Buried-eyed caecilians

(Scolecomorphidae)

Class: Amphibia

Order: Gymnophiona

Family: Scolecomorphidae

Thumbnail description
Small to medium-sized caecilians with a recessed mouth, tentacular apertures far forward on the snout, and eyes attached to the base of tentacles, which move with the tentacles; usually dark lavender-gray above and cream to flesh-colored below

Size
Adult scolecomorphids range in size from 6.3 to 18.2 in (160–463 mm) in total length

Number of genera, species
2 genera; 6 species

Habitat
Tropical rainforests and deforested areas, generally in mountainous regions

Conservation status
Not classified by the IUCN; population data unknown

Distribution
Western (Cameroon) and eastern (Malawi and Tanzania) equatorial Africa

Evolution and systematics

In 1968 E. H. Taylor showed that caecilians are far more diverse than their rather uniform external morphologic features had suggested. At the time, caecilians were placed in a single family, the Caeciliidae. Taylor described two new families but left the majority of genera in the Caeciliidae, noting that various genera eventually might have to be removed from that family. In 1969 Taylor removed the genus Scolecomorphus from the Caeciliidae and placed it in a new family, the Scolecomorphidae. Species of this new family have several unusual and highly derived characteristics that set them broadly apart from all other caecilians, including a mobile eye and lack of a stapes (sound-conducting bone) in the middle ear. No subfamilies are recognized.

At the time Taylor described the Scolecomorphidae, he recognized six species in a single genus. In 1981 R. A. Nussbaum described a seventh species of Scolecomorphus, and in 1985 he partitioned the latter genus into two genera, Scolecomorphus with three species (two were lost to synonymy) and a new genus, Crotaphatrema, with two species. Subsequently (2000), D. P. Lawson described a third species of Crotaphatrema.

Scolecomorphids, along with another caecilian family (Caeciliidae), are evolutionarily derived compared to the "tailed caecilians" (Rhinatrematidae, Ichthyophiidae, and Uraeotyphlidae). Species of the former two families are advanced in several ways. They lack tails and have fewer and more solidly fused skull bones. The skin segmentation is reduced; only primary and secondary annuli (rings or folds) occur and often only primaries. There are also fewer scales and usually no larval stage in the life cycle. Scolecomorphids are perhaps the most specialized of the two advanced families. They differ from caeciliids in several uniquely derived features, including lack of a stapes in the middle ear and presence of an eye that is attached to the base of the tentacle, which can be moved outside the skull when the tentacle is protruded.

Physical characteristics

Scolecomorphids lack stapes and internal processes (bony projections) on the pseudoangular bones of the lower jaw. There are no secondary annuli; the number of primary annuli ranges from 120 to 153 and the number of vertebrae from 131 to 165. There are typically no dermal scales in the annular folds; rarely, a few tiny vestigial scales may be present in the posterior folds. Annuli are usually complete anteriorly but may be fused dorsally, ventrally, or both, especially along the middle and posterior parts of the body. The mouth is subterminal. The vestigial eyes are attached to and move with the tentacles and may be exposed when the tentacles are extruded. Normally, the eyes are under bone, as there are no orbits. The tentacular apertures are positioned ventrolaterally on the snout, below the nostrils and even with, or slightly anterior to, the anterior margin of the mouth. Each tentacle has an external subglobular base that is surrounded partly or entirely with a groove and a central opening through which the body of the tentacle passes when it is extruded or retracted. There is no tail; instead, there is a terminal shield without annuli. The terminus is bluntly rounded and flattened ventrally. The longitudinal vent lies in a shallow, oval depression only a few millimeters longer than the vent. The tongue lacks narial plugs. Temporal openings may be present (Scolecomorphus) or absent (Crotaphatrema). Some species have calcified spines on the phallodeum (penis), but this has not been seen in all species.

R. A. Nussbaum noted that some scolecomorphids have an interesting pattern of sexual dimorphism in which the females are the larger (longer) sex. This occurs because females have considerably more vertebrae (and primary annuli) than males; it may be advantageous, because the elongation of the body provides more space for developing fetuses. Males have larger heads than females of comparable size. R. A. Nussbaum and M. Pfrender found this to be true of several species of caecilians in different genera and families. It may be related to male combat in competition for mates or territory, as suggested by bite marks found on caecilians in captivity and in nature.

Distribution

Scolecomorphids are restricted to eastern and western equatorial Africa. Scolecomorphus occurs in Tanzania and Malawi, whereas Crotaphatrema is restricted to Cameroon. No caecilians of any kind have been found in central equatorial Africa. This distribution pattern is anomalous, because the vast region of the upper Congo seems ideally suited for caecilians. Caecilians likely occur in this region, and scolecomorphids are among the most likely candidates to be found there.

Habitat

Like most caecilians, scolecomorphids inhabit tropical rainforests and adjoining deforested areas. They usually are found in moist areas under logs and in leaf litter on the forest floor. They also can be dug up from moist soil. Most specimens were seen in hilly or mountainous regions. In Tanzania and Malawi, they have been found in turned soil and piles of vegetation in farming regions.

Behavior

These caecilians are seen rarely, and little is known about their behavior. They are excellent burrowers, and they pump their tentacles in and out when they are moving and otherwise investigating their environment. As with all caecilians, their tentacles are thought to be chemosensory organs used for "tasting" their immediate surroundings.

Feeding ecology and diet

Little is known about the feeding habits of scolecomorphids. Soil, earthworms, and insects have been found in their digestive tracts. They readily eat earthworms and small crickets in captivity.

Reproductive biology

The three species of Scolecomorphus are viviparous. The young are retained in the oviducts, were they are thought to feed on "uterine milk," a nutritious substance secreted by their mother's oviducts and ingested with the aid of specialized embryonic or fetal teeth. These teeth are comblike, with multiple crowns, and they also may be used to stimulate the oviduct to secrete "milk" near the mouth of the feeding fetus. This remains to be established. The reproductive biology of the three species of Crotaphatrema is unknown. Because they have large, yolky, ovarian eggs, it seems likely that they are egg layers with direct development (lacking a larval stage) and female parental care.

Conservation status

Not classified by IUCN—population data are unknown.

Significance to humans

None known.

Species accounts

Resources

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

Lawson, D. P. "A New Caecilian from Cameroon, Africa (Amphibia: Gymnophiona: Scolecomorphidae)." Herpetologica 56 (2000): 77–80.

O'Reilly, J. C., R. A. Nussbaum, and D. Boone. "Vertebrate with Protrusible Eye." Nature 382 (1996): 33.

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. ——. "Scolecomorphus lamottei, a New Caecilian from West Africa (Amphibia: Gymnophiona: Scolecomorphidae)." Copeia (1981): 265–269. ——. "Systematics of Caecilians (Amphibia: Gymnophiona) of the Family Scolecomorphidae." Occasional Papers of the Museum of Zoology, University of Michigan, no. 713 (1985): 1–49.

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

Taylor, E. H. "A New Family of African Gymnophiona." University of Kansas Science Bulletin 48, no. 10 (1969): 297–305.

[Article by: Ronald A. Nussbaum, PhD]

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