets highlight their cultural and recreational significance.
• As bioindicators, amphibians' moist, permeable skin makes them highly sensitive to environmental changes like drought and pollution, providing critical insights into ecosystem health.

3.0Evolution of Amphibians

• Amphibians first appeared about 370 million years ago. They probably evolved from lobe-finned fish that came on to land to crawl from pond to pond.
• The earliest amphibians evolved during the Devonian period from sarcopterygians—lung-equipped fish with bony, limb-like fins that facilitated adaptation to life on land.
• Amphibians flourished and became dominant during the Carboniferous and Permian periods but were later overshadowed by reptiles and other vertebrates.
• Over time, their size and diversity diminished, leaving only the modern subclass Lissamphibia as their descendants.
• Ichthyostega, one of the earliest primitive amphibians, showcased key transitional features:
• • Nostrils and efficient lungs for breathing air.
  • Four sturdy limbs, a neck, and a tail with fins.
  • A skull resembling the lobe-finned fish Eusthenopteron, linking its aquatic ancestry to terrestrial adaptation.

• As amphibians evolved, their lungs became more efficient, and their skeletons grew stronger to support body weight on land.
• They developed "hands" and "feet" with five or more digits, enhancing mobility on solid ground.
• Their skin adapted to retain body fluids, reducing the risk of desiccation.
• The hyomandibular bone in the hyoid region diminished and evolved into the stapes, an essential adaptation for hearing in terrestrial environments.
• These adaptations enabled amphibians to spend extended periods out of water, marking a significant evolutionary leap toward terrestrial life.

4.0Characteristics of Amphibians

• Amphibians can live in both aquatic and terrestrial environments.
• They are cold-blooded vertebrates.
• Their skin is smooth or, in some cases, rough, lacking scales but containing glands that keep it moist and slimy.
• The body is divided into a head and a trunk, with the tail being present or absent depending on the species.
• Amphibians have a complex life cycle, including distinct larval and adult stages.
• They are sexually dimorphic, with separate sexes, and fertilization is usually external, though salamanders exhibit internal fertilization.
• Breeding typically occurs in water, and males lack copulatory organs.
• Eggs are released without mating, usually as clear, jelly-like structures.
• The young undergo metamorphosis, transforming from gill-breathing larvae to lung-breathing adults.
• Amphibians breathe through their skin, and some species, such as small terrestrial salamanders and frogs, lack lungs entirely, relying on cutaneous respiration.
• Some adult amphibians retain external gills.
• They lack paired fins, though unpaired fins may be present.
• Many species are vocal and use calls for communication.
• Adults typically develop four legs.
• Amphibians have a three-chambered heart.
• Their kidneys are mesonephric, and they excrete ammonia and urea.
• Amphibians possess ten pairs of cranial nerves.
• A lateral line system is present during their developmental stages.

5.0Classification of Amphibia

According to G. Kingsley Noble (1931) classified Amphibians into six orders. Three are extinct and three are living. The classifications of three living orders are of class Amphibia is given below:

1. Subclass Lepospondyli: A small group of Paleozoic amphibians. These are thought to be more closely related to amniotes than to modern amphibians (Lissamphibia).
2. Subclass Temnospondyli: A diverse group of amphibians that thrived during the Paleozoic and early Mesozoic eras
3. Subclass Lissamphibia: Includes all modern amphibians such as frogs, toads, salamanders, newts, and caecilia.

Order 1. Anura (Salientia)

• There are around 3400 species of Anura in the world.
• They have four limbs. The front limbs are elongated and modified to jump.
• The head and trunk are fused together.
• The tail is present only in the larval stage and is lost in the adults.
• Fertilization is external and the eggs are laid in water. Eg., frogs and toads

Order 2. Caudata (Urodela)

• These are the organisms with a tail.
• The body is elongated with four equally sized limbs.
• The skin is smooth with poison glands.
• Fertilization is internal.
• They feed on insects and worms. Eg., Salamanders
• They are found under leaf litter, in the soil, or in water.
• In the southern US, they reproduce primarily in winters.
• Very little differences between male and female.
• Spermatophores are utilized for internal fertilization.
• They possess hidden gills.
• E.g- Salamander, Ambystoma, Necturus, Amphiuma

Order 3. Gymnophiona (Apoda or Caecilia)

• Apoda means “without legs”. These are limbless organisms with scales on their body.
• They are also known as “blind-worms” because their eyes are covered by skin or bone.
• The tentacles on their head are the chemosensory organs that help them to detect the
• underground prey. Eg., Caecilians
• They possess venom glands.
• They secrete mucus to reduce water loss.
• E.g.-icthyophyis