Distribution & Ecology: Wallis & Futuna (Wallis); Austral (?Rapa); Marquesas (Tahuata, Hatu Iti); Society (Moorea, Tahiti); Tuamotu (Fakarava, Hao, Raroia, Takapoto, Tikehau) - Marine; Shoreline to 10 m (0-30m); Algae associate.

Regional Records: WALLIS & FUTUNA - Menaethius monoceros - Poupin & Juncker, in preparation. - FRENCH POLYNESIA - Menaethius tuberculatus Dana, 1852b: 123; 1855, pl. 5, fig. 1a-c (Tuamotu). - Menaethius monoceros - Nobili, 1907: 382 ("Ohura" = Hao). - Rathbun, 1907: 64 (Fakarava). - Holthuis, 1953: 4 (Raroia). - Forest & Guinot, 1961: 14, fig. 9a-b (Tahiti; Syn.); 1962: 56 (Biogeography "Tahiti-Tuamotu, Marquesas"). - Peyrot-Clausade, 1977a, annex of the species: 25 (Moorea); 1977b: 212; 1989: 112, 115 (Moorea, Tikehau; 30m). - Odinetz, 1983: 208 (Moorea, Tahiti, Takapoto). - Guinot, 1985: 452 (List). - Odinetz-Collart & Richer de Forges, 1985: 201 (Moorea and/or Tahiti, Takapoto). - Poupin, 1996a: 27 (Checklist) - New material, MNHN - Coll. P. Frouin, det. J. Poupin (Tahiti). - Unpublished sp., coll. & det. G. Paulay, Tikehau, July 10, 2001, Outer end of Passe Tuheiava and adjacent outer reef slope on W. side of atoll (FLMNH-UF ?1641). - Unpublished sp. - Austral, Rapa Island, Haurei bay, coll. BENTHAUS Expedition (Warén), November 8, 2002, prov. det. J. Poupin November 25, 2003 (4 sp. in MNHN); Moorea fieldwork December 2006, coll. Poupin (Papetoai lagoon, 0.1 m, fringing reef, coral rubbles), det. Poupin, specimens (MNHN) and color photograph. - Moorea Biocode project. - AAMP, Marquesas, 2012, Leg 2, coll. Evans, sample 0510, St 35, Tahuata, 15 m (with Huenia brevifrons, ?heraldica); Leg 3, St. MQ27-B, Hatu Iti, 5-22 m, coll. Albenga et al., sample LC342.

Depth distribution: 0 - 30 m