15/03/2014 Amphibians Amphibians include the only living vertebrates that have a transition from water to land in both their ontogeny and phylogeny Movement onto land Air Environment and Water Environment: a short comparison Index Water Environment Air Environment Comparison Density liter : kg liter : 1.25 gr WE >800 times AE Viscosity Water is approximately 50 times as viscous as air Max 50 ml/ liter (often around 10 ml) Oxygen content Heat Capacity and Heat Conductivity Electrical Conductivity 209 ml/ liter AE 20 times WE The specific heat of water: 3400 times that of air/ water conduct heat 24 times as fast as air Electrical conductor No Aquatic animals can use electricity to detect the presence of others and as an offensive or defensive weapon Movement onto land Important physical differences between aquatic and land environment: (1) oxygen content, (2) density, (3) temperature regulation, (4) habitat diversity, 15/03/2014 Movement onto land Important physical differences between aquatic and land environment: oxygen content Oxygen is at least 20 times more abundant in air, and it diffuses much more rapidly through air than through water Movement onto land Important physical differences between aquatic and land environment: density Air has approximately 800 times less buoyant density than water and is approximately 50 times less viscous Movement onto land Important physical differences between aquatic and land environment: temperature regulation Air fluctuates in temperature more readily than does water: terrestrial environments experience harsh and unpredictable cycles of freezing, thawing, drying, and flooding 15/03/2014 Movement onto land Important physical differences between aquatic and land environment: habitat diversity, Despite its hazards, the terrestrial environment offers a great variety of habitats: Early evolution of terrestrial vertebrates About 400 million years ago (Devonian period) Fishes have an important combination of characteristics that evolved originally in aquatic habitats, now gave its possessors some ability to explore terrestrial habitats: - an air-filled cavity, which functioned as a swim bladder - paired internal nares, which functioned in chemoreception - bony elements of paired fins, modified for support and movement on underwater surfaces, gained sufficient strength to provide support and movement of the body on land Early evolution of terrestrial vertebrates The early tetrapode evolved the characteristic tetrapod adaptations: - air breathing, including increased vascularization of the air-filled cavity with a rich capillary network to form a lung, and a double circulation - modify their limbs to their evolutionary movement onto land 15/03/2014 10 MODERN AMPHIBIANS Class Amphibia: 7,246 species (Mar 15, 2014, http://amphibiaweb.org) + Order Gymnophiona (jim'no-fy'o-na) (Gr gymnos, naked, ophioneos, of a snake) (Apoda): caecilians + Order Urodela (yur'uh-del'uh) (Gr oura, tail, delos, evident) (Caudata): salamanders + Order Anura (uh-nur'uh) (Gr an, without, oura, tail) (Salientia): frogs, toads 11 Order Gymnophiona (Apoda) Body elongate; limbs and limb girdle absent; dermal scales present in skin of some; tail short or absent; 95 to 285 vertebrae; pantropical; 10 families, 35 genera, 199 species (Mar 15, 2014, http://amphibiaweb.org) 12 15/03/2014 Order Urodata (Caudata) Body with head, trunk, and tail; no scales; usually two pairs of equal limbs; 10 to 60 vertebrae; predominantly Holarctic 10 families, 67 genera, 659 species (Mar 15, 2014, http://amphibiaweb.org) 13 Order Anura (Salientia) Head and trunk fused; no tail; no scales; two pairs of limbs; large mouth; lungs; to 10 vertebrae including urostyle (coccyx); cosmopolitan, predominantly tropical 55 families, 441 genera, 6388 species (Mar 15, 2014, http://amphibiaweb.org) 14 Characteristics of Class Amphibia Skeleton mostly bony, with varying numbers of vertebrae; ribs present in some, absent or fused to vertebrae in others 15 15/03/2014 Characteristics of Class Amphibia Body forms vary greatly among species: salamanders usually have distinct head, neck, trunk, and tail; adult frogs have a compressed body with fused head and trunk and no intervening neck; caecilians have an elongated trunk not strongly demarcated from the head and a terminal anus 16 Characteristics of Class Amphibia Limbs usually four (quadrupedal) in two pairs with associated shoulder/hip girdle, although some forms have a single pair of limbs and others no limbs; webbed feet often present; no true nails; forelimb usually with four digits but sometimes five and sometimes fewer 17 Characteristics of Class Amphibia Heart with a sinus venosus, two atria, one ventricle, a conus arteriosus; double circulation through the heart in which pulmonary arteries and veins supply lungs (when present) and return oxygenated blood to heart; skin abundantly supplied with blood vessels 18 15/03/2014 Characteristics of Class Amphibia Skin smooth, moist and glandular; integument modified for cutaneous respiration; pigment cells (chromatophores) common and of considerable variety; granular glands (poison glands) 19 Characteristics of Class Amphibia Respiration by skin and in some forms by gills and/or lungs; presence of gills and lungs varies among species and by developmental stage of some species; forms with aquatic larvae lose gills at etamorphosis in frogs; many salamanders retain gills and an aquatic existence throughout life 20 Characteristics of Class Amphibia Ectothermic, body temperature dependent upon environmental temperature and not modulated by metabolically generated heat Excretory system of paired mesonephric or opisthonephric kidneys; urea main nitrogenous waste 21 15/03/2014 Characteristics of Class Amphibia Ear with tympanic membrane (eardrum) and stapes (columella) for transmitting vibrations to inner ear 22 Characteristics of Class Amphibia 10 For vision in air, cornea rather than lens is principal refractive surface for bending light; eyelids and lachrymal glands protect and wash eyes 23 Characteristics of Class Amphibia 11 Mouth usually large with small teeth in upper or both jaws; paired internal nostrils open into a nasal cavity lined with olfactory epithelium at anterior part of mouth cavity and enable breathing in lung-breathing forms 24 15/03/2014 Characteristics of Class Amphibia 12 Ten pairs of cranial nerves 25 Characteristics of Class Amphibia 13 Separate sexes; fertilization mostly external in frogs and toads but internal via a spermatophore in most salamanders and caecilians; predominantly oviparous, some ovoviviparous or viviparous; metamorphosis usually present; moderately yolky eggs (mesolecithal) with jellylike membrane coverings 26 Amphibians 27 ... the terrestrial environment offers a great variety of habitats: Early evolution of terrestrial vertebrates About 400 million years ago (Devonian period) Fishes have an important combination of... sufficient strength to provide support and movement of the body on land Early evolution of terrestrial vertebrates The early tetrapode evolved the characteristic tetrapod adaptations: - air breathing,