Morphological and molecular methods were used to describe a new species of Trapania Pruvot-Fol, 1931 from shallow water kelp forests on the north-central coast of Peru. The new species, Trapania huarmeyana sp. nov., is distinguished from other species along the Eastern Pacific by external morphological characters such as its translucent white body with brown stripes and small spots on the dorsum, blotches on the base of the extra-branchial processes, extra rhinophoral processes and gill branches. Internally, T. huarmeyana sp. nov. is distinguishable by several morphological characteristics of the radula, jaws and genital organs. Phylogenetic trees recovered using Bayesian Inference and Maximum Likelihood analysis of DNA sequences support its distinct status and clarify its relationship to other species from the Eastern Pacific. This new species constitutes the first record of Trapania from the Humboldt Current Ecosystem, contributing to our understanding of the distribution of the genus in the South-eastern Pacific.

Although nudibranchs are common and attractive animals, our understanding of these marine gastropods in Vietnam remains limited. Prior research has suggested that combining morphological examination with molecular analysis results in more accurate identification of nudibranchs. However, previous studies in Vietnam have typically relied solely on morphological methods for nudibranch identification. In this study, the nudibranch species Halgerda batangas was recorded in Vietnam for the first time based on both morphological and molecular approaches. Halgerda batangas was characterized by a network pattern consisting of orange lines, relatively low dorsal tubercles with red-orange caps and white basal rings, and an orange line along the foot margin. Molecular analysis corroborated the morphological findings. These results suggest that integrating morphological and molecular methods is an effective approach for identifying nudibranchs.

We report 37 species of heterobranch sea slugs from the National Park Alacranes reef, located in the Campeche Bank, at the south-eastern Gulf of Mexico, of which 18 species are reported for the first time in this reef. With this contribution, the species richness in this national park increases to up to 67. The species Nakamigawaia felis, Petalifera sp., Elysia flava, Elysia pratensis and Bulbaeolidia sp. are reported in the Gulf of Mexico for the first time. The geographical distribution ranges for Oxynoe azuropunctata, Elysia ornata, Placida kingstoni, Aphelodoris antillensis and Tritonia hamnerorum are extended westward. Those for Elysia flava, Pleurobranchus areolatus and Felimare ruthae are extended both northward and westward.

Members of the genus Spurilla are nudibranchs with circumtropical distribution. Spurilla neapolitana is reported for the first time inhabiting subtidal soft-bottom habitats of the central coast of Peru. Adult specimens were found in Bahía Ferrol (09°04′S 78°35′W) at 10 m depth in November of 2009. Spurilla neapolitana is characterized by the presence of a long body with numerous cerata along the dorsal side of the body and a distinctive orange colouring with white dots. This finding extends the geographical range of Spurilla spp. into the south-eastern Pacific and adds a new contribution to the macro-mollusc diversity thriving in the Humboldt Current Upwelling Ecosystem.