Chapter - Amphibians

Amphibians

  • Amphibians are vertebrates that are adapted to life in water, land, or both

    • amphi = double, bios = life

  • Three living taxa (orders):

    • Frogs (toads) = Anurans

    • Salamanders = Caudates

    • Caecilians = Gymnophiona (naked snake)

Evolution of Amphibians

  • Intense competition in aquatic environments often drives the evolution of unique adaptations among amphibians, such as specialized reproductive strategies and diverse feeding mechanisms that help them thrive.

  • Selection favored groups that could exploit the terrestrial environment

  • Led to all terrestrial vertebrates (tetrapods = four limbs)

  • Tetrapod traits didn’t evolve because they would later be useful on land, but rather because they were useful to animals still in water

  • Momentous step in evolution

  • Extremely difficult to switch from water to land:

    • Why?

      • They needed more muscle because the water would no longer hold them up

      • Oxygen is more abundant, but extraction from a different medium

      • Temperature fluctuates more on land than in water

      • less water and ions

      • Other issues: dryness, UV, etc…

  • Tiktaalik - transitional species that could spend time on the shore with eyes that look up

  • Early (Devonian) Tetrapods

    • paired appendages - elongate, supported by flesh and bone; allowed them to squirm between swamps

  • Rudimentary lungs (modified swim bladders)

  • “preadapted” for transition to land

  • Late Paleozoic tetrapods

    • Large (1m), salamander-shaped

    • Rudimentary necks, heavy muscular tails

    • The vertebral column is much stronger than that of fish

    • Fins transformed into lets (not powerful)

    • Legs move in the horizontal plane, not the vertical

    • undulations more feed forward

  • Caecilians (Gymnophiona)

    • legless, wormlike, burrowing

    • confined to the tropics

    • hard to study because they are burrowing

  • Salamanders (Caudata)

    • retain primitive body form

  • Frogs (and toads; Anura)

    • short, tailless trunk

    • powerful hind legs (hopping, jumping)

General Characteristics

  • Vertebrates

  • over 8,300 species

    • 7,300 Frogs

    • 760 Salamanders

    • 214 Caecilians

Thermoregulation:

  • Ectothermic

    • obtain heat from external sources

  • Hibernation

  • Antifreeze (glycerol)

    • pump this into their cells to keep them safe while they are frozen solid

  • Aestivation

    • during hot and dry periods

    • cocoon formation allows amphibians to avoid dehydration by resting in a protective layer until conditions improve.

    • Sirens do this

Osmoregulation

  • Skin is very permeable to water

  • Mucus glands help retain water

    • Major adaptation to land is the ability to withstand water loss

  • Can lose 50% of body mass in water and survive

  • Mostly live in or near freshwater

  • a few frequent brackish waters

  • at least one species can live in true salt water

Morphology

  • moveable eyelids and tear glands protect and cleanse the eye

  • stronger skeleton than fish

  • Aquatic larvae have gills; lost during metamorphosis

  • Most gas exchange via moist membranes in lungs and skin; this adaptation allows amphibians to thrive in both aquatic and terrestrial environments, supporting their ability to inhabit diverse ecosystems.

  • Sensory Abilities:

    • adults don’t see much besides shape or movement

    • frogs have excellent hearing (calls); other groups do not

    • Electroreceptors, mechanoreceptors in larvae and aquatic adults

    • lateral line occurs in aquatic narvae, lost during metamorphosis

Reproduction

  • Most are oviparous; some viviparous, and a few are ovoviviparous (females fertilize eggs inside the female and hatch in the female)

  • external and internal fertilization

  • No copulatory organs (one exception)

  • Most frogs are external (amplexus)

  • Exception to the Rule: Tailed Frogs (ascaphus) - used to do internal fertilization

  • Most Salamanders are internal (spermatophore) - males leave a jelly sac of sperm and the females will insert it into their cloaca, the eggs are fertilized as the eggs leave the body

    • Exception to this is the Hellbender, they fertilize externally under rocks in streams like other stream fish

Parental care

  • most species do not exhibit parental care

  • defense of eggs often depends on type of fertilization

  • Frogs have external, male will tend to defend

  • Salamanders are internal, females will tend to defend

  • Male bullfrogs defend oviposition sites from predators, other males, and environmental threats, ensuring the safety of the eggs until they hatch.

  • Other species carry eggs on their back, or mouth frood

  • Terrestrial Salamanders females surround eggs to prevent desiccation and protect from egg predators and parasites

    • Exception - hellbender, aquatic salamander with external fertilization, males will defend the eggs

Growth and Development

  • Complex lifecycle: Adult → Embryo→ larva or tadpole → metamorphosis → adult

  • Obligate Metamorphosis - most primitive

    • All frogs, some salamanders

  • Obligate Paedomorphosis

    • paedo=juvenile; morph=form

    • adults retain larval traits, never metamorphose into their terrestrial adult forms, allowing them to thrive in aquatic environments throughout their lives. (Hellbenders are an example)

    • Always aquatic, more advanced than obligate metamorphosis

    • Some salamanders (sirens, mudpuppy, hellbender)

  • Facultative Paedomorphosis

    • some metamorphose, other become paedomorphic

    • depends on the environment x genotype (phenotypic plasticity)

    • more advanced than obligate paedomorphosis

    • some salamanders (mole family, newts)

  • Direct Development

    • Eggs laid on land

    • mini-adult hatches out (metamorphosis in egg)

    • Most advanced of egg producers

    • some frogs and salamanders

  • Regeneration

    • unparalleled among vertebrates

    • includes bone and nerve tissue

    • con do it multiple times

    • legs and tail

  • Antipredator Defenses

    • Tain autotomy

    • Dropping their tail as self defense

    • biting

    • displays, colorful underside as a warning

    • Toxins & warning coloration

  • Foraging behavior

    • most swallow prey whole

    • diet varies with life stage and taxa

      • frogs: tadpoles omnivorous, adults carnivorous

      • Salamanders: carnivorous

    • Tend to feed on anything moving

    • salamander larvae can be highly selective (size)

Why Should We Care About Amphibians?

  • Integral parts of many ecosystems

    • dominant presence in some ecosystems

    • important predators and prey

  • Warning Signals of environmental health

    • good indicators of ecosystem health because:

    • Complex life cycles: exposed to stress and/or pollutants in both habitats (double jeopardy)

    • Permeable membranes: skin and eggs are in intimate contact with the environment

  • Moral/Ethical arguments

    • We should not be causing the mass extinction to these species

  • Six major threats to amphibians:

    • Habitat Loss: Destruction of natural habitats due to urbanization, deforestation, and agriculture reduces the living space available for amphibian populations.

    • Environmental Pollution: Chemicals and waste products in water bodies can be toxic to amphibians, impacting their health and reproductive success.

    • Disease and Parasites: Amphibians are highly susceptible to various diseases and parasites, which can decimate populations and disrupt local ecosystems.

    • Global Climate Change: Rising temperatures and shifting weather patterns alter the habitats and breeding cycles of amphibians, further threatening their survival and leading to population declines.

    • Invasive Species: The introduction of non-native species can disrupt the balance of local ecosystems, often leading to increased competition for resources and heightened predation risks for native amphibian species.

    • Unsustainable Use: Overexploitation of amphibian populations for food, pets, and traditional medicine can lead to severe declines, undermining conservation efforts and threatening biodiversity.

  • Amphibian Deformities

    • extra limbs sticking out of the abdomen - parasite that attaches to developing tadpoles

      • pollutants, UV-B, or Parasites?

    • Pollution, Snails, and Ribeiroia Trematodes. - the parasite will mess up the frog then the birds will easily catch them and then it will infect them and then the snails in the water

    • Global Nitrogen Use

  • Chytrid (Chytridiomycosis) - Is the disease

    • Batrachochytrium dendrobatidis (Bd) - causes the disease

      • is a pathogenic fungus that has led to declines in amphibian populations worldwide, often causing skin lesions and impairing the ability of frogs to regulate their hydration.

    • Batrachochytrium salamandrivorans (Bsal) - causes the disease