The genus Micromelo (family Aplustridae) occurs in almost all tropical and subtropical waters across the globe, with the exception of the Eastern Pacific. Most authors consider Micromelo undatus (Brugui ere, 1792) as the only valid species in this genus. This study examines populations of specimens identified as M. undatus across its geographic range, using morphological and genetic data from two mitochondrial genes (16S and CO1) and one nuclear gene (Histone H3). The results reveal that M. undatus is a complex of four species with consistent genetic and anatomical differences. A literature review and evaluation of type material indicates that available names exist for three of the four species. Micromelo guamensis, M. undatus and M. scriptus are resurrected for species found in the Western Pacific, the Atlantic Ocean, and widespread in the Indo-Pacific, respectively. A new name is introduced for another species found in the Western Pacific, namely Micromelo barbarae sp. nov. and a neotype is designated for the type species of the genus M. undatus.

The study of newly collected, live specimens of Pupa (Acteonidae) from New Caledonia and French Polynesia, revealed the presence of six distinct species in this region, including an undescribed species. All these species are re-described and/or named using molecular and morphological evidence as well as a review of the literature and examination of photographs of the type material of described species. Moreover, a review of the literature and type material suggests the existence of six additional species from other geographic regions across the Indo-Pacific. Illustrations of all these species are provided as well as remarks on possible synonymies. This study is the first species-level review of this group using modern taxonomic techniques.