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Diversity and Adaptations of Amphibians

Extant Vertebrate Biodiversity

• Approximately $70,000$ vertebrate species exist in total.

• Vertebrate Groups and Species Counts:

  • Crocodylia: $25$

  • Testudines: $348$

  • Snakes: $3,619$

  • Aves (Birds): $10,806$

  • "Lizards": $6,459$

  • Lepidosauria (Lizards and Snakes): $10,078$

  • Mammalia (Mammals): $5,416$

  • Cyclostomata (Jawless fishes): $124$

  • Chondrichthyes (Cartilaginous fishes): $1,303$

  • Actinopterygii (Ray-finned fishes): >$34,600$

  • Coelacanthiformes: $2$

  • Dipnoi (Lungfishes): $6$

  • Lissamphibians (Living Amphibians):

    • Anura (Frogs and Toads): $7,350$

    • Gymnophiona (Caecilians): $205$

    • Urodela (Salamanders and Newts): $750$

  • Tuatara: $2$

Groups of Living Amphibians

Frogs and Toads (Anura)

• Evolution and Ecology:

  • Approximately $6,000$ living species.

  • Found in almost all terrestrial habitats, excluding the driest and coldest.

• Notable Traits:

  • Undergo dramatic metamorphosis from larval (tadpole) to adult stages.

  • Maintain a relatively consistent body form.

  • Possess permeable skin.

  • Are sophisticated vocalizers.

  • Includes the smallest vertebrate, Paedophryne amanuensis.

• Interactions with Humans:

  • Xenopus (African Clawed Frog) is an important model organism in research.

  • Frog legs are consumed in some cultures.

  • Frogs are currently facing a significant extinction crisis due to disease and habitat destruction.

Salamanders and Newts (Caudata)

• Evolution and Ecology:

  • Over $600$ species.

  • Primarily have a temperate distribution.

  • The highest species diversity is concentrated in Appalachia.

  • Hugely abundant in local forest ecosystems.

• Notable Traits:

  • Some species, like lungless salamanders, entirely lack lungs.

  • Exhibit a wide range of sizes.

  • Some species are paedomorphic, retaining larval traits into adulthood (e.g., axolotl).

• Interactions with Humans:

  • Are the subject of many human legends often involving fire and poison.

  • Like frogs, salamanders are also facing an extinction crisis.

Caecilians

• Evolution and Ecology:

  • Around $190$ species.

  • Have a tropical distribution.

• Notable Traits:

  • Limbless, giving them a worm-like appearance.

  • Burrow underground, using their heads to push through soil.

  • Possess small eyes and limited vision.

  • Some species exhibit maternal dermatophagy, where offspring eat their mother's skin for nourishment.

• Interactions with Humans:

  • Represent a poorly understood group of vertebrates.

Highlights of Amphibian Body Systems and Evolution

Integumentary System (Skin)

• Adaptations for Locomotion:

  • Adhesive toe pads: Aid in grasping and climbing.

    • Scanning Electron Microscopy (SEM) imagery reveals hexagonal columns with mucous in between them.

    • Intercalary bones in some frogs allow the entire toe disk to interface effectively with surfaces.

  • Extensive webbing: Found between feet in some species to facilitate gliding and swimming (e.g., Wallace's Flying Frog).

• Permeability and Respiration:

  • Amphibian skin is typically permeable to water, ions, and gasses.

  • It serves as a primary site of gas exchange, especially for lungless species (e.g., lungless red-backed salamander).

  • Most amphibians desiccate easily and require humid habitats.

  • Some species (often called toads) have thicker, less permeable skin, enabling them to inhabit drier areas.

• Defensive Glands:

  • Poison glands: Produce noxious or toxic secretions as a defense mechanism.

  • Mucus glands: Produce mucus to keep the skin moist.

• **Skin Layers and Components (Cross-section of poisonous frog skin):

  • Epidermis (outer layer)

  • Dermis (inner layer)

  • Chromatophores: Pigment-containing cells responsible for skin coloration.

  • Blood Vessels

  • Poison Gland

  • Mucus Glands

Diversification of Poisonous Amphibians

• Skin Poisons and Aposematism:

  • Dendrobatid frogs (poison dart frogs): Native to Central and South America, with ~$200$ species.

    • Some species are extremely toxic and brightly colored.

    • Others are drab and produce no poison.

    • Aposematism (warning coloration): Bright coloration warns predators, which learn to avoid them. The most colorful frogs often make no effort to conceal themselves and are active during the day.

  • Pharmacologically potent substances: Found in frog skin.

    • Epibatidine: From Epipedobates, it has $200$x the analgesic potency of morphine but with severe side effects.

    • Highly toxic alkaloids are abundant in the skin of many toxic frogs.

• Origin of Poisons (Diet Restriction Experiments):

  • Poison dart frogs do not produce poisons themselves.

  • They obtain these poisons by consuming arthropods (ants, termites, beetles, millipedes) that produce alkaloids.

  • Some ingested alkaloids are chemically modified within the frog's body to increase their toxicity.

• Correlation Between Toxicity and Color:

  • Why be colorful? Three main hypotheses:

    • Sexual selection: To attract mates.

    • Thermoregulation: For easier warming.

    • Aposematism: As a warning signal for toxicity.

  • Testing the aposematic hypothesis: If aposematic, more toxic frogs should be more brightly colored.

    • Phylogenetic Comparative Methods (PCMs): Statistical tools used to account for shared traits and trait combinations due to common ancestry when analyzing the evolution of traits. They help distinguish independent evolutionary events from traits inherited from a common ancestor. This method was used to determine how many independent times color and toxicity evolved in sync.

    • Study by Summers and Clough (2001, PNAS): Investigated the correlation between toxicity and coloration in Dendrobatid frogs.

      • Measurements: Total toxicity (composite of diversity, quantity, lethality of toxins) and coloration (scored by observers and computer software) for various species across genera like Allobates, Colostethus, Dendrobates, Epipedobates, Minyobates, and Phyllobates.

      • Analysis: Phylogeny-controlled regressions were performed using two models of character evolution:

        • Gradual model: Characters evolve gradually over time along branches of a phylogenetic tree.

        • Punctuational model: Characters evolve in rapid spurts, primarily at speciation events.

      • Results: As expected, more colorful frogs were significantly more poisonous.

        • Gradual model: $R^2 = 0.71$, $p = 0.00$

        • Punctuational model: $R^2 = 0.83$, $p = 0.00$

        • The strong effect remained consistent regardless of the character evolution model chosen.

        • Conclusion: The aposematic hypothesis is strongly supported.

• Diversification Rates in Toxic Species:

  • Computational tools: Advances in genetic and ecological data, alongside computational tools, allow for the construction of massive phylogenetic trees to study diversification processes.

  • Diversification rates: Simple metrics used to understand how quickly a clade diversifies, considering speciation rate and extinction rate.

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