General Biology

Caecilians (Table 3-1) are elongate, limbless amphibians and may be confused with snakes, eels, or worms because of this morphology. Extant caecilians are genetically most closely related to salamanders.³ They are in the order Gymnophiona and are currently classified into 10 families comprising 192 species; Caeciliidae (42 species), Chikilidae (1 species), Dermophiidae (14 species), Herpelidae (9 species), Ichtyophiidae (53 species), Indotyphlidae (21 species), Rhinatrematidae (11 species), Scolecomorphidae (6 species), Siphonopidae (22 species), and Typhonectidae (13 species).¹ They inhabit tropical climates in Southeast Asia, India, Africa, Mexico, and South America and may be found in subterranean (fossorial), semi-aquatic, and aquatic environments. Most are nocturnal. In one survey of captive animals, the most common species reported were Dermophis mexicanus, Geotrypetes seraphini, Ichthyophis kohtaoensis, Schistometopum thomense, and Typhlonectes natans.¹² Longevity in captivity appears to be approximately 11 years according to medical records available to one author (NDM); anecdotally life span may be up to 20 years.

Caecilians have cylindrical bodies with annular rings and typically range in length from 90 to 1600 mm.²⁸ Dermal scales are covered by skin. The integument is vascular and permeable to water. In many species, specialized skin glands produce toxins, which may cause mucous membrane irritation¹³ and hemolysis (in vitro).²¹ Mucus from Typhlonectes sp. may be toxic to fish, although fish have also been kept successfully with the species.²⁸ Schistometopum sp. may have caused mortality in Feylinia sp. housed in the same enclosure.⁷ Caecilians do not have vestigial limbs or shoulder or pelvic girdles, and the ribs do not support the body.¹⁴ Caecilians lack functional bone marrow, and hematopoiesis occurs in the liver, kidney, spleen, and thymus.³⁸

Most have a functioning right lung and a vestigial left lung,¹⁸ although two lungless species have been described.^29,33 A tracheal lung is described in Typhlonectes natans and a review of the upper respiratory anatomy is available.¹⁸ The respiratory cycle is a single, long exhalation with a series of short inhalations through buccopharyngeal pumping.^2,5,18 This pattern prevents the mixing of inspiratory and expiratory airflow, unlike in the majority of amphibians.^5,18 Respiration may occur by pulmonic, buccopharyngeal, and cutaneous mechanisms, and the normal respiratory rate of T. natans is 4 to 7 breaths per hour.^5,18 Overall, oxygen uptake appears to rely on pulmonic respiration, whereas carbon dioxide elimination is via cutaneous respiration in the species studied.^5,38 In T. natans, respiratory rate approximately doubled in response to aerial hypoxia; aquatic hypoxia had no effect on respiration rate.⁵ Aquatic hypercapnia nearly tripled respiratory rate, whereas aerial hypercapnia had minimal effect on respiratory rate.⁵ Large gills are transiently present in larval caecilians immediately after birth but regress within 48 hours. The metabolic rate in caecilians appears lower than the rate in other amphibian orders.²³

As is common with other amphibians, terrestrial caecilians appear to be uricotelic and aquatic caecilians ammonotelic.²³ Many species have a bilobate bladder. A renal portal system is believed to exist.

Caecilian locomotion varies with species and substrate, but all species studied may utilize concertina movement.⁶ Hydrostatic pressure generated between the skin and the muscle layer are important for burrowing.^6,14 Some species, primarily in the larval stage, have lateral lines with ampullary organs that detect water movement and weak electric fields.^15,22,25 The tentacle (a chemosensory and tactile organ) protrudes from an opening between each eye and nostril and is attached to the tear duct and the vomeronasal (Jacobson’s) organ. Innervation is via the trigeminal nerve, and the tentacle serves as the major means of environmental perception, as ocular development is poor. Rod-type structures and visual pigments are present in the retina; no cone photoreceptors have been identified.¹⁰ Caecilians appear to sense light intensity but not images. The eyes are covered by skin or bone in most species. Prey is detected via olfaction and grasped in the jaws; bicuspid teeth help retain prey and break prey into smaller pieces.^15,28 Most species lack outer and middle ear cavities and audition is limited to detection of low frequency vibrations. Vocalizations are rare and may be heard as squeaks or clicks.²²

Special Housing Requirements

Glass aquaria are most appropriate as enclosures and should have tight-fitting lids for all species. Full-spectrum lighting may be provided on a diurnal light cycle. Fossorial caecilians (Siphonops sp., Pseudosiphonops sp., and Ichthyophis kohtaoensis) live in rich soils with forest litter. In general, 70% to 80% humidity is usually recommended for terrestrial caecilians, but a thorough review of natural habitat is appropriate, as some species may be adapted to dryer environments. Soil substrates should be organically rich and pH balanced (neutral). Soil depth should be 3 to 10 cm and kept moist (except with dry soil species), but not wet, to allow for tunnel formation. Soil should be lightly compacted. Some animals require moisture gradients within the soil, and aquaria may be tilted so that the bottom layers have standing water and the top layers are drier. Soil and leaf litter should be sterilized to prevent introduction of infectious agents or arthropods. Vermiculite is a potential foreign body and should be avoided in the substrate. Peat moss and manure are acidic and may cause skin irritation and should therefore also be avoided. Soil should be replaced every few months. Scolecomorphus sp. do not burrow and need flat surfaces with hiding spots. Enclosures for semi-aquatic species such as Geotrypetes seraphini and Siphonops annulatus should have standing water. Water for semi-aquatic and fully aquatic species such as Typhlonectes sp. and Potomotyphlus sp. should be maintained following freshwater fish water chemistry parameters, and aquariums should have appropriate life support systems, including filtration (mechanical and biologic) and aeration. Dissolved oxygen should be greater than 80% to maintain the biologic filter.³¹ Undergravel filters may be used, but tubing components must be small enough to prevent animal access. A 10% to 20% water change with aged or dechlorinated water should be done every 1 to 2 weeks. Aquatic caecilians depend on breathing air for adequate respiration, and aquariums should provide ready access to the air–water interface; shallow water is generally appropriate. This is particularly important with neonatal larvae. Aquatic species should be provided with terrestrial areas to allow for the full range of natural behavior. The website AmphibiaWeb (amphibiaweb.org) is a routinely updated centralized source of research information and may be consulted for species-specific natural history information.

Feeding

All caecilians are carnivorous. Free-ranging diet includes annelids (earthworms), platyhelminths, arthropods (amphipods, crayfish, orthopterans, lepidopteran larvae, coleopteran larvae, hemipteran larvae, termites, ant eggs), frog eggs, tadpoles, and anoline lizards.^{7,13,16,25,26,28} One African species (Boulengerula sp.) eats only termites,⁸ but most species are less selective.

Successful captive diets include earthworms, turtle gel diets, mealworms, waxworms, tadpoles, chopped fish (e.g., smelt, goldfish), mussels, neonatal mice (pinkies), Tubifex sp., shrimp, tadpoles and frog eggs, and beef or pig heart.

Restraint and Handling

Manual restraint should be limited to reduce disruption of the protective mucus layer, skin, or both. Standard protocols for handling amphibians should be followed, but additional focus on atraumatic capture and restraint such as utilizing clear containers appears warranted with these species during capture and physical examinations.³⁷ For example, clear plastic “bag” nets or simple plastic bags may be used to remove aquatic animals from enclosures and facilitate examinations.⁴⁰ Damp flannel cloth pre-coated with a water-soluble gel or a moistened foam rubber sponge may be useful for further immobilization.³⁷ Alternatively, the animal may be allowed to crawl into a syringe case or smooth clear tube such as those used for snake restraint and be held in place. Medication was successfully administered to Dermophis sp. by restraining in a wet hand, opening the mouth, and medicating with a metal avian gavage tube. Gentle technique is critical; iatrogenic jaw fractures have been anecdotally reported.

Immobilization is achieved with buffered MS-222 (tricaine methane sulfonate) at 1 to 3 grams per liter (g/L) for induction followed by 100 to 200 milligrams per liter (mg/L) for maintenance.³⁰ Unbuffered solutions should not be used as the solution is acidic and may cause skin irritation and escape behavior.⁴⁰ Other sedative and anesthetic drugs have not been reported in the veterinary literature.

Surgery

Standard surgical protocols described for other amphibians are applicable to caecilians.^24,39 In aquatic caecilians, dermatitis and subsequent osmotic imbalance appears to be a major complicating factor that may increase morbidity.^12,24 Clinicians should address preexisting dermatitis prior to surgery and proactively minimize skin disruption during handling.

Persistent cloacal prolapse in T. natans was noted to cause morbidity and mortality in one review.¹² Osmotic imbalance secondary to dermatitis was suspected to be responsible for mortality after management via reinsertion (n = 1) and anastomosis and resection (n = 1).¹² One case of coelomic exploratory surgery in Typhlonectes sp. has been published.²⁴ The animal did not recover from surgery, and the case authors speculate that sepsis and fluid overload secondary to preexisting dermatitis complicated recovery.²⁴ One author (NDM), successfully completed coelomic exploratory in a T. natans. The animal survived for 5 days, and the incision site appeared to be healing normally, but preexisting renal disease led to mortality. Successful surgical debridement and management of a severe tail wound has been reported.¹⁷