A new species of the family Tritoniidae is described from the tropical South Atlantic Ocean. The animal was found off north-east Brazil. Tritonia khaleesi sp. nov. is up to 12 mm long, with a slender white body, of which the notum is covered with one broad white band extending from between the eyes and veil to the tail; veil with 4 velar appendages; retractable white rhinophores; rhinophoral sheath with fleshy extension; seven pairs of branchial plumes; the anus is located between the 3rd and 4th gills on the right side, and the genital opening is under the 2nd gill. Internally, T. khaleesi sp. nov. is distinguished from other tritoniids by jaws with 10–14 rows of denticles on the inner lips, absence of stomach plates and the radular formula 32 × 2–5.1.1.1.2–5 teeth. Tritonia khaleesi sp. nov. is the only Tritonia that possesses a unicuspid rachidian tooth as an adult.

A new species of dendronotid nudibranch, Marionia abrahamorum sp. nov., is described here. This species was found on Príncipe Island (Gulf of Guinea, eastern Atlantic Ocean), and its description is based on its morphological characteristics as well as molecular data from two mitochondrial (cytochrome c oxidase subunit I and 16S rRNA) markers and one nuclear (histone-3) marker. This species differs from all known Tritoniidae in terms of its size, color pattern, notum pattern, number of velar processes, number of gills, presence of stomach plates, and radular formula. The phylogenetic results support the results of the morphological analysis, confirming its placement within the genus Marionia. Sea slugs from the family Tritoniidae (Nudibranchia, Cladobranchia, Dendronotidae) span a variety of sizes and colors, but they feed exclusively on octocorals (McDonald and Nybakken 1999). The eight currently valid genera are Tritonia,

A new species of the family Tritoniidae is described for the tropical South Atlantic Ocean. The animal was found off north-east Brazil. Marionia limceana sp. nov. is up to 31 mm long, with a sturdy white body, of which the notum is covered with two rows of red polygons running from the rhinophores to the tail; notum with tubers; bi-lobed veil with 16 velar papillae; retractable white rhinophores; 11–14 pairs of branchial plumes; the anus is located below the 4th gill on the right side, and the genital opening is under the 3rd gill. Internally, M. limceana sp. nov. is distinguished from other tritoniids by jaws with three or four rows of denticles on the inner lips, a belt of 18 stomach plates; and the radular formula 26 × 26–32.1.1.1.26–32 teeth. The animal was found feeding on octocorals of a species of the new genus Stragulum, and is the first reported nudibranch to feed on this genus.

The phylogeny of the family Tritoniidae has recently been studied with significant advances, but the relationships between genera are still controversial. In our study, we investigated the phylogeny of Tritoniidae using the most diverse taxon sampling possible. We applied an integrative approach based on new sequences of two mitochondrial genes (COI and 16S), a nuclear gene (H3) and morpho-anatomical characters. The monophyly of the family Tritoniidae was not recovered in our phylogenetic analyses. In view of our results, we propose a new rearrangement at the subfamily and genus levels. The plate-bearing genera are raised to the subfamily level as the monophyletic Marioniinae subfam. nov., formed by Marionia and the reinstated Marioniopsis. The remaining plate-less genera are raised to the subfamily level as the monophyletic Tritoniinae subfam. nov.. The genus Myrella is reinstated to assign the Antarctic and sub-Antarctic tritoniids. The taxonomic status of the monotypic Tritonidoxa is confirmed. Species delimitation analyses reveal a new large Marionia species from the Gulf of Cádiz (south-west Spain, Atlantic Ocean) and evidence for the pseudocryptic speciation of numerous Atlantic–Mediterranean and Indo-Pacific tritoniids. Finally, taxonomic notes are provided for several species according to the proposed rearrangement.