Shell medium-sized for the subgenus, up to 17 mm in height at maturity, spinose, lightly built. Spire high with 1.5 protoconch whorls, and 6 broad, angulate, strongly shouldered teleoconch whorls. Suture impressed. Protoconch large, whorls rounded, smooth, glossy. Terminal varix low. Axial sculpture of teleoconch whorls consisting of low lamellae, ending as broad, long, blunt, broadly open spines at shoulder. First and second whorl with 7 lamellae, third to last whorl with 6 or 7. Spiral sculpture consisting of numerous, narrow, striae. Aperture small, ovate. Columellar lip narrow, smooth; rim adherent. Outer lip smooth, thin, smooth within. Siphonal canal medium sized, broad, broadly open, 3 or 4 previous canals retained. Colour milky white. Height of the holotype: 17 mm. Operculum and radula not examined.

Actinotrophon tenuis is the third known species of Actinotrophon, and the second species from the western Pacific. A. fragilis Houart, 1996 was described from New Caledonia, and was initially compared with the type species, A. actinophorus (HOUART, 1996: 60). Since its description, A. fragilis has now been collected off Vanuatu, and Wallis and Futuna .
A. tenuis differs from A. fragilis in having a shorter, broader siphonal canal relative to shell height. The carinal spines are broader, shorter, more broadly open, less acute, more backwardly recurved, and more adapically bent. Moreover, the teleoconch whorls are higher and more adapically carinate. On the last teleoconch whorl of adult P. tenuis, the preceding axial lamellae almost entirely cover the succeeding spine on the sutural ramp: this lamellae is narrower, and less expanded in A. fragilis.

TYPE LOCALITY. — SW Pacific, Waterwitch Bank, 12°30' S, 176°41' W, 325-400 m, [MUSORSTOM 7, sta. DW537]. DISTRIBUTION. — SW Pacific, Waterwitch Bank, and Vanuatu, living at 400-405 m.