Authored by Christina Miller RVT, BSc, of Companion Animal Hospital, Canada.
They aren’t reptiles: The awesomely unique world of Amphibians
Amphibians are actually a unique group of animals, quite different from the reptiles they’re commonly categorized with.
Amphibians seem to get the shaft when we’re casually talking about herpetology. Often “lumped into” this group, with reptiles stealing the limelight. When it comes to their biology, the only thing they have in common with reptiles is the fact that they’re tetrapods, and ectotherms like most reptiles. Amphibians are actually a unique group of animals, quite different from the reptiles they’re commonly categorised with.
1: Amphibians are not reptiles
What differentiates amphibians and reptiles? Classifying organisms has evolved significantly from the days of early taxonomists, like Carl Linnaeus. Once upon a time, reptiles, amphibians, and sharks were classified as “herpetiles” [sic] (Schmidt 2003). Despite this being later determined as inaccurate, the terms “herp” and “herptile” persist in modern zoology (minus the sharks) as words collectively referring to reptiles and amphibians as a group.
Herpetofauna is another accepted term. The word “herpeton” means “creeping animal” in ancient Greek, possibly a reference to their stature.
Herpetologists are zoologists who study herps, and herpetoculturists are people who keep and breed herps in captivity.
Early biologists would make observations on animals and then create “family trees” based on shared characteristics to better classify organisms. Species with more traits in common are generally assumed to be more closely related. While this has functioned to an extent to help us better understand evolutionary relationships, it’s not a foolproof technique. Just as two people aren’t necessarily related because they share the same eye or hair colour, we see traits occur in distantly related species that evolved independently (such as what occurs with convergent evolution).
Enter phylogenetics: Observations of genetically inherited traits such as DNA sequences or evolutionary models of morphological traits. Phylogenetics allow us to better refine our animal “family trees” and better understand evolutionary relationships, so the trees are created based on shared ancestry instead of simply traits shared in common (a classification science called phenetics). Linnaeus thought reptiles, amphibians, and sharks were closely related because of their cardiopulmonary anatomy (Schmidt 2003). We of course now know this is inaccurate: Sharks are a group of cartilaginous fishes (class Chondrichthyes) who are only distantly related to amphibians and reptiles in that they are all vertebrates (Hickman et al. 2001).
Coming back to tetrapod vertebrates. This is where reptiles and amphibians “diverge” as groups. Tetrapoda is a superclass of vertebrate animals where the common ancestors of four-limbed species of amphibians, reptiles, birds, and mammals are classified. This early “stem group” of animals is where the ancestors of modern amphibians diverged from the early ancestors of modern reptiles, birds, and mammals (the amniote vertebrates) (Hickman et al. 2001). Class Amphibia contains almost 7,400 species (AmphibiaWeb 2015a). A summary of some physical differences between reptiles and amphibians may be found below:
Table 1.1: Differences between amphibians and reptiles (Vitt and Caldwell 2009)
|Taxonomic classification||Kingdom Animalia, phylum Chordata, subphylum Vertebrata, superclass Tetrapoda, class Amphibia||Kingdom Animalia, phylum Chordata, subphylum Vertebrata, superclass Tetrapoda, class Reptilia|
|Thermal metabolism||Mostly heterothermic ectotherms or poikilotherms||Mostly heterothermic ectotherms|
|Egg characteristics||Non-amniotic (very permeable to water, not drought resistant)||Amniotic (semipermeable, comparatively more drought-resistant)|
|Skin||Permeable, very glandular||Mostly non-permeable, few glands
|Life history||Metamorphosis: Larval and adult life stages||No metamorphosis: Same body form throughout their life
2: There are three extant groups of amphibians
Class Amphibia has three living orders: Anura, Caudata and Gymnophiona. These groups are thought to have a common ancestor, and are grouped into the clade Lissamphibia (Vitt and Caldwell 2009).
Anurans, order Anura, are the frogs, toads and tree frogs. This group is sometimes called Salientia, a reference to the stem clade that includes extinct proto-frogs as well as anurans. This is perhaps the most well-known group of amphibians. They are tailless and typically have well-developed hind legs adapted to saltatory (jumping) locomotion, although there are exceptions (Vitt and Caldwell 2009). There exist relationships between limb length and locomotory style, e.g. long back limbs with short front limbs are adapted for a jumping lifestyle, short rear and front limbs are adapted for a crawling, hopping, burrowing lifestyle, etc. (Pough et al. 2005).
Caudates, order Caudata, are the tailed amphibians. The order includes salamanders, newts, sirens, amphiumas, and others. Sometimes called Urodela, which is the clade within Caudata that contains all extant species. Most have a lizard-like appearance, but there are many species with reduced or no limbs. Caudates typically have four toes on the front limbs and five toes on the hind limbs (Vitt and Caldwell 2009).
Caecilians, order Gymnophiona, are secretive, limbless and comparatively poorly studied amphibians. There are few species in the pet trade, and are often incorrectly referred to as “rubber eels.” They are either fossorial (adapted to a burrowing lifestyle) or completely aquatic. Caecilian reproduction is a fantastic subject; they seem to break all of our preconceived “rules” about amphibians. Most are viviparous (fetuses of many species feed on a special trophic uterine lining), and there is plenty of maternal care. The neonates of some species appear to be altricial, absolutely requiring some maternal involvement after birth for proper development (Vitt and Caldwell 2009).
3: Amphibians undergo metamorphosis
Metamorphosis is one of the classically defining traits of amphibians (but remember: organisms are classified based on shared ancestry, not common characteristics!). Metamorphosis is defined as a change in body form and function from immature to mature body stages, in two or more distinct stages (Allaby 2009). Most amphibians hatch from eggs in the water as aquatic larvae (called tadpoles in anurans), then undergo a change in body form to become terrestrial or semi-aquatic adults. Aquatic larvae develop limbs, lungs and may lose their tail (in the case of anurans). Note, amphibians are the only vertebrate animals to experience metamorphosis (Vitt and Caldwell 2009).
There are many exceptions and variations to the “rule” that amphibians always have a larval and adult life stage. Some caudates will metamorphose inside of a terrestrial egg hidden within a humid micro-habitat, hatching as miniature adults. This occurs in species who live in areas where water bodies may not remain long enough to sustain an aquatic larval stage. The Marbled Salamander, Ambystoma opacum (Ambystomatidae) (Behler 1995, Conant and Collins 1998, Kowalski 2002) and the Dunn’s Salamander, Plethodon dunni (Plethodontidae) (Nauman et al. 1999) are examples. Neoteny is observed in some caudate species. Neoteny, is the delay of physiological development of the animal—in reference to amphibians, this is a retention of larval traits such as gills (Chandra 2008).
Facultative neoteny may occur depending on environmental factors. If the terrestrial environment is inhospitable (for example, too dry or too cold), these caudates may maintain an aquatic lifestyle to survive and reproduce
There are several types of neoteny observed in amphibians:
Obligate neoteny, where larval characteristics such as gills are always retained into adulthood (to varying degrees). It is observed in Amphiumidae (amphiumas), Cryptobranchidae (the Hellbender, Cryptobranchus alleganiensis and Asian giant salamanders), Proteidae (mudpuppies and waterdogs) and Sirenidae (sirens) (Allaby 2009, Chandra 2008, Vitt and Caldwell 2009). These animals are essentially completely aquatic from birth to death.
Facultative neoteny may occur depending on environmental factors. If the terrestrial environment is inhospitable (for example, too dry or too cold), these caudates may maintain an aquatic lifestyle to survive and reproduce. Species in the Ambystomatidae (mole salamanders) (Sprules 1974), Dicamptodontidae (Pacific giant salamanders), Hynobiidae (Asiatic salamanders) and Plethodontidae (lungless salamanders) may display facultative neoteny (Allaby 2009, Chandra 2008, Vitt and Caldwell 2009).
There is another small, third category of inducible obligate neoteny within the Ambystomatidae and some Plethodontidae. By manipulating the thyroid gland of these animals in the laboratory, metamorphosis can be induced. The Axolotl, Ambystoma mexicanum, is famous for this (Chandra 2008, Tompkins and Townsend 2005, Vitt and Caldwell 2009).
4: Amphibians have multiple modes of respiration
Amphibians have four modes of respiration: Cutaneous (skin), pulmonic (lungs), buccopharyngeal (oral cavity) and branchial (gills). Most species will employ a combination of two or more of the above methods (O’Malley 2005), see Table 2 for a summary. The mode employed by different species is a result of evolutionary adaptation to a specific habitat or lifestyle, including environmental features such as the presence of permanent water bodies, temporary water bodies, and the dissolved oxygen content of said water (Wright 2001).
Table 2: Modes of respiration in Amphibia
|Anura||Yes||Yes||Yes||Yes, as larvae|
|Caudata||Yes, except Plethodontidae||Yes||Yes||Yes, as larvae and in neotenic sp.|
Amphibian lungs are simple, delicate, sac-like structures. Mammalian lungs contain a network of tiny bubble or sac-like structures called alveoli to maximize surface area, allowing for greater gas exchange with each breath. In amphibian lungs there are no alveoli, and gas exchange is not very efficient compared to mammalian lungs. It is, however, sufficient as an amphibian’s metabolic rate is comparatively lower (requiring less oxygen and producing less carbon dioxide) than a similarly sized mammal and they rely on multiple modes of respiration to provide enough gas exchange.
NECROPSY PHOTO FROG LUNG
Amphibians lack a diaphragm, instead relying on combination of limb muscle movement for pulmonic respiration, and a process called buccal pumping: Air is sucked into the oral cavity through the nostrils then forced into the trachea and lungs by the muscles of the buccal pouch (O’Malley 2005, Whitaker and Wright 2001a). This also exposes the buccophayngeal tissue (another respiratory surface) to air for supplementary gas exchange.
Cutaneous respiration is present in all amphibians, although the degree of its efficiency is variable with the skin’s surface area and vascularization. Oxygen is diffused into the bloodstream, and carbon dioxide is diffused out. Note that many species have anatomical adaptations, such as skin grooves or folds, to allow for a greater surface area for gas exchange (Wright 2001). This is especially pronounced in completely aquatic species (Wright 2001). Perhaps the most elaborate example of this is the Titicaca Water Frog (Telmatobius culeus, Leptodactylidae), found in the poorly oxygenated waters of Lake Titicaca in South America (AmphibiaWeb 2004).
Branchial respiration occurs in the larvae of Anura and Caudata, and any neotenic adults of Caudata who retain gills (Wright 2001). Gills often have a “feathered” appearance, another adaptation to increase the surface area exposed for gas exchange.
Amphibians have permeable, moist skin, and as a result they usually live in humid environments, or within close proximity to sufficiently humid microhabitats
5: Their skin is sensitive, and multifunctional
Amphibian skin is an extremely important, multifunctional organ. As we have just explored, it is used in gas exchange, and also plays a role in osmoregulation (basically, the maintenance of fluid and electrolyte balance). The skin is also a defensive organ: It may serve as camouflage or as a predator deterrent, and generally contains poison glands.
Amphibians have permeable, moist skin, and as a result they usually live in humid environments, or within close proximity to sufficiently humid microhabitats. They are always at a risk of evaporative water loss, and the skin must be moist for gas exchange to occur (O’Malley 2005). Most water uptake occurs through the skin, as water absorption from the digestive tract is negligible in most species (Wright 2001).
Anurans possess a small patch of skin located at the posterior ventrum (in the pelvic or inguinal region) called the drink patch. This patch of skin has a relatively large blood supply and in many species, it is estimated that 90% of their fluid uptake occurs at this highly vascularized, thin-skinned site. Note that cutaneous diseases may show up first at the drink patch, so it is worth observing on a physical exam (Wright 2006a).
Anurans may urinate in defense and this can present a risk for dehydration if the animal is already stressed. The amphibian bladder can act something like a water storage organ. Water (and sodium) is reabsorbed through the bladder wall. This may minimize the amount of water lost to urination to expel nitrogenous wastes (waste products from protein digestion and metabolism) (O’Malley 2005, Wright 2001).
Water conservation techniques in amphibians largely involve managing the skin as a permeable surface. Retreating to a humid or wet microhabitat is the most widespread “technique” among amphibian taxa. Arboreal anurans have perhaps the widest range of behavioural and physiological adaptations with regards to preventing water loss. Many species will adjust their posture, skin colour, and skin secretions to limit evaporative water losses. The most extreme example of this can be seen in the Waxy Monkey Leaf Frog, Phyllomedusa sauvagii (Leptodactylidae, Phyllomedusinae), and a few other species in this genus: These animals possess specialized skin glands (alveolar glands) that produce a lipid material the frogs wipe all over exposed their skin surfaces to significantly reduce evaporative water loss (Blaylock et al. 1976). These animals are
also some of the few uricotelic amphibians (Shoemaker and McClanahan 2005), as opposed to the more common ureotelism and ammonotelism.
Amphibian skin contains three types of glands (not counting the specialized glands in Phyllomedusinae). As mentioned previously, mucous glands help to keep the skin moist by providing a water-conserving mucous barrier. Granular glands produce various toxins for defense. They may be dispersed throughout the skin or localized (such as the parotid glands of Bufonidae, the true toads). Hedonic glands, found only on the chins of some caudates, produce pheromones and are important during courtship (O’Malley 2005, Wright 2001a).
Many poisonous amphibians boast aposematic colouration, obvious and uncryptic colouration, as a deterrent to predators as it acts as a warning signal the prey animal is dangerous. Some harmless species boast false aposematic colouration to benefit from learned avoidance from predators, a form of Batesian mimicry. Conversely, there are many species that employ cryptic colouration and skin ornamentation to avoid predation by camouflage (Vitt and Caldwell 2009). The global ecological disaster that is amphibian population declines and extinctions can be partially attributed to their sensitivity to environmental toxins. Their skin is quite permeable, and environmental contaminants can easily diffuse into the body. Pollution, cleaning products and organic waste are all threats to wild and captive amphibians (O’Malley 2005, Wright 2001).
Cage cleaning and disinfection must be done with special care taken to select products that will not harm the animals inside. Dish soap is an effective cleaning agent. Disinfectants such as povidone iodine, alcohol, chlorhexidine and quaternary ammonium compounds are known to cause skin irritation and even lesions. Bleach (sodium hypochlorite) is effective, but residue may leach out of aquarium silicone and potentially harm cage inhabitants. Hydrogen peroxide (or accelerated hydrogen peroxide) is effective and generally safe. All cleaning and disinfecting products must be rinsed extremely well to avoid potentially harmful residues (Whitaker and Wright 2001).
If handling is necessary, it is imperative you wear non-powdered, dampened nitrile gloves. This protects the animal from your skin excretions (sweat and oils) and any skin products you may be wearing, and reduces the friction on the amphibian’s delicate skin. Latex may potentially be harmful to amphibian skin, although there are only a few accounts in the literature (Whitaker and Wright 2001).
6: Amphibian populations are in serious danger
Various environmental and climatological changes are threatening amphibian populations worldwide. Their declines have been noted since the 1980s, starting with the Monteverde Golden Toad, Bufo periglenes, in Costa Rica (National Geographic 2010).
The threat to amphibian populations is multi-faceted (AmphibiaWeb 2009):
Disease: Several diseases threaten various populations, either introduced by people or alien species, or their effects exacerbated by environmental factors.
Pollution and pesticides: Considering the permeable nature of amphibian skin, environmental contaminants can have severely damaging effects.
Noise pollution: Interference with male mating calls in urban areas reduces breeding success.
Habitat destruction and degradation: Reduction in the quantity and quality of habitat causes direct mortalities and affects reproductive success.
Collection for the pet trade: This often happens secondary to habitat destruction (animals who survive forest clear-cutting are easy pickings), but it is also an economically important trade in developing countries. As long as there is a demand for wild caught animals, there will be a supply.
Increased UV exposure from ozone layer degradation: DNA damage can occur from increased exposure to UV-B radiation, especially in amphibian eggs.
Chytridiomycosis, a fungal disease involving infection with the pathogen Batrachochytrium dendrobatidis (often abbreviated as Bd) in anurans and Batrachochytrium salamandrivorans (or Bsal) in caudates has been linked to amphibian deaths on every continent except Antarctica (where there are no amphibians) (AmphibiaWeb 2009, 2015). It is thought the aquatic fungus Bd is originally from Africa, as the fungus has been found on museum specimens of Xenopus laevis, the African Clawed Frog (Weldon et al. 2004).
In the 1930s, these frogs were exported to medical labs worldwide for use in human pregnancy tests. Xenopus were released by the thousands into non-native habitats once commercial, non-frog pregnancy tests were developed. A few species appear to be less affected or unaffected by the fungus (allowing these animals to be reservoirs) however to most anurans it is devastating. Recent research has uncovered that crayfish may be the main reservoir and vector for transmission of Bd between inland water bodies (McMahon et al. 2012).
In late 2013, it was determined that some micro-predators such as rotifers and ciliated protozoa prey on the zoospores of Bd. Greater concentrations of these microorganisms correlated with lower rates of infection in anuran tadpoles, providing some hope for chytrid fungus biocontrol (Schmeller et al. 2013).
The fungus infects the keratin layer of the skin, causing hyperkeratosis (a proliferation of the tough keratin outer layer of the skin). This affects various physiological functions such as respiration, osmoregulation (including electrolyte homeostasis and fluid balance) and resistance to other infections. If they do not succumb to other infections first, the loss of electrolytes eventually causes cardiac arrest (Bradley et al. 2006, Trenton et al. 2006).
Bsal affects caudates differently than how Bd affects anurans. Thought to have originated from Asian native species that were imported into Europe and the USA for the pet trade, Bsal causes ulcerating lesions that contribute to severe electrolyte loss and leave the animal susceptible to secondary infections (AmphibiaWeb 2015b). In the Netherlands, this fungus has been linked to the deaths of an estimated 99.9% of the Fire Salamander (Salamandra salamandra) population since 2013 (Nelsen 2016), and is the reason for the current trade restrictions in the United States in order to protect native American caudates (Zimmer 2016).
Amphibians may be promising sources of new pharmaceuticals: Epibatidine, for example, is an analgesic 200 times more potent than morphine, and it was discovered in the tiny poison dart frog Epibates tricolor
Amphibian extinctions are a serious problem, as they are happening at an alarming rate. Wildlife and biodiversity certainly have an intrinsic value. Biodiversity is inherently important, as the more diverse an ecosystem the more stable it is (Fridley 2001). Amphibians have been regarded as an “indicator species;” because of their vulnerability to environmental changes, their population declines are suggestive of serious environmental problems. Amphibians may be promising sources of new pharmaceuticals: Epibatidine, for example, is an analgesic 200 times more potent than morphine, and it was discovered in the tiny poison dart frog Epibates tricolor (this particular drug unfortunately has some serious gastrointestinal side effects in humans, limiting its use) (Wikipedia 2013).
There are many things that you, as a hobbyist, can do to help wild amphibian populations:
- Purchase captive-bred animals from reliable breeders, and do not encourage the wild caught pet industry.
- If you are interested in breeding your animals, do it responsibly. Maintain contact with other breeders to help maintain viable pet populations that are genetically healthy (a rather complex topic that even zoological institutions have difficulty mastering).
- Do not release unwanted pets into the wild, even if they are “technically native” species: You can be introducing alien pathogens into an ecosystem that may not be able to adapt.
- Do not use bait amphibians for fishing: They are important disease vectors that carry disease into new ecosystems. Observe wild herps responsibly: Disinfect herping equipment (like snake hooks and your hiking or water footwear) in between trips to different locales.
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