Vertebrate Zoology: The Amphibians

Vertebrate Zoology

Unit 3: The Amphibians

Superclass: Tetrapoda

  • Characteristics of Tetrapoda:

    • Evolved characteristics beneficial for survival in shallow water environments.

    • Advantages of terrestrial activity include:

    • Escape from aquatic predators.

    • Access to additional food supplies.

    • Dispersal opportunities for juveniles.

    • Basking in the sun to help elevate body temperature.

Class Amphibia

  • Skin Characteristics:

    • Skin is moist, scaleless, and permeable.

    • Functions include protection, respiration, osmoregulation, and thermoregulation.

    • Presence of granular (poison) glands.

    • Webbing between toes aids in propulsion and air gliding.

    • Contains chromatophores:

    • Melanophores: Black and brown pigments.

    • Iridophores: Produce iridescence through purine.

    • Xanthophores: Produce yellow pigments.

  • Color Visualization:

    • In green frogs, blue light is refracted by iridophores back through xanthophores, resulting in a green appearance.

    • Blue frogs lack xanthophores, hence do not exhibit this green coloration.

  • Color Change Mechanism:

    • Color change occurs via shifting pigment granules within cells.

    • Regulated by Melanocyte-stimulating hormone (MSH).

    • Melatonin causes aggregates of melanin in the dermis, leading to a lighter color appearance.

  • Skeletal System:

    • Increased ossification compared to ancestors.

    • Modification of the appendicular skeleton enables terrestrial mobility.

    • Generally features four well-developed limbs.

    • Exhibits an autostylic jaw where the upper jaw connects directly to the skull.

    • Many species possess a true sternum.

  • Muscular System:

    • There is a decrease in axial musculature since locomotor functions have transitioned to appendages.

    • The levator bulbi muscle located on the floor of the eye orbit:

    • Causes the eyes to bulge, enlarging the buccal cavity.

    • Functions oppositely to the retractor bulbi muscle.

  • Cardiovascular System:

    • Heart consists of two atria and one ventricle.

    • Ventricular trabeculae: Ridges in the ventricle wall separate oxygenated and deoxygenated blood.

  • Respiratory System:

    • Four primary methods for gas exchange:

    • Cutaneous: Through moist skin.

    • Buccopharyngeal: Through the mouth and pharynx.

    • Branchial: Using external gills.

    • Pulmonary: Utilizing lungs.

    • Many species lose a significant amount of CO2 through their skin.

Digestive System

  • Most species exhibit a sticky tongue, which is attached to either the anterior or posterior end.

  • Teeth Characteristics:

    • Pedicellate teeth: Composed of a dentine crown and pedicel.

    • Homodont dentition: All teeth are similar.

    • Pleurodont dentition: Teeth affixed to the inner side of the jawbone.

    • Polyphyodont dentition: Constant replacement of lost teeth.

    • Teeth are primarily utilized for grasping and holding food rather than chewing.

Feeding Habits

  • Lifecycle Feeding:

    • Larval stage can be herbivorous or carnivorous.

    • Adult stage is typically carnivorous.

Sense Organs

  • Neuromast Organs: Present in both larval and adult aquatic amphibians for detecting movement.

  • Ear Structure:

    • Comprised of outer (tympanum), middle, and inner portions to facilitate sound perception.

    • Operculum-Columella Complex: Bone structure transmitting sound to the inner ear.

  • Middle Ear Formation:

    • First pair of pharyngeal pouches develops into the middle ear in frogs and toads.

    • Inner ear functions: Used for hearing and maintaining equilibrium.

    • Papilla amphibiorum: Detects frequencies below 1000 Hz.

    • Papilla basilaris: Detects frequencies above 1000 Hz.

  • Eyes Characteristics:

    • Typically large and well-developed.

    • Capable of retraction into the orbit to aid in swallowing.

    • Orbital glands (Harderian & Lacrimal): Produce oil and tears for eye cleansing, representing an evolutionary first.

    • Tapetum lucidum: Assists in nocturnal vision.

    • Contain green rods for efficient blue light absorption.

    • Certain cave-dwelling and subterranean species (salamanders & caecilians) are anophthalmic (blind).

  • Nasal Structure:

    • Double olfactory system is present.

    • Olfactory epithelium: Samples chemicals in airflow.

    • Jacobson's (Vomeronasal) organ: Recognizes food and detects pheromones, another evolutionary first.

  • Taste Receptors:

    • Taste buds located on the dorsal surface of the tongue, floor of the mouth, jaws, and palate.

Endocrine System

  • Hormonal regulation is crucial for physiological and morphological changes during metamorphosis.

  • Hormones are responsible for regulating reproductive cell development and associated behaviors, including secondary sex characteristics.

Reproductive Categories

  1. Aquatic: Eggs deposited in water where larvae develop.

  2. Semi-Terrestrial: Eggs laid outside aquatic environments, larvae develop in water.

  3. Terrestrial: Eggs and young are independent of standing water, influenced by temperature and precipitation (most breed late winter/early spring; some in summer/fall).

    • Sexual maturity occurs between 1 and 8 years of age.

Development

  • Embryonic Development: Varies from 1 day to 9 months, heavily temperature-dependent.

  • Hatching Mechanism: Embryos produce hatching enzymes to digest membranes; certain frogs possess an egg tooth.

  • Parental Care Behaviors:

    • Displayed by many species through guarding, aerating, and transporting eggs, alongside protecting young.

  • Growth & Metamorphosis:

    • Eggs in aquatic environments develop into gill-bearing larvae (tadpoles).

    • Terrestrial eggs undergo direct development into miniature adult forms.

    • Metamorphosis Trigger: Induced by the thyroxine hormone, effects include physical changes and alterations in life-support systems.

Order Anura (Frogs and Toads)

  • Species Diversity: Approximately 5400 species found on every continent except Antarctica.

  • Skeletal and Locomotion Features:

    • Possess a short vertebral column with numerous fused bones, providing a stable anchor for hind limb muscles.

    • Tree frogs utilize “wet adhesion” for sticking to surfaces; mucus on toe discs creates surface tension and viscosity.

  • Vocalization:

    • Male frogs and toads exhibit well-developed voices, often featuring vocal sacs or resonating chambers.

  • Dermis and Epidermis Features:

    • Comprise mucous and poison glands; structures like warts and parotoid glands in toads, along with dorsolateral ridges in certain frogs.

    • Fluid Uptake: Around 70% of water uptake occurs through the pelvic patch, characterized by thinner, vascularized skin.

Sensory Structures

  • Tympanic Membrane:

    • Well-developed and usually larger in males, allowing more sensitive hearing capabilities.

  • Eyes:

    • Contain a nictitating membrane.

Reproductive Behaviors

  • Most breeding occurs at night, utilizing species-specific calls for mate attraction.

  • Males and females respond to different components of mating calls.

  • Vocal Sac Utilization:

    • Males can use single or paired sacs.

    • Amplexus: Male grasps female behind her forelimbs to aid expulsion of eggs, while sperm is deposited onto eggs simultaneously.

  • Fertilization Methods:

    • Most fertilization is external, though a few species exhibit internal fertilization through a cloacal extension (intromittent organ).

    • Eggs typically laid in water, in masses, or unique habitats like bromeliads and foam masses, with some species featuring dorsal pouches (e.g., Surinam Toad, Spix Frog).

Anuran Larval Development

  1. Premetamorphosis: Characterized by rapid growth.

  2. Prometamorphosis: Hind legs begin to emerge; growth slows down.

  3. Metamorphic Climax: Emergence of fore limbs and degeneration of the tail.

    • Approximately 800 species skip the tadpole stage via direct development into froglets.

  • Viviparity:

    • Known in 5 species, showcasing diverse reproductive strategies.

    • Parental care exhibited by some species by maintaining eggs/tadpoles in vocal sacs or stomachs and ensuring their safety.

Order Caudata (Salamanders)

  • Sensory Features:

    • Lack tympanic membranes; sound waves transmitted through the lower jaw and front limbs to the inner ear.

    • Nasal structures feature well-developed smell capabilities, with structures like nasolabial grooves and cirri aiding olfactory functions.

  • Reproductive Traits:

    • Exhibit extensive courtship behaviors, including rubbing and nuzzling.

    • Sexual dimorphism is present in certain species where male feces, resulting from a high-quality diet, are attractive to females.

  • Fertilization:

    • Internal fertilization is utilized by 90% of species; the male deposits a spermatophore on submerged structures which the female retrieves.

    • Eggs attached to underwater vegetation or rocks hatch into gill-bearing larvae.

Development in Salamanders

  • Neoteny: Retention of juvenile or embryonic characteristics even after reaching sexual maturity.

    • Examples include Necturus, Axolotl, and certain adult sirens, which possess both gills and lungs.

  • Paedogenesis: The phenomenon of reproduction occurring in larval forms.

Order Gymnophiona (Caecilians)

  • General Characteristics:

    • Legless, inhabiting burrowing or aquatic environments, typically in tropical regions.

  • Sensory Adaptations:

    • Eyes are reduced and concealed beneath the skin.

    • Possess tentacles within openings on either side of the head serving tactile and chemoreceptive roles—unique among vertebrates.

  • Feeding Habits:

    • Diet consists primarily of termites, earthworms, and insect larvae.

Reproductive Mechanisms in Caecilians

  • Generally exhibit internal fertilization, with males using an intromittent organ.

  • Oviparous Species:

    • Lay eggs in mud near water; some females provide parental care.

  • Viviparous Species:

    • Offspring may feed on a combination of maternal skin cells or oviduct lining and “uterine milk.”

Vanishing Amphibians

  • Causes of Decline:

    • Habitat loss due to deforestation and agricultural expansion.

    • Chemical contamination from industrial activities (mining/oil extraction).

    • Climate change leading to adverse environmental conditions.

    • Acid deposition, notably with a pH < 5, detrimental to embryo survival and growth.

    • Ultraviolet radiation linked to ozone layer depletion.

    • Disease—specifically chytrid fungus, which penetrates skin and hampers cutaneous respiration and water absorption.

Human Relevance of Amphibians

  • Amphibians serve various roles:

    • Food Source: For humans and other organisms.

    • Companionship: Often kept as pets.

    • Ecological Role: Assist in insect control.

    • Medical Research:

    • Painkillers derived from poison dart frog alkaloids.

    • Development of antibiotics, anti-tumor agents, and anti-inflammatory compounds.

    • Indicator Species: Useful for monitoring environmental stressors in ecosystems.

    • Limb Regeneration Studies: Offer insights into regenerative medicine.

    • Educational Tools: Frequently utilized for dissection and research purposes.