Shell small for the genus, adult up to 14.6 mm in length, slender, lanceolate, biconical, squamous. Spire high. Suture impressed. Protoconch of sinusigera type, conical, of 3 smooth whorls. Terminal lip raised, curved. Teleoconch sculpture consisting of low, narrow, rounded, frondose varices, each with short, blunt spinelets. No intervarical sculpture except for numerous axial growth lamellae. Spiral sculpture of squamous primary, secondary and tertiary cords as follows: first teleoconch whorl with P1 and P2 visible; second with P1, s1, P2; third with IP, P1, s1, P2; fourth and fifth with adis, IP, abis, P1, s1, P2; last whorl with adis, IP, abis, P1, s1, P2, s2, P3, s3, P4, s4, P5, s5, P6, s6, ADP, MP. Last whorl with primary, secondary and tertiary cords of same strength in some specimens. Aperture small, ovate, almost round. Columellar lip narrow, flaring, smooth except for one elongate knob abapically (holotype). Lip partially erect, adherent at adapical extremity. Anal notch shallow. Outer lip erect, crenulate, with weak denticles within. D1 and D2 very low, almost indistinct, D3 and D4 low, D5 strongest. Siphonal canal moderately long, narrow, dorsally recurved. Yellowish white (juvenile paratype) or ivory white with slightly darker protoconch and first teleoconch whorl. Tip of siphonal canal pale brown in the holotype and adult paratype. Operculum and radula not studied. Height of holotype: 14.6 mm.

Murexsul and related groups have been revised by Merle & Houart (2003). Four species resemble
M. merlei and are probably related to it: M. tokubeii Nakamigawa & Habe, 1964, occurring widely in the West Pacific, M. purpurispina (Ponder, 1972), from Queensland and New South Wales, Australia, M. reunionensis Houart, 1985, from Réunion, in 75-280 m, and M. spiculus (Houart, 1986) from the Coral Sea. Although none of them has a multispiral protoconch with sinusigeral notch as in M. merlei, all of them have a paucispiral protoconch of 1.5 to 2 whorls.
Murexsul purpurispina, except for its tilted protoconch of 1.5 smooth, very slightly shouldered whorls (Ponder 1972: 242), is most similar to M. merlei in having the same outline and colour, even the darker first whorl and light brown tip of the siphonal canal. However, not only are the protoconch whorls different, but the shell of M. purpurispina also has a comparatively larger and broader aperture with a more strongly prominent abapical columellar knob, a comparatively shorter siphonal canal, almost half the length of that in M. merlei, and a broader shoulder.

Tonga and Fiji.

TYPE LOCALITY. —Tonga, N Ha'apai group, 19°03'S, 174°19'W, 523-806 m [BORDAU 2: stn DW1595].
DISTRIBUTION. —Tonga, Fiji, Coral Sea, and Vanuatu, living in 314-319 m, empty shells in 319-523 m. Murexsul merlei was previously known from the holotype and 2 paratypes, described from Tonga and Fiji, dredged alive at 314-377 m. The bathymetric range in Coral Sea for living specimens is similar to that recorded in Fiji.