L9- Amphibians

  • Darwin was the first to describe amphibians to scientists, but they were known to local communities e.g. Darwin Frog

    • found that those in South America lived in a high diversity of environments but those in Europe all had similar life histories

Shared Derived Features of Amphibians:

  1. Skin→

    • have a smooth, moist, permeable skin

    • breathe through their skin→ facilitates oxygen uptake and carbon dioxide extraction

      • has a thin epidermis

    • suffer high rates of water loss

      → have a semi-aquatic life

  2. Ear→

    • use different parts of the inner ear to detect different frequencies (amphibian papilla, basilar papilla)

    • have an extra bone→ function is controversial, may be a connector to forelimbs (operculum columella)

  3. Vision→

    • rod (light) and cone (colour) photoreceptors are specialised neurons that allow vision and complement each other

    • have 4 types of photoreceptors (1 more than usual vertebrates) that allow perceiving light at intensities that other vertebrates cannot

      • are the only vertebrates that can do this

      • has many functions

  4. Teeth→

    • are the only group of recent tetrapods to have pedicellate teeth

      • are complex teeth→ function is debated

      • non-calcified connective tissue sandwiched in by two calcified dentine elements

  5. Muscle→

    • have a levator bulbi muscle

      • moves the eye as a whole→ retract inside and put back again

      • is important for feeding→ overcomes the restraint of opening the mouth

There are 3 groups of Amphibians:

  1. Anurans→ Frogs and Toads

  2. Urodeles→ Newts and Salamanders

  3. Gymnophonia→ Caecilians


Caecilians:

  • 150 species

  • live underground→ difficult to study and find

  • have internal fertilisation

  • have three reproductive modes:

    • oviparous→ free-living aquatic larvae eggs

    • oviparous→ direct development eggs

    • viviparous→ terrestrial and aquatic live young

  • energy for offspring growth is first supplied by yolk then by mother→ evidence of parental care

    • viviparous→ provided through uterine milk, scrape secretions from the oviduct

    • oviparous→ feed off the skin of the mother, outer epidermal cells have more fat for nutrients

    • young have specialised foetal teeth to feed


Newts and Salamanders:

  • 760 species, highly threatened

  • mostly internal fertilisation

    • males deposit their spermatophore in ground that has a sperm cap at the tip, the female uptakes this to fertilise her eggs

    • there are many complex courtship behaviours with elaborate secondary sexual characters to make sure the female takes their spermatophore

    • pheromones are transferred in many ways e.g. tail fanning, biting, rubbing…

  • most species lay eggs under water that develop into aquatic larvae

  • the purely terrestrial species are ovoviviparous/viviparous (produce eggs that hatch inside the body and the young are born live)

    • females retain embryos/eggs until the end of gestation, are fully developed juveniles

  • paedomorphosis→ adults retain larval traits during adult stage

    • some groups keep traits depending on the environment


Frogs and Toads:

  • 5000 species, huge diversity

  • all are carnivorous

  • have a reduced skeleton with less ossification that is specialised for jumping:

    • elongated hind limbs and pelvis

    • reduction or no tail

    • urostyle→ shock absorber for the pelvis

    • shortened or no ribs→ solid path for transmission of thrust

    • tibio-fibula→ tibia and fibula are fused together

Reproductive Modes of Anurans:

  • generalised life cycle (eggs, tadpoles, growth)→ only in temperate regions

  • is actually very complex and diverse

  • traits differ between groups e.g. egg and clutch characteristics, larval development, parental care, site of egg-laying…

  • there are 2 patterns of the evolution of reproduction:

    1. there is higher diversity of reproductive modes in the tropics

    2. there is a progression from aquatic to terrestrial reproduction

  • looked at selective pressures driving these two patterns:

    • terrestrial reproductive modes are more common in warmer, wetter environments in the tropics

    • locations with low precipitation and seasons support aquatic reproductive modes more or have adaptations to resist desiccation

    • move from aquatic to terrestrial to reduce predation threat

  • however, must now protect young from desiccation and terrestrial predation, so there is a trade off:

    • terrestrials have less but larger eggs with more water inside→ more costly, need longer development, need parental care, but have higher survival rates

Adaptations for terrestrial reproduction:

  1. Foam Nests-

    • are secretions of the oviduct

    • protect the eggs from desiccation and predation

    • has evolved convergently in 6 lineages, only in tropical continents

  2. Development of eggs on leaves over-hanging water-

    • when eggs hatch, they will fall into the water

    • evolved convergently in 3 groups

    • sometimes, male surround leaf to ensure there is no predation

  3. Some species have ovo/viviparity (produce eggs that hatch inside the body and the young are born live)-

    • is present in 2 genera in Africa, 1 in Indonesia and 1 in Puerto Rico

    • 1 species has eggs and tadpoles developing in the female oviduct

    • some species show true viviparity→ embryos are nourished by secretions

    • one species raised tadpoles in their mouth and stomach

    • e.g. Darwin’s Frog:

      • females lay eggs, males remain near until they are ready to hatch

      • larvae contract muscles to stimulate males to swallow eggs where they hatch inside and develop and are later expelled

Recent Taxonomic Discoveries:

Amphibians are highly understudied but highly threatened

  • a new species found in the south of Ecuador

  • a new family was found in 2003

  • a new species was found in New York city wetlands