Comprehensive DescriptionRead full entry
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 Bergmanns Rule, increasing in size as sample sites increase in elevation (Measey and Van Dongen 2006).
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).
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)
Phylogenetic Relationships:Schistometopum thomense is in the family Caeciliidae, a taxon that includes many derived African and South American genera. 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, only one other species is currently described: S. gregorii, from the eastern coasts of Kenya and Tanzania. 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).
Etymology: Schistometopum - from Latin neuter noun meaning split forehead; thomense - reference to geographic range (Nussbaum and Pfrender 1998)