Shell very small for the genus, up to 5.3 mm in height at maturity. Height/width ratio 1.9 (holotype). Slender, lanceolate, biconical, narrow, very weakly spinose, lightly built. Substural ramp narrow, weakly sloping, very slightly convex. Entirely white, translucent white or orange (paratype MNHN).
Spire high with 1.5 protoconch whorls and teleoconch of three broadly-convex, elongate, weakly shouldered whorls. Suture impressed. Protoconch large, broad, whorls rounded, smooth, maximum width 600 µm. Terminal lip delicate, narrow, weakly erect, very weakly curved.
Axial sculpture of teleoconch whorls consisting of very low, weak, quite indistinguishable, narrow, lamellose ribs and low, weak, narrow varices. First whorl with eight or nine ribs, second and third with ribs and varices of same strength as each other. Apertural varix high, broad, rounded with five very short, blunt, broadly open spinelets, extending from P1-P5- Spiral sculpture of low, rounded, narrow primary and secondary cords. First teleoconch whorl starting with P3 only, starting P1 and P2 at the end of the whorl, second whorl with P1-P3, occasionally with P4 almost entirely covered by following whorl, last whorl with P1, sl, P2, (s2), P3, P4, s4, P5, s5, P6, ADP, (MP). P1-P5 almost similar in strength and height, P6 narrower.
Aperture moderately large, ovate. Columellar lip narrow, strongly flaring, smooth, rim partially erect, adherent at adapical extremity. Anal notch shallow, broad. Outer lip erect, smooth within. Siphonal canal moderately long, narrow, weakly dorsally recurved, open.
Operculum and radula not examined.

Murexsul mananteninaensis is part of a small group of species with only three or four teleoconch whorls but that seem to have reached adult size: Murexsul asper Houart, 2004 from Western Australia, M. queenslandicus n. sp. Houart, 2004 from Queensland, Australia, M. leonardi (Houart, 1993) from Christmas Island, M. charcoti (Houart, 1991) and M. micra (Houart, 2001) from New Caledonia. Many specimens have been examined from Australia and New Caledonia and none of these reaches a height of more than 7.5 mm.
Compared to M. mananteninaensis, M. asper has a larger shell, reaching 8.6 mm in height, with broader spiral cords and higher IP and abis.
Murexsul queenslandicus is broader and heavier with broader spiral cords, no secondary spiral cords and a broader, higher apertural varix.
Murexsul leonardi differs markedly and does not need to be compared here.
Murexsul charcoti is larger, reaching a height of 7.2 mm, and broader with broader, more obvious axial varices, broader spiral cords and small, blunt spinelets on axial varices and on the siphonal canal.
Murexsul micra is smaller, reaching a height of 3.9 mm, with a lower spire, broader spiral cords, more obvious axial varices with short, blunt spinelets and a smaller aperture.
Curiously, Murexsul mananteninaensis also resembles species o( Enatimene Iredale, 1929, a genus confined to eastern Australia, comprising three species. The shell morphology of E. lanceolatus Houart, 2004 is particularly close to Murexsul mananteninaensis n, sp., but E. lanceolatus is twice as large for the same number of teleoconch whorls, has a rounded protoconch and a first teleoconch whorl with P1-P3. The genus Enatimene is currently classified in Trophoninae sl. Cossmann, 1903 and its radula characters are distinct from those of the Muricopsinae Radwin & D'Attilio, 1971.

TYPE LOCALITY. — South Madagascar, Manantenina sector, 24°23'S, 47°32,E, 307-319 m [ATIMO VATAE: stn DW3524].
DISTRIBUTION. — Southern Madagascar, taken alive at 87-307 m, shells at 220-307 m.