Section 12.3 Amphibians

Evolution of Amphibians About 350 million years ago a lineage of lobe-finned fish were the first vertebrates to make the transition from water to land – This was about 100 million years after the first plants made the same movement These fish had specialized characteristics that allowed for life on land – Limb like fins for crawling – Primitive lungs for breathing air for short periods From these ancient fish there would be descendents who could stay out of water for longer and longer and eventually we got the first amphibians Amniotes Amphibians Greerpeton Hynerpeton lchthyostega Acanthostega Metaxygnathus Elginerpeton Eusthenopteron Panderichthys Lungfishes Coelacanths Ray-finned fishes Paleozoic SilurianDevonianCarboniferousPermian To present Millions of years ago

Class Amphibia Is represented by about 4800 species which are divided into 3 orders: – Legless amphibians, order Apoda Include the wormlike caecilians – Tailless amphibians, order Anura Include frogs and toads – Tailed amphibians, order Urodela Include salamanders and newts Amphibians have moist skin that helps with gas exchange along with their lungs Most adults are carnivorous

Order Apoda As mentioned are legless Most are nearly blind Most species burrow in moist forest soil in the tropics

Order Anura Out of all the orders are the most specialized for life on land – Adults with powerful legs to hop on the land – Use of sticky tongues to catch insects Also have many more adaptations to reduce predation – Camouflage – Secrete distasteful or even poisonous mucus from their skin (some advertise this via bright colouring as a warning, e.g. poison dart frogs)

Order Urodela Some species are entirely aquatic Others are on land as adults or throughout their life Most are small organisms, but also includes the largest amphibian the giant salamanders found in Japan and China

Amphibian = “two lives” Amphibian means two lives and this is the reference to the common metamorphosis of aquatic larvae to terrestrial adults – For this reason it is as if the organism has 2 lives – Most common in toads and frogs (tadpole and adult) This does not mean all amphibians have a dualistic life as many do not – Some larvae of some look like adults – Some species are strictly aquatic or terrestrial

Amphibian Reproduction Most have external fertilization and lay their eggs in fresh water – Oviparous = lay eggs Some species though are ovoviparous (eggs hatch within parent) or viviparous (give birth to live young) In any case for reproduction for most water is key as the eggs lack shells and dehydrate quickly in dry air Most are familiar with reproduction of the the order Anura (frogs and toads)

Case of the Tadpole Egg hatch as tadpole – As tadpole most are herbivores – In some cases though tadpoles are omnivores Gas exchange via gills And have tail for movement Overtime tadpole undergoes metamorphosis developing limbs and loses its tail, and gills

Amphibians Tie to Water As mentioned is key for most eggs to survive – Also most lay their eggs in water For this reason amphibians are usually near water or found in damp habitats – This is also due to the fact most rely heavily on their moist skin to carry out gas exchange over their lungs Those adapted to drier habitats spend much of their time in burrows or under moist leaves where humidity is higher

Amphibian Circulation Adults have a 3 chambered heart, unlike fish Blood from the body enters the right atrium – Then flows into the single ventricle Blood from the lungs enter the left atrium – Then flows into the single ventricle Since the heart only has one ventricle both oxygenated and deoxygenated blood leave the heart together as they have mixed in the ventricle This makes amphibians circulation system to not be very efficient

Amphibian Respiration For most species the young that live in water use gills Meanwhile for most adults they use the following for gas exchange: – their lungs Their lungs have a simple structure with some internal folding to help increase their surface area for gas exchange – their moist skin The use of their lungs and skin help make up for their less efficient circulation

Amphibian Decline For the past 25 years, zoologists have been documenting a rapid and alarming decline in amphibian populations throughout the world. Several causes that have been proposed include environmental degradation (especially acid rain) and the spread of a pathogen, a chytrid fungus. This decline is also part due to the fact amphibians have been found, especially frogs, to be very sensitive to environmental contaminants – This is part to the fact that frogs are found on land and water and can absorb gas via their skin – Also they eat a variety of food including plant and animal material Thus from this many scientists can at times evaluate a habitats contamination and condition based on the amphibian population

Homework Read pages Answer Questions – 1-2, 5 page 460