Gymnophiona

(Caecilians)

Class Amphibia

Order Gymnophiona

Number of families 5

Number of genera, species 33 genera, 165 species

Evolution and systematics

The fossil record for caecilians is primarily of a few vertebrae and other small elements; however, exquisite fossils from the Kayenta formation of Arizona include nearly complete skeletons that have small limbs, a moderately long tail, the vertebral column, and complete skulls. The fossils are late Jurassic in age, and have features that clearly identify them as gymnophiones. Vertebrae of approximately the same age have been found in the Sahara of Africa, and in Paleocene, Miocene, and Quaternary sites in South America and Central America, respectively. The Kayenta fossils clearly demonstrate a transition toward the limblessness, body elongation, and tail reduction characteristic of extant taxa. Only the Quaternary vertebra from southern Mexico has been assigned to a family, the Caeciliidae, definitively.

Five families are recognized according to current phylogenetic treatments (e.g., Hedges, et al. 1993). They include the basal Rhinatrematidae of South America, which with the family Ichthyophiidae of southeast Asia constitutes the sister group to all other caecilians, the Indian Uraeotyphlidae, the pantropical and phylogenetically poorly known Caeciliidae, and the east and west African Scolecomorphidae. However, most experts on caecilian biology also recognize a sixth family, the Typhlonectidae, because of its special features and because of the multiple paraphyly of the family Caeciliidae. Resolution of the relationships of genera now included in the Caeciliidae is likely to result in the designation of additional families.

Physical characteristics

Caecilians are limbless, elongate, usually tailless amphibians. They have a somewhat flattened head with a large, usually underslung mouth, anterior nostrils, and an extrusible organ called the tentacle between the nostril and the eye region. The tentacle is a chemosensory and tactile structure, composed largely of "remnant" elements of the eye, but with a fluid-filled channel that runs from the tip to a sensory chamber that opens into the olfactory lobe of the brain. The eyes of caecilians are small and covered by skin; they are also covered by skull bone in some species. The eyes of many species have lost some to all of their musculature, and some have modified or lost the lens, and reduced the retina and the optic nerve. However, most caecilians apparently can distinguish light and dark.

The mouths of caecilians have two rows of teeth on the upper jaw, and one or two on the lower. The tooth crowns have different shapes among species, but all are hinged, and usually recurved backward, apparently to prevent the loss of prey items. The features of the bones, and to a lesser degree, the muscles, and other organ systems provide characters for identifying species, as well as understanding the biology of caecilians. The bodies of caecilians are encircled by grooves, correlated loosely with the number of vertebrae. There is always one groove per vertebra beginning a few vertebrae behind the head; some species have two grooves per vertebra, especially on the posterior, and the most basal species have three grooves per vertebra the length of their bodies. The basal species also have short tails, which also bear grooves.

Most caecilians are a dark gray to gray-brown to deep purple color, often with a lighter head and venter. However, some are more brightly colored; for example, a species on the African island of Sao Tome, Schistometopum thomense, is bright yellow. A South American species, Siphonops annulatus, is deep blue-purple with bright white body grooves. Several of the basal species are dark gray to gray-brown to brown-black with bright yellow side stripes. Most adult caecilians are between 11.8 and 23.6 in (300 and 600 mm) long; however, some

species are much smaller or larger. The smallest adult caecilians, the miniaturized Idiocranium russeli, are about 4.5 in (115 mm) long. The largest, Caecilia thompsoni, are more than 63 in (160 cm) long.

Distribution

In the New World, caecilians occur from mid-Mexico through Peru, Bolivia, Paraguay, and northern Argentina. The aquatic or semi-aquatic "typhlonectids" inhabit primarily the Orinoco and Amazonian drainages, but occur from Colombia to northern Argentina and Uruguay. Old World species are found in East and West Africa south of the Sahara and north of the Temperate Zone, the Seychelles Islands, India, Sri Lanka, Southern China, Cambodia, Laos, Vietnam, and much of Malaysia to the southern Philippines, but are not known from Madagascar, much of central Africa, and the Australian-Papuau Region.

Habitat

Caecilians typically are fossorial, living in moist organic soil, leaf litter, and (rarely) in the axils of plants just above the substrate. The "typhlonectids" are aquatic or semi-aquatic, and nose about in the substrate to find food, or scrape it from logs, rocks, and the like in the water or stream banks.

Behavior

The skins of caecilians have mucous and serous glands; the latter secrete a substance ("poison") that is toxic to many potential predators.

Feeding ecology and diet

Caecilians are carnivores; terrestrial species prey on animals that they can reach in the substrate, such as earthworms, termites, orthopteran instars, and many other invertebrates. Larger caecilians are known to eat lizards and baby rodents. Snakes and birds are predators that feed on caecilians, and some snakes apparently specialize on them.

Reproductive biology

Members of the two basal families, the Rhinatrematidae and the Ichthyophiidae, as well as the Uraeotyphlidae, one of the two genera of the Scolecomorphidae, and several members of the large family Caeciliidae, are egg layers with free-living larvae that have small gills and tail fins. So far as is known, eggs are laid in burrows or under grass or litter on land, the mother guards the clutch, and the newly hatched larvae wriggle into nearby streams which they inhabit until they metamorphose, when they again become terrestrial. Some caeciliid species have direct development, in which the embryos of the land-laid clutch develop through metamorphosis, so that juveniles hatch and the aquatic larval period is avoided. Derived species of caeciliids and scolecomorphids, and the "Typhlonectidae," are viviparous, retaining the developing eggs in the female's oviducts, where she provides nutrients to the embryos after the yolk of the eggs has been resorbed. Pregnancies are seven to 11 months, depending on the species, and fully metamorphosed juveniles are born. The embryos of species with larvae have three pairs of gills, which are reduced at hatching, though one or more gill slits remain open. Embryos of terrestrial direct-developing and viviparous species also have triramous gills; in the "typhlonectids," the gills are fused during development and form large sac-like structures that may aid in intraoviductal transport of gases and nutrients. The fetuses of viviparous species, and apparently some with direct development or larvae, have a fetal dentition in which the tooth crowns have a distinct morphology and distribution that is quite different from that of the adults. The live-bearers apparently use the dentition to ingest the nutrient material secreted by the epithelium of the oviducts and to stimulate its production. The fetal teeth are shed at or near birth. Several studies have added new information about the developmental biology of various species, following nearly 100 years of little research on caecilian development.

Resources

Books

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Himstedt, W. Die Blindwuehlen. Marburg: Die Neue Brehm-Buecherei Bd. 630, Westart Wissenschaften and Heidelberg: Spektrum Akademischer Verlag, 1996.

Taylor, Edward H. The Caecilians of the World. Lawrence, KS: University of Kansas Press, 1968.

Wake, Marvalee H. "The Osteology of Caecilians." In Amphibian Biology. Vol. 5, Osteology, edited by Harold Heatwole and Margaret Davies. Chipping Norton, Australia: Surrey Beatty and Sons.

Periodicals

Duenker, Nicole, Marvalee H. Wake, and Wendy M. Olson. "Embryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table." Journal of Morphology 243 (2000): 3–34.

Hedges, S. Blair, Ronald A. Nussbaum, and Linda R. Maxson. "Caecilian Phylogeny and Biogeography Inferred from Mitochondrial DNA Sequences of the 12S rRNA and 16S rRNA Genes (Amphibia: Gymnophiona." Herpetology Monographs 7 (1993): 64–76.

Jenkins, Farish A., and Dennis M. Walsh. "An Early Jurassic Caecilian with Limbs." Nature 365 (1993): 246–250.

Nussbaum, Ronald A., and Mark Wilkinson. "On the Classification and Phylogeny of Caecilians (Amphibia: Gymnophiona), a Critical Review." Herpetolgy Monographs 3 (1989): 1–42.

Savage, J. M., and M. H. Wake. "A Re-evaluation of the Status of Taxa of Central American Caecilians (Amphibia: Gymnophiona), with Comments on Their Origin and Evolution." Copeia 2001 (2001): 52–64.

Wake, Marvalee H., and David B. Wake. "Early Developmental Morphology of Vertebrae in Caecilians (Amphibia: Gymnophiona): Resegmentation and Phylogenesis." Zoology—Analysis of Complex Systems 103 (2000): 68–88.

Marvalee H. Wake, PhD