Description
Diagnosis: Schistometopum – “Caeciliids with eye not under bone, in socket formed mostly by squamosal, anterior border formed by maxilla; no temporal fossae; mesethmoid exposed dorsally between separated frontals; splenial teeth present; secondary annuli present; scales present; tentacular aperture closer to eye than to external naris; no unsegmented terminal shield; no narial plugs; no diastema between vomerine and palatine teeth; no terminal keel” (Nussbaum and Pfrender 1998).

S. thomense – “A Schistometopum with dorsolateral coloration ranging from immaculate bright yellow to darker yellow with heavy brown freckling in life, yellow color fades to light tan or cream in preservative; 89-105 primary annuli; 94-109 vertebrae; 12-25 splenial teeth” (Nussbaum and Pfrender 1998).

Description: An elongate, limbless amphibian of uniform contour (total length: trunk diameter of mature adult = 21:1, Barboza du Bocage 1873) and striking yellow color, with head sloped from crest of forehead toward terminus of upper mandible, ventral and recessed mouth, vestigial eyes visible through skin, primary and secondary annular grooves, dermal scales present especially toward anterior third of body, and lacking true tail. Size range of adult males reported to be 135-344 mm (n=92), of adult females 129-340/350 mm (n=160) and of neonates 91-118 mm (n = 27 born in captivity) (Nussbaum and Pfrender 1998). Largest individual recorded at 375 mm (Measey and Van Dongen 2006; G.J. Measey, pers. comm.). At birth, young are miniature replicas of the adult form (Nussbaum and Pfrender 1998).

Coloration: In life, background color ranges from bright yellow to darker yellow with no to varied amounts of purple-brown flecking throughout. In preservative, background color ranges from cream to pale yellow or tan with no to varied amounts of darker flecking throughout. Some specimens fade to grayish blue (Nussbaum and Pfrender 1998).

Variation: Chromatic variation exists in degree of flecking: clearer morphs appear more common in the north of the island and heavily flecked morphs appear more common in the south of the island, though geographic assignment of morphotype is not absolute (Haft 1992; Loumont 1992; Schätti and Loumont 1992; Fahr 1993; Haft and Franzen 1996; Nussbaum and Pfrender 1998; Stoelting 2006). The single, purported mainland specimen is described as clear of flecking with meristic and morphometric character states similar to those of S. thomense collected on the island (Nussbaum and Pfrender 1998). Nussbaum and Pfrender (1998), among other authors, report interpopulation variation in mean total length, as well as in degree of head dimorphism between males and females. Per population, mean total length and mean mass follow Bergmann’s Rule, increasing in size as sample sites increase in elevation (Measey and Van Dongen 2006).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Sao Tome and Principe

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (4 records).

Schistometopum thomense is found on São Tomé Island (Republic of São Tomé and Principé, Gulf of Guinea, West Africa), from 0 m to at least 1440 m (R.C. Drewes, pers. comm.). The origin of one specimen purportedly from the Rwenzori region of the Democratic Republic of the Congo (DRC) awaits confirmation (Nussbaum and Pfrender 1998; however, a biologist visiting Ituri Province, DRC, in 2008, reported that local Congolese recognized this species in photographs: G.J. Measey, pers. comm.).

On São Tomé, habitats frequented include secondary low-land forest, plantations (cacao, banana, oil palm, coffee and other crops), small agricultural plots, primary mid-elevation forest, as well as rocky, coastal areas (Haft 1992; Schätti and Loumont 1992; Loumont 1992; Fahr 1993; Haft and Franzen 1996; Nussbaum and Pfrender 1998; Delêtre and Measey 2004; Measey and Van Dongen 2006; Stoelting 2006). Haft and Franzen (1996) reported a sighting at 0 m on sandy shore.

Within these habitats, S. thomense can be found under rotting vegetation, in trash piles, or by digging superficially through loose soils, especially after rains (Schätti and Loumont 1992; Loumont 1992; Fahr 1993; Haft and Franzen 1996; Nussbaum and Pfrender 1998). In drier conditions, these animals are found deeper in the soil (Nussbaum and Pfrender 1998; reported up to 20 cm by Schätti and Loumont 1992, Loumont 1992 and Fahr 1993). They also are present in harder packed, root-infused soils, though here are more difficult to sample and as a result may appear less common (Under experimental conditions, S. thomense showed preference for softer soils and pre-existing burrows over moderate-hard packed [penetrometer readings ≥ 0.078 kg/cm2, Ducey et al. 1993] or unexcavated terrain [Ducey et al. 1993; Teodecki et al. 1998]).

Soil temperatures reported for collection sites ranged from 21.4-25.2 °C in mid-June 1988 (early afternoon across 12 sites, approx. 850 m, in “degraded” habitat; Nussbaum and Pfrender 1998) and 18.3-27.1 °C in October 2002 and September 2003 (13 sites from 15 m to 1360 m, across a variety of habitats; Measey and Van Dongen 2006).

A number of authors have reported S. thomense to be uncommon in the drier, northwestern portion of the island. The distribution in the central southwest of the island has been poorly documented due to limited accessibility and steep terrain.

Life History, Abundance, Activity, and Special Behaviors
Movement/ Burrowing: These thigmotrophic, surface-venturing soil dwellers appear to use rotted-out root tunnels for passage underground, as well as excavate their own burrows (pers. obs.; see Ducey et al. 1993, Haft and Franzen 1996, Teodecki et al. 1998 and Nussbaum and Pfrender 1998 for a variety of comments about burrowing behavior in the laboratory and limited observations in the field). Haft and Franzen (1996) reported that, within a few days of introduction to captivity, S. thomense constructed a large burrow network, which remained mostly unchanged for months thereafter. These caecilians were observed to move backward and forward through passages of 11-14 mm constant diameter (Haft and Franzen 1996). Herrel and Measey (2010) described use of a combination of lateral undulation and internal concertina locomotion for movement above ground (i.e., on a “wet towel”) and a combination of internal and whole-body concertina locomotion below ground. In the wild, observations of surface locomotion include the 1440-foot elevation record where, “Quintino had one crawl over his foot” (R.C. Drewes, pers. comm.), and a photographed instance when an adult female was discovered moving across leaf litter (Fig. 4 of Wollenberg and Measey, 2009; G.J. Measey, pers. comm.).

Reproduction: Reproductive mode in this species is viviparous and is not tied to water. As in all other caecilians, the mature male of S. thomense has an extrusible phallodaeum which is used for internal fertilization. Captive females reproduce biennially (Teodecki et al. 1998). Commentaries on fetal development and intraoviductal feeding have been provided by Parker (1956) and Müller and Measey (2006). Newborn S. thomense appear as miniature adults, having no independent, larval stage and no gill scars apparent (Nussbaum and Pfrender 1998). Nussbaum and Pfrender (1998) reported observing 2-7 neonates per captive brood and collecting gravid individuals in June 1988, which gave birth within days of capture. Haft and Franzen (1996) reported birth in captivity in late January, finding young caecilians loosely knotted together in an enlarged tunnel with a diameter of approximately 25 mm). Nussbaum and Pfrender (1998) documented captive-born neonates ranging from 91-118 mm in total length (n=27).

Growth: Haft and Franzen (1996) reported captive neonates doubling in size within nine months, but slowing growth after two years. At approximately 130 mm total length, the sexes begin to exhibit dimorphism in head shape, with males expressing larger, blunter heads than females; Nussbaum and Pfrender’s (1998) regressions of head length and width against body length for males and females showed significant dimorphism at approximately 150 -200 mm total length (Figure 8, p. 15). Measey and Delêtre [2004] categorized specimens < 130 mm total length as sex indeterminate (i.e., “juveniles”).

Demography: Demography of the species is not well documented. Delêtre and Measey (2004) reported 30% juveniles in one sample in northern São Tomé in Oct-Nov 2002, but 0% juveniles at a higher elevation, primary forest site during roughly the same period. Pooling the 2002 data with September 2003 collections, Measey and Van Dongen (2006) reported 8/13 sites occupied by juveniles and found the overall sex ratio of adults to be female-biased (1: 1.3 for 13 pooled sites).

Abundance: Using standardized methods to sample leaf litter to soil depth of 30 cm, Measey (2006) found an average density of 0.298 S. thomense per m2 at four occupied sites in October 2002. From larger-scale sweeps, he calculated that an average of 255 S. thomense could be expected per hour spent searching one linear kilometer of ground (no. surveys = 8, Measey 2006).

Predation: Very little is known of predation involving this species. Nussbaum and Pfrender (1998) reported a few cases of possible predation and/ or scavenging on S. thomense by ants and other small arthropods. R. C. Drewes reported one observation of S. thomense, in the beak of a cattle egret (Bubulcus ibis) (R. C. Drewes, pers. comm.). The striking coloration of S. thomense suggests an aposematic or camouflage function (see discussion by Wollenberg and Measey 2009); Teodecki et al. (1998) reported that these animals are noxious or distasteful (p. 159).

Diet: Delêtre and Measey’s (2004) stomach content analysis found predominantly earthworms in the diet, but also included centipedes, ants, larvae, mites and other organic matter (Delêtre and Measey 2004, samples pooled across three habitat sites). Jones et al. (2006, cited in Delêtre and Measey 2004) expanded on these findings and reported a mix of epigeic and endogeic species in the earthworm dataset, with juvenile stomach contents revealing only endogeic species.

Feeding behavior: Evidence points to a sit-and-wait predation strategy, with feeding activity focused toward evening hours. Haft and Franzen (1996) describe a particular posture observed in captive individuals at night: S. thomense positioned partway in its burrow, head raised slightly above and perpendicular to the ground with neck cocked forward, remaining motionless. They tentatively attributed this behavior to hunting. Upon prey capture, Measey and Herrel (2006) recorded S. thomense using long-axis rotation of prey items, purportedly to subdue and reduce size of prey for ingestion. Experiments on respiratory capacity reveal an extremely low resting rate and quick recovery after exertion, as would be expected of a sit-and-wait predator (Smits and Flanagin 1994). According to Measey and Herrel (2006), captive S. thomense showed most interest in feeding between 1800 hrs and midnight.

Intraspecific communication: Biting and scraping with teeth across individuals appears common based on scarification of wild-caught and captive individuals (Teodecki et al. 1998; Nussbaum and Pfrender 1998; Delêtre and Measey 2004). Because the frequency of bite scars was ubiquitous (not significantly tied to sex or age class), Teodecki et al. (1998) suspected biting to be a form of chemical communication, tied to vomeronasal activity of the tentacle, and not singularly used for territorial/ dominance interactions between males, or female stimulation and/ or suppression during copulation. They observed individuals rubbing tentacles against each other and “intermittent bouts of rapid buccal pumping accompanying this ‘sampling behavior’” (Teodecki et al. 1998). Interestingly, Teodecki et al. (1998) reported that “no bites observed on any of the field-collected caecilians in this investigation appeared to inflict serious damage to the tentacle organ”. Captive males confined in limited quarters apparently bit each other aggressively, especially when disparate size classes were kept together (with large males inflicting bites on small males more frequently than vice versa) (Teodecki et al. 1998).

As in other amphibians, highly vascularized skin allows for efficient cutaneous respiration in addition to pulmonary respiration. Observation of apnea for up to three hours in some captive individuals is thought to reflect low metabolic demand at rest and highly efficient cutaneous respiration (Smits and Flanagin 1994).

Trends and Threats
Field reports by most authors included here state that S. thomense is quite abundant, especially in comparison to other caecilian species, and especially in proximity to human habitation (plantation, small scale agricultural plots, etc.). Repeated collections over the past 20 years have supported these observations, with some localities turning up large samples in museum collections (California Academy of Sciences, University of Michigan’s Museum of Zoology, and the British Museum of Natural History in London, among others). Nussbaum and Pfrender (1998) remark that S. thomense actually may benefit from limited deforestation and transition to low-intensity agriculture through gains in soil moisture, litter layer and/ or invertebrate prey base. However, due to the lack of demographic data on this species, recent focus of collection, and increases in development – expected along with that of newly accessed off-shore oil deposits (Dallimer et al. 2009) – S. thomense's range, habitat and abundance could be impacted negatively in the future.

Locals reported that these caecilians were apparently often confused with snakes and thus killed upon sight when found in agricultural plots (pers. comm., 2000).

Relation to Humans
S. thomense appeared in the pet trade in the 1990s (Measey et al. 2004).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities
Intensified agriculture or grazing
Urbanization
Local pesticides, fertilizers, and pollutants
Intentional mortality (over-harvesting, pet trade or collecting)

Comments
São Tomé is an oceanic island approximately 220 km distant from the western coast of Africa. Based on genetic divergence within the island and known dates of human settlement, it appears that Schistometopum thomense arrived on São Tomé prior to habitation by humans (Stoelting 2006). A rafting hypothesis has been put forth to detail possible mechanisms by which a potential source population on the African mainland may have colonized the island (Measey et al. 2007).

Phylogenetic Relationships: Schistometopum thomense is in the family Dermophiidae, although it has been placed in Caeciliidae previously. Current phylogenetic analyses show Schistometopum to be most closely related to Dermophis, a South American genus (Hedges et al. 1993; Wilkinson et al. 2003; Loader et al. 2007; Roelants et al. 2007). Within Schistometopum, one species is from the eastern coasts of Kenya and Tanzania, S. gregorii; a second species, S. ephele is cryptic on Sao Tome and has a hybrid zone with S. thomense (O'Connell et al. 2021). Expanded sampling and continued phylogenetic work may reveal more structure within this genus and in its relation to other African genera.

Within São Tomé, phylogenetic and population genetic analyses of mtDNA (ND4) showed the pattern of genetic variation to be congruent with two geographic clades on the island with an apparent admixture zone present at the clade boundary (Stoelting 2006). These geographic clades may be related to historical patterns of volcanic activity on the island (Stoelting 2006).

Species Authority:
Barboza du Bocage 1873 (describes Siphonops thomensis)
Peters 1874 (describes Siphonops brevirostris)
Peters 1879 (designates genus Dermophis and places S. thomensis and S. brevirostris within)
Peters 1880 (demotes D. brevirostris to junior synonym of D. thomensis)
Parker 1941 (designates genus Schistometopum and places D. thomensis within)
Taylor 1965 (designates Schistometopum ephele and resurrects S. brevirostre [Peters])
Nussbaum and Pfrender 1998 (revise genus Schistometopum, demoting S. ephele and S. brevirostre to junior synonyms of S. thomense)
Zhang and Wake 2009 (report partial mt genome for Schistometopum thomense, CAS 219292, GenBank Accn GQ244476)

Etymology: Schistometopum – from Latin neuter noun meaning split forehead; thomense – reference to geographic range (Nussbaum and Pfrender 1998)

This species was featured in News of the Week June 7, 2021:

O'Connell et al. (2021) investigated speciation in caecilians from the small oceanic island of São Tomé off the west coast of Central Africa in the Gulf of Guinea. Although small, the island of São Tomé exhibits dramatic variation in habitat type, with dry habitats in the north and rainforest in the south. They found two genetically distinct lineages of caecilians corresponding to yellow populations in the north (Schistometopum thomense) and brown spotted populations in the south (Schistometopum ephele). Further, they found evidence of hybridization at the transition of the two habitat types. The authors proposed that late Pleistocene volcanic activity separated caecilians on the island and favored local adaptation to wet and dry climates, leading to speciation and the maintenance of a hybrid zone. This study is one of the first to document speciation histories in caecilians and lays the groundwork for future studies of reproductive isolation in this group. (Written by Rayna Bell)

References

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Measey, G. J., and Van Dongen, S. (2006). ''Bergmann’s rule and the terrestrial caecilian Schistometopum thomense (Amphibia: Gymnophiona: Caeciliidae).'' Evolutionary Ecology Research, 8, 1049-1059.

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Peters, W. (1874). ''Über neue Amphibien (Gymnopis, Siphonops, Polypedates, Rhacophorus, Hyla, Cyclodus, Euprepes, Clemmys).'' Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin, 1874, 616-625.

Peters, W. (1880). ''Über die Eintheilung der Caecilien und insbesondere über die Gattungen Rhinatrema und Gymnopis.'' Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin, 1879, 924-943 .

Peters, W. (1880). ''Über neue oder weniger bekannte Amphibien des Berliner Zoologischen Museums (Leposoma dispar, Monopeltis (Phractogonus) jugularis, Typhlops depressus, Leptocalamus trilineatus, Xenodon punctatus, Elapomorphus erythronotus, Hylomantis fallax).'' Monatsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin, 1880, 217-224 .

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Schätti, B., and Loumont, C. (1992). ''Ein Beitrag zur Herpetofauna von São Tomé (Golf von Guinea) (Amphibia et Reptilia).'' Zoologische Abhandlungen Staatliches Museum für Tierkunde Dresden , 47(4), 23-36.

Smits, A. W., and Flanagin, J. I. (1994). ''Bimodal respiration in aquatic and terrestrial apodan amphibians.'' American Zoologist, 34, 247-263.

Stoelting, R.E. (2006). Tomé Caecilian, Schistometopum thomense (Gymnophiona: Caeciliidae) Master's Thesis. San Francisco State University, San Francisco, CA, USA.

Taylor, E.D. (1965). ''New Asiatic and African caecilians with redescriptions of certain other species.'' University of Kansas Scientific Bulletin, 46, 253-302.

Teodecki, E.E., Brodie, Jr., E.D., Formanowicz, Jr., D.R., and Nussbaum, R.A. (1998). ''Head dimorphism and burrowing speed in the African caecilian Schistometopum thomense (Amphibia: Gymnophiona).'' Herpetologica, 54(2), 154-160.

Wilkinson, M., Loader, S. P., Gower, D. J., Sheps, J. A., and Cohen, B. L. (2003). ''Phylogenetic relationships of African caecilians (Amphibia: Gymnophiona). Insights from mitochondrial rRNA gene sequences.'' African Journal of Herpetology, 52, 83-92.

Wollenberg, K. C., and Measey, G. J. (2009). ''Why colour in subterranean vertebrates? Exploring the evolution of colour patterns in caecilian amphibians.'' Journal of Evolutionary Biology, 22, 1046-1056.

Zhang, P. and Wake, M.H. (2009). ''A mitogenomic perspective on the phylogeny and biogeography of living caecilians (Amphibia: Gymnophiona).'' Molecular Phylogenetics and Evolution, 53, 479-491.

Species Account Citation: AmphibiaWeb 2021 Schistometopum thomense: São Tomé Caecilian <https://amphibiaweb.org/species/1937> University of California, Berkeley, CA, USA. Accessed Nov 24, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 24 Nov 2024.

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