Description (holotype): Shell turriform, with high spire. Protoconch bulbous, of little over 1.5 smooth convex whorls. Protoconch/teleoconch transition marked by single rather strong prosocline axial rib, after which definitive sculpture appears. Protoconch diameter 0.7 mm, exposed height 0.45 mm. Teleoconch of 5.75 convex whorls strongly angulated at shoulder. Suture shallow, wavy in accordance to axial ribs. Subsutural ramp narrow, weakly concave. Axial sculpture represented by strong, sharp on top, slightly arcuate and weakly opisthocline ribs, running from suture to suture but fading at transition of shell base to canal, 8 on first teleoconch whorl, 10 on second, 9 on penultimate whorl and last whorls. Interspaces between ribs nearly twice ribs' width. Spiral sculpture of indistinct spiral cords, poorly visible in interspaces between ribs, but forming nodules at intersections with axial folds, most distinct on shoulder. On last whorl each rib with 5 such nodules, diminishing from shoulder abapically. Base evenly convex, concave at passing to canal. Aperture oval, with short broad leftward inclined siphonal canal. Anal sinus very shallow, subsutural, hardly discernible. Inner lip with nearly straight columellar part and very weakly convex parietal. Parietal callus very thin and narrow. Outer lip angulated at shoulder and weakly convex below. Shell glossy, semitransparent, off white. Measurements (holotype largest specimen): SL 6.2 mm, AL (with canal) 3.3 mm, SW 2.2 mm. Radula and operculum not available for studies.

Remarks. The species is rather constant in adult shell characters, but with clearly observed ontogenetic variability. In a small specimen with SL 4.0 mm the shell looks much stouter, with sharply narrowing shell base in comparison with larger ones.
Presently the genus Iwaoa remained monotypical and the type species is rather different from the new one. It is much larger, reaching 4 cm, it has better pronounced spiral cords, while the axial ribs are more numerous and smoothened. Nevertheless there are several still unnamed (and mostly unsequenced) deep-water species of Horaiclavidae, that have intermedíate morphology between I. reticulata and I. invenusta sp. nov. At the moment, it seems to be the most suitable for the new species. Therefore we prefer to place the new species with
in Iwaoa rather than describing one more new genus. The three sequenced samples of I. invenusta sp. nov have almost identical COI sequences, and in the COI tree, I. invenusta sp. nov. is not sister to I. reticulata, but as explained before the relationships between most species of Horaiclavidae are unsupported, and more data are needed to clarify the genera boundaries within Horaiclavidae.
Etymology: invenusta (Lat.) - unattractive, referring to inconspicuous shell.
Distribution: The species presently is endemic to Papua New Guinea, 505-780 m.