Amphibians – an introduction

23 Amphibians – an introduction



Introduction


There are over 4000 species in the three orders of the class Amphibia. Many are endangered in the wild, particularly due to worldwide spread of the fungal infection chytridiomycosis (Batrachochytrium dendrobatidis) that may also affect captive amphibians. Anurans (frogs and toads) are most commonly kept as pets, although members of the orders Caudata (salamanders and newts) or Gymnophiona (caecilians) may be seen from time to time.



Taxonomy


Taxonomic classification of amphibians commonly seen as pets is given in Table 23.1.



Table 23.1 Taxonomic classification of amphibians commonly seen as pets

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Biology


The thyroid gland is predominantly responsible for controlling metamorphosis, and also ecdysis. Mexican axolotls (Ambystoma mexicanum) are obligate neotenic species, never metamorphosing in nature but doing so in captivity when thyroxine is administered. Tiger salamanders (Ambystoma tigrinum) are a facultative neotenic species, and metamorphose when TSH (thyroid-stimulating hormone) is produced in response to deteriorating environmental conditions.


As ectotherms, it is essential to house amphibians at an appropriate environmental temperature (both at the owner’s home and at the veterinary clinic). Metabolic requirements vary depending on the environmental temperature and level of activity (increasing with illness or recovery from surgery). The preferred optimum temperature range (POTR) required may change according to species, age, season and current bodily functions (e.g. digestion or reproduction). Amphibians do not depend solely on the environmental temperature for thermoregulation. They can vary their body temperature by postural and locomotor changes, peripheral vasodilation and constriction, and changes in skin colour.


Anurans and caudates that hibernate have additional physiological adaptations to lower the freezing point of tissues, and permit ice formation in extracellular compartments. Although many species can survive at very broad temperature ranges, they adapt over a period of time. Rapid fluctuations in temperature should be avoided, as potentially fatal thermal shock may result.


Moisture is essential to amphibians. Physiology differs between terrestrial and aquatic amphibians. The former need to protect against water loss to the environment, while the latter need to cope with the threat of over-hydration. Aquatic species thus excrete excess water and conserve plasma solutes.


These animals have various physiological means of preventing dehydration, such as secretion of water-proofing substances (e.g. South American tree frogs, Phyllomedusa spp), dried mucus on the skin, behavioural changes such as posture variation and limiting activity unless humidity is high. Some species, such as axolotls and mudpuppies, are completely aquatic. Some anurans have a ‘drinking patch’, an area of modified skin on their ventral pelvic region that is responsible for up to 80% of their water uptake.


Amphibian skin is relatively permeable, and is used for gas exchange. Desiccation may occur in excessively dry environments. Terrestrial animals require a relative environmental humidity of 60–80% for temperate species and 70–90% for tropical species. Water quality is extremely important to amphibian health, and parameters should be monitored. Water should be dechlorinated, and filtration or regular replacement of water is necessary for aquatic species or larval forms.


Amphibian kidneys cannot concentrate urine more than the concentration of plasma solutes. In aquatic anurans, ammonia is excreted via the kidneys as well as diffused across the skin. Most terrestrial anurans excrete urea.


Amphibian hearts are three-chambered, comprising two atria and one ventricle. The interatrial septum may be fenestrated, permitting mixing of oxygenated and deoxygenated blood. The lymphatic system is well developed, with lymph hearts (or lymph sacs) that return lymph (blood without erythrocytes) back to the heart.


The haematolymphopoietic system of amphibians differs from other taxonomic groups. Some species have bone marrow, although its function is not the same as in mammals. The spleen is the site of erythropoiesis and myelopoiesis. The thymus remains throughout life (although it may involute with chronic stress or malnutrition). The function of lymph nodes (which are absent in amphibians) is performed by gut-associated lymphoid tissue.


The respiratory system varies between orders and species, and intraspecies depending on life-stage and environment. Amphibians do not have a diaphragm. Most larval stages are aquatic, have external gills and rely on branchial respiration. Gills are often lost when amphibians metamorphose. Adult anuran respiration may be cutaneous, buccopharyngeal and/or pulmonic. In adult salamanders, branchial, cutaneous, buccopharyngeal or pulmonic respiration may occur. Neotenic species such as axolotls (Ambystoma mexicanum) have external gills. Sirens have lungs and gills. Adult caecilians have pulmonic (except for one lungless species), buccopharyngeal and cutaneous respiration.


Amphibian teeth are shed and replaced throughout life. Most anuran and salamander tongues can extend to capture prey. The intestinal tract is relatively short.


Gonads are paired in amphibians, with seasonal fluctuations in reproductive activity. Sexual dimorphism (e.g. nuptial pads developing in male White’s tree frogs (Pelodryas caerulea) during the breeding season) is rarely present. Anurans are external fertilizers, most salamanders are internal fertilizers (via spermatophores), while all caecilians fertilize internally (copulating via a phallodeum). Many amphibians are seasonal breeders; visual and environmental cues are important for reproduction.


Anurans have well-developed senses. These include auditory structures, ocular structures (apart from caecilians), as well as taste, touch and olfaction. Amphibians have a Jacobson’s organ for chemodetection.



Husbandry


Most species require a vivarium with a terrestrial area as well as shallow water. Good water quality should be maintained. Most species return to water to breed. Hide areas should be provided. Temperature and humidity gradients should be present. UV light is required by many species. Deep leaf litter is a good substrate for many amphibians. The vivarium should mimic the native environment for the species as closely as possible.


Many tadpoles are herbivorous (eating natural microflora on pond weed when small), converting to carnivores at metamorphosis. Adult amphibians are all carnivorous, mostly insectivores (being fed on earthworms, crickets, ants and mealworms in captivity), although larger species consume vertebrates (such as pinkies). Invertebrates should be supplemented with vitamins and calcium. Nutritional secondary hyperparathyroidism (NSHP) is common in captive amphibians if an inappropriate diet is consumed.



Anura


Adult anurans do not possess a tail, nor do they have external gills (Fig. 23.1). The hindlimbs are usually longer than the forelimbs, and toes are often webbed. To improve jumping performance, the vertebrae are fused (including the pelvic girdle), with presacral, sacral and postsacral regions. There is no sacrum. Although tadpoles can regenerate lost limbs, generally adults cannot.


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Figure 23.1 Schematic to show major organs in an anuran (ventral view).

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Amphibians – an introduction

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