br26820_07_frogs-i

Introduction

  • Lecture on Lemur Leaf Frog (Hylomantis lemur)

  • Focus on Class Amphibia

Definition of Amphibians

  • Etymology: Greek "amphi" (both) and "bios" (life) indicates aquatic and terrestrial existence.

  • Subclass Lissamphibia includes three extant orders with shared derived characters in skin, eyes, ears, and teeth.

Life Cycle of Amphibians

  • Biphasic Life History:

    • Characterized by metamorphosis.

    • Aquatic Larvae: Obligatorily aquatic larvae (tadpoles) transition to facultatively terrestrial adults.

    • Tadpoles have:

      • Lateral lines for sensing.

      • External gills in embryonic stage; internal during metamorphosis.

      • Fish-like circulatory systems.

    • Example: Common Frog (Rana temporaria).

Evolutionary History

  • Paleozoic Amphibians:

    • Abundant larval forms in fossil records, especially lower Permian (thousands of Apateon larvae fossils).

    • Fossils indicate amphibian larvae's role in trophic chains from Carboniferous- Permian lakes.

  • Classical Paleozoic Example: Archegosaurus (1847)

    • Documented evolution from larval (15cm) to adult (1.5m) stages showing reduction of external gills and skeletal development.

    • Possessed bony dermal scales unlike modern lissamphibians.

Fossil Record Gaps

  • Romer's Gap:

    • Notable absence of amphibian fossils during a 20-30 million year window in the Early Carboniferous.

    • Discovery of Scottish fossil sites in the 1980s revealed early Carboniferous amphibians (e.g., Balanerpeton).

Evolutionary Origins of Modern Amphibians

  • Possible ancestors include:

    1. Temnospondyls: e.g., Apateon, Balanerpeton, Archegosaurus (Carboniferous - Early Cretaceous).

    2. Lepospondyls: e.g., various forms during the Carboniferous - Early Permian.

Early Lissamphibians

  • Triadobatrachus:

    • Earliest undisputed lissamphibian from the Early Triassic (-251-247 mya) in Madagascar.

Anuran Origins and Traits

  • Paedomorphism Hypothesis:

    • Retention of juvenile larval characteristics in adult forms.

    • Skull features in early Triassic temnospondyls reveal similarities to current anuran morphology.

    • Characteristics include:

      • Miniaturization of skull features.

      • Proportionally larger orbits.

Characteristics of Lissamphibians

  • Derived Character: Smooth (liss) skin lacking scales.

  • Skin features include:

    • Moisture retention by mucus glands.

    • Permeability to oxygen and carbon dioxide.

    • Presence of poison glands.

Gas Exchange Mechanisms

  • Amphibians obtain 20-85% of oxygen and excrete two-thirds of CO2 through skin.

  • Cutaneous gas exchange essential for survival in water and during hibernation in temperate climates.

    • Example species include: Common Frog (Rana temporaria) and Northern Leopard Frog (Lithobates pipiens).

Amphibian Skin Structure

  • Two primary layers: Epidermis and Dermis.

  • Epidermis:

    • Comprised of multiple layers with limited water resistance.

    • Characterized by periodic moults (sloughing).

    • Contains keratinocytes that provide barrier functions.

  • Dermis:

    • Houses mucus and poison glands.

Amphibian Skin Diseases

  • Chytridomycosis:

    • Caused by the chytrid fungus Batrachochytrium dendrobatidis.

    • Affects 50% of amphibian species since 1997.

    • Leads to increased sloughing and decreased osmotic regulation.

    • Recognized as one of the worst infectious diseases affecting vertebrates.

Glandular Functions in Amphibians

  • Mucus Glands:

    • Produce slippery mucus, aiding in gas exchange and reducing water loss.

    • Increased discharge under higher temperatures (e.g., basking).

  • Poison Glands:

    • Also known as granular glands, contain bioactive compounds.

    • Over 800 alkaloids and 2000 bioactive peptides identified, with many more undiscovered.

Functions of Bioactive Compounds

  • Notable properties include:

    • Anti-oxidation and anti-microbial effects.

    • Analgesic properties and anti-predation mechanisms.

    • Regulate reproductive processes and immune responses.

Chromatophore Function in Skin Coloration

  • Types of Chromatophores:

    • Xanthophores (yellow), Erythrophores (red), Iridophores (reflective, blue), Melanophores (brown/black).

  • Chromatophore units allow for rapid color changes in response to environmental interactions.

Color Changes in Frogs

  • Diurnal Color Change:

    • Example: Lemur Leaf Frog (Hylomantis lemur) changes from green during rest to brown when active.

  • Transparent Skin Variants:

    • Japanese Brown Frog mutants with altered pigmentation show varying degrees of transparency.

    • Documented research on skin mutation highlights complexity in chromatophore inheritance.

Identification of Common Frog

  • Common Frog (Rana temporaria): Adults typically 6-7 cm with variable coloration.

  • Features smooth, moist skin, often with dark eye markings.

  • Breeding males display distinct throat colorations and thick-frontal legs with nuptial pads.

Aposematism in Poison Frogs

  • Strawberry Poison Frog (Dendrobates pumilio):

    • Bright coloration signifies toxicity to warn potential predators, demonstrating evolutionary adaptation.

Summary of Amphibian Characteristics

  • Class Amphibia:

    • Contains three extant orders under subclass Lissamphibia.

    • Biphasic life-history involving metamorphosis.

    • Key fossils reveal archaic amphibian diversity during Carboniferous and Permian.

    • Essential skin traits include mucus and poison glands that contribute to survival and resilience.