Diagnosis: Shell yellowish, slender, fusiform, surface nodulous; aperture elongated, height equals half shell height; a few white varices present on last whorls.
Description: Shell elongate, high-spired, rApH = 0.47, with narrow, elongate aperture. Sculpture of many fine small rounded knobs; a few varices. Two columellar folds, colour pale tan. varices and outer lip white. Protoconch paucispiral, dome-shaped, smooth, glossy; n = 1,75 , d = 0.3 mm, D = 1.4 mm, e. h. = 1.3 mm; transition to teleoconch not clearly marked. Teleoconch has 5.5 whorls, suture impressed, with a very narrow, round shoulder. Axial ribs are formed by the rows of whitish nodules, diameter about 0.3 mm; there are 30, 30, 31 axial ribs on the 3rd to last teleoconch whorl. Very fine growth lines, about 18/mm, occur all over the shell surface; they are slightly opisthocline. Spiral sculpture on lst- 5th teleoconch whorl: 5, 6, 7, 8, 9 rows of nodules: on LW 20 spiral rows of nodules on the crossings with the axial ribs. Yet. neither the axials nor the spirals are clearly indicated as such by an axial rib or a spiral line; they are only formed by the ordered succession of the nodules. The peristome is thick and white, except the columellar inductura, which is thin and transparent. Starting at 1,75 teleoconch whorls, temporary growth stops are marked on the spire by the presence of the exposed rim of a thin outer lip; on later occasions a varix is formed, at first only distinguishable by a slightly higher axial rib, then by a paler colour of the shell, later a real varix and then a strong varix at the sigmoid outer lip. There is a total of five growth stops, not counting the apertural varix. Outer lip white, with about 17 lirae on the everted part of the outer lip. starting strong near the adapical angle, and gradually diminishing in strength in the anterior part of the aperture. The lirae are short and do not continue inside the aperture. Inner lip covered by thin transparent colourless callus. A strong columellar fold is formed at the height of the end of the last varix. A weaker fold follows anteriorly: further on. the columella is straight to the siphonal canal (best visible when the shell is slightly rotated axially. In apertural view, two or three small nodules on the anterior part of the columella give the impression of very small columellar folds , which they are not.
The anteriormost part of the columella is solid, white, covering the umbilical region: it looks as if several layers of callus have been deposited there. No umbilicus.

Mericella rosadoi n. sp. has the general shell shape of the genus Mericella, and especially of M. jucunda height up to 15 mm) and M. paschalis (up to 17 mm, shells of which were figured by Verhecken & Bozzetti, ( 2006: pl. 1. figs 1-6 and 7-9), but M. rosadoi grows larger (up to 28.9 mm). Mericella bozzettii Petit & Harasewych, 1993, from off Somalia. 200-205 m, shells up to 37 mm, is similar in sculpture to M rosadoi, but the sculpture of the former is somewhat coarser. M. bozzettii has a much larger protoconch (AV0019: n = 2, d = 0.6 mm, D = 2.1 mm, e.h. = 2.5 mm) than M. rosadoi, the two columellar folds are stronger and it has 2-3 small nodules near the anterior end of the columella. M. bozzettii has a sculpture of nodules like that of M. rosadoi, but the other Mericella species differ markedly from these two in having a sculpture of fine axial and spiral lines. M. rosadoi has a strong similarity to Gerdiella alvesi (Lima et al., 2007), known from off NE Brazil, 08°46.5'S, 34°44.5'W, 690 m, and SE Brasil 22°06.5'S, 40°33.2'W (Petuch, 2013: 189). Both species are very much alike in general shell and protoconch shape, but G. alvesi has numerous microscopic spiral threads on the protoconch, visible in SEM photos but not in an optical photo of the holotype (Lima et al., 2007: figs 4 and 6-9, resp.). The callus of the peristome, the sigmoid outer lip and its lirations are similar in both species. The outer lip has its largest distance from the shell axis in the adapical half of the aperture in M. rosadoi, but at half aperture height in G. alvesi. The two columellar folds are weaker in M. rosadoi than those in G. alvesi. In apertural view, G. alvesi also presents a slight thickening of the columella abapically from the smallest fold, probably comparable to that in M rosadoi. In contrast to M. rosadoi, G. alvesi has no varices on the spire (Lima et al., 2007: 102). The colour of the shell is white for G. alvesi and yellowish tan for M. rosadoi. The narrow axial and spiral lines, on the crossings of which the nodules are formed, are clearly visible in a SEM photo of G. alvesi (Lima et al., 2007, fig. 14); similar lines are absent in M. rosadoi (see description above, no SEM image). The slenderness (H/W) of M. rosadoi is 2.52 (holotype) and 2.60 (paratype); the holotype of G. alvesi has 2.65, based on the dimensions given, but its figure (Lima et al, 2007: fig. 1, reconstructed from two SEM photos) has 2.24; The rApH for M. rosadoi is 0.48 (holotype), 0.56 (para¬type); it is 0.51 for G. alvesi (PL 4, Fig. 1). In G. alvesi, the number of axial ribs on the lst-5th teleoconch whorl (as given by Lima et al., 2007: 100) is 20-22; 20; 22; 26; 32-33; and 32 on the LW. For the M. rosadoi holotype these figures are: 27; 26; 30: 30; 31; 25. The number of spiral rows of nodules on said teleoconch whorls of G. alvesi is: 6-7; 7; 7; 7; 4; 4, respectively; and 15 on the base. For the M. rosadoi holotype they number 5; 6; 7; 8; 9; 20. In the rectangles between axials and spirals, G. alvesi has finer microscopic growth lines (Lima et al, 2007: fig. 14) than in M. rosadoi. All these differences are rather small. Hence, there is no reason to maintain the separation between Atlantic Gerdiella and Indian Ocean - northwestern Pacific Mericella, mentioned above in the discussion of these genera. Therefore, the Brazilian species G. alvesi is hereby transferred to the genus Mericella.

Type locality: Southern Tanzania, between Dar es Salam and Rovuma River, trawled by Portuguese trawler for deepwater crayfish on muddy bottom at 330-360 m, the area where Galeodea keyteri (Kilburn, 1975) is captured, in November 2005. Distribution: Only known from the type locality.