American tailed caecilians

(Rhinatrematidae)

Class Amphibia

Order Gymnophiona

Family Rhinatrematidae

Thumbnail description
Characterized by the presence of a tail and a terminal mouth; some species are more or less uniform lead gray in coloration, while others are lead gray with yellowish lateral stripes

Size
Adults range in size from 7.7 to 12.9 in (195–328 mm) in length

Number of genera, species
2 genera; 9 species

Habitat
Leaf litter and burrows in tropical rainforests

Conservation status
Not classified by the IUCN; population data unknown

Distribution
Northern South America

Evolution and systematics

Before 1968 all caecilians were placed in a single family. In 1968 E. H. Taylor described two new families, Ichthyophiidae and Typhlonectidae; the former family included caecilians now placed in the family Rhinatrematidae. Taylor's Ichthyophiidae was characterized by the presence of a tail (all other groups have no tails) and breeding habits that included egg laying and an aquatic, feeding larval stage in the life cycle. Because no information was available about the life history of the majority of species, the features of egg laying and a larval stage in the life cycle were highly speculative. Even today nothing is known about the life history of most species of Taylor's "Ichthyophiidae."

The "Ichthyophiidae," as envisioned by Taylor, included caecilians from Southeast Asia and South America. This posed an interesting biogeographic question, because "ichthyophiids" were not known to occur in the geographic regions between southwestern Asia and South America (Africa, Madagascar, Seychelles). In the 1970s R. A. Nussbaum embarked on a broad-based anatomical investigation of caecilians with the aim of gaining a better understanding of their evolutionary and biogeographical relationships. An important result of these studies was the discovery that Taylor's "Ichthyophiidae" contained two highly distinct, relatively primitive groups, one (Ichthyophiidae) restricted to Southeast Asia and the other (Rhinatrematidae fam. nov.) restricted to northern South America. Nussbaum published these results in 1977, along with information about the rediscovery of the dual jaw-closing mechanism, which characterizes all caecilians and hence identifies the Gymnophiona.

All vertebrates, except caecilians, have a single pair of muscles used for closing the jaws. These are the paired adductor mandibulae muscles that pull up on the lower jaws via their attachment in front of the jaw joint. Caecilians have this primitive mechanism of jaw closure, but they also have a second, novel component of jaw closure that enlists (in the evolutionary sense) a new muscle to assist in closing the jaws. This new muscle, the interhyoideus posterior, pulls down on a part of the lower jaw (retroarticular process) that projects behind the jaw joint, causing the forward, toothed portion of the lower jaw to close by rotating upward. The action is much like that of a teeter-totter.

The two genera of the Rhinatrematidae have a relatively primitive dual jaw-closing mechanism, in which the retroarticular

process is short and straight and the interhyoideus muscle is relatively small and unmodified. The ichthyophiid genera (sensu Nussbaum) have a longer and dorsally curved retroarticular process of the lower jaw and a well-developed interhyoideus muscle, suggesting a more highly developed and evolutionarily advanced mechanism. Other attributes of the rhinatrematids also suggest that they are the basal or most ancestral group of caecilians and that, by comparison, the Southeast Asian ichthyophiids are advanced. For example, rhinatrematids have terminal mouths in which the lower jaw is as long as the upper jaw; in ichthyophiids, the lower jaw is recessed, presumably for better burrowing efficiency. The rhinatrematids also have sensory tentacles in the presumed ancestral position adjacent to the eyes, compared with ichthyophiids, in which the tentacles are positioned somewhere between the eyes and the nostrils. No subfamilies are recognized.

Physical characteristics

Rhinatrematids are medium-sized caecilians with true, albeit short, tails. Radiographic studies show that rhinatrematids have a few vertebrae posterior to the cloacal opening, hence a true tail. Other caecilians have fleshy "terminal shields" that project behind the cloaca, but these shields contain no vertebrae and therefore are not true tails. Rhinatrematids have primary annuli, which are subdivided by secondary and tertiary annuli. All annuli are complete and orthoplicate (straight folds). The mouth is terminal, and the tentacles are adjacent to the eyes. The skulls are distinctly zygokrotaphic (with temporal openings through which the adductor mandibulae muscles bulge). The lower jaws have short, straight retroarticular processes, which attach to a weakly developed interhyoideus muscle that aids in jaw closure. The hyoid apparatus of adults has only two or three elements, which decrease in size posteriorly. The ground color is either lead gray (purplish lead gray in life) or lead gray with yellowish lateral stripes along the body.

Distribution

Northern South America, including parts of Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru, Surinam, and Venezuela.

Habitat

All species are denizens of tropical forests, where they occur in moist habitats associated with leaf litter, rotten logs, and burrows in soil. Larvae occur mainly in streams but occasionally are found in sluggish waters associated with streams.

Behavior

Nothing is known of the behavior of rhinatrematids beyond the facts that they burrow in soil and leaf litter and that they sometimes twist their bodies rapidly when subduing prey organisms that have been grasped in the mouth.

Feeding ecology and diet

As with most caecilians, nothing much is known about the feeding habits of rhinatrematids. Digestive tracts of museum specimens contain large amounts of soil, suggesting that earthworms have been ingested; occasionally undigested earthworms have been noted. Remains of arthropods, mostly insects, also have been found in their guts.

Reproductive biology

There are no studies available concerning the reproductive biology of rhinatrematids. The absence of developing embryos in the oviducts of museum specimens and the presence of large eggs in the ovaries suggest that most species are egg layers. Recently, larvae of Rhinatrema bivittatum were discovered in French Guiana. At least one species of Epicrionops, E. marmoratus, also has indirect development with functional larvae in the life cycle. All records of caecilians that deposit eggs in terrestrial nests indicate that the females engage in maternal care, coiling around the developing eggs. Although this has not been recorded for rhinatrematids, it likely occurs, based on phylogenetic history.

Conservation status

Although not threatened according to the IUCN, caecilians (all species) are rarely observed. For the most part, it is unclear whether this is because they are rare or because they are highly secretive and difficult to find. For this reason, and because so few people have directed their efforts toward caecilian research, the conservation status of most caecilians is completely unknown.

Significance to humans

None known.

Species accounts

List of Species

Marbled caecilian

Epicrionops marmoratus

taxonomy

Epicrionops marmoratus Taylor, 1968, Santo Domingo de los Colorados, Ecuador.

other common names

None known.

physical characteristics

Relatively large (up to 11.8 in [300 mm] in length), stocky rhinatrematid with a long tail (up to 0.9 in, or 22 mm, long). The dorsal ground color is dark lavender with scattered yellowish blotches; the sides and ventral surfaces are yellow with scattered dark lavender spots.

distribution

Occurs on the Pacific slope of Ecuador.

habitat

Inhabits pristine rainforest at middle elevations and also lives along streams in deforested areas.

behavior

The behavior of this species, other than burrowing and feeding in captivity, is completely unknown. These caecilians can form their own burrows in moist soil in terraria. They discover earthworms and crickets by scent and lunge forward to grasp them in their jaws. Larger earthworms, capable of struggling when grasped, elicit a twisting response, in which the caecilian rapidly spins on its longitudinal axis. This often results in the earthworm being twisted in half; the grasped part then is swallowed. This behavior, which is characteristic of many caecilians, is reminiscent of the technique used by crocodilians to subdue and rip apart their prey.

feeding ecology and diet

Earthworms and small litter and soil arthropods have been found in the digestive tracts of marbled caecilians.

reproductive biology

As with most caecilians, the reproductive biology of this species is poorly understood. Larvae of this species have been found in leaf litter and stone rubble on the bottoms of small streams. Along with the absence of embryos in the oviducts of adults, this finding suggests that this is an egg-laying species.

conservation status

Not threatened.

significance to humans

None known.

Two-lined caecilian

Rhinatrema bivittatum

taxonomy

Caecilia bivittatum Guérin-Méneville, 1829, Guyane [Cayenne], French Guiana.

other common names

None known.

physical characteristics

These are relatively small (7.4–8.5 in [188–215 mm] in length), slender rhinatrematids with a transverse cloacal opening and a short tail (less than 0.11 in[2.8 mm] long). They are lead gray in coloration with paired yellowish lateral stripes.

distribution

These caecilians occur in French Guiana, Surinam, Guyana, and Brazil.

habitat

They inhabit the litter layer and soil in tropical rainforest.

behavior

Nothing is known about their behavior.

feeding ecology and diet

The stomachs of museum specimens contain remains of earthworms and insects.

reproductive biology

Their reproductive habits are largely unknown. Larvae of this species have been discovered, showing that the species has indirect development.

conservation status

Not threatened. The IUCN and CITES do not classify these caecilians. Although they are rare in collections, reports suggest that the species is not uncommon in French Guiana.

significance to humans

None known.

Resources

Books

Taylor, Edward Harrison. Caecilians of the World: A Taxonomic Review. Lawrence: University of Kansas Press, 1968.

Periodicals

Nussbaum, R. A. "The Evolution of a Unique Jaw-closing Mechanism in Caecilians (Amphibia: Gymnophiona) and Its Bearing on Caecilian Ancestry." Journal of Zoology (London) 199 (1983): 545–554.

——. "Rhinatrematidae: A New Family of Caecilians (Amphibia: Gymnophiona)." Occasional Papers of the Museum of Zoology, University of Michigan no. 683 (1977): 1–30.

Nussbaum, R. A., and M. S. Hoogmoed. "Surinam Caecilians, with Notes on Rhinatrema bivittatum and the Description of a New Species of Microcaecilia (Amphibia, Gymnophiona)." Zoologische Mededelingen 54, no. 14 (1979): 217–235.

Nussbaum, R. A., and M. Wilkinson. "On the Classification and Phylogeny of Caecilians (Amphibia: Gymnophiona): A Critical Review." Herpetological Monographs 3 (1989): 1–42.

Ronald A. Nussbaum, PhD