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Solenogastres (Mollusca, Aplacophora) are widespread across all oceans and depths, yet remain among the least understood molluscs. Despite frequent collection in deep-sea surveys, only 10% of species have been described from depths below 3500 m, largely due to the labor-intensive nature of their identifications. This study examines specimens from the DIVA 3 expedition (Me 79/1, 2009) in the Brazil Basin and describes three new abyssal species of Wirenia Odhner, 1921 (W. bertae sp. nov., W. meteori sp. nov., and W. opistodenticulata sp. nov.), the first formally described Gymnomeniidae from the abyss and the southern hemisphere. Using an integrative morphological approach (SEM, light microscopy, histology), we highlight the value of external sclerite characters for species delimitation. We also present a synthesis of known abyssal solenogaster diversity focused on the formally described taxa and outlining recent progress and future research priorities. Our findings emphasize the importance of deep-sea exploration and taxonomy in closing major knowledge gaps in marine biodiversity. 

Solenogastres is a group of mollusks with evolutionary and ecological importance. Nevertheless, their diversity is underestimated and knowledge about the distribution of the approximately 300 formally described species is limited. Factors that contribute to this include their small size and frequent misidentification by non-specialists. Recent deep-sea explorations have resulted in the collection of numerous specimens through effective methods such as epibenthic sledges. However, this is a costly, labor-intensive, and destructive methodology. In contrast, Autonomous Reef Monitoring Structures (ARMS) offer a novel, non-destructive approach, by providing a substrate for benthic organism colonization. This study is the first to describe Solenogastres collected using ARMS, demonstrating that they are an effective tool for biodiversity assessment and characterizing rare marine invertebrates. Following an integrative taxonomic approach, two new solenogaster species are described:Dondersia tweedtaeFarris, Olson & Kocot,sp. nov.(Dondersiidae) andEleutheromenia bullescensCobo,sp. nov.(Pruvotinidae). The diagnosis of the family Dondersiidae is amended and the necessity of reassessing the validity of the current diagnostic characters for Pruvotinidae, and its classification is emphasized. The two newly described species exhibit distinct external characteristics;D. tweedtaesp. nov.has a striking pink color with a bright yellow dorsal keel andE. bullescenssp. nov.has a unique, discontinuous dorsal keel with nearly spherical protrusions. The presence of cnidocytes in the digestive systems of both species indicate that they feed on cnidarians. It is hypothesized that, like in some nudibranchs, their coloration and body features reflect defensive adaptations related to their diet. This study shows that while habitus alone is typically insufficient for accurate identification in solenogasters, it can sometimes simplify the process. For this, live observations and photographs are essential. 

Solenogastres and Caudofoveata (Aplacophora) remain some of the least known molluscs, despite ubiquity in the marine environment and importance in understanding molluscan evolution. The use of new morphological techniques and development of DNA barcode libraries have helped make specimen identification easier. However, for solenogasters, using histology for identification and adequate description of species remains necessary in most cases. This, together with the facts that knowledge about solenogaster species distributions is biased and that most species were described from one or very few individuals, explains why many open questions about the actual distribution, intra- and interspecific variability, etc., remain. We performed an integrative taxonomic study of eight specimens of solenogasters from the South China Sea (West Pacific Ocean) thatresulted in the identification of four new species of Proneomeniidae. Species identification and description following the established diagnostic characters were straightforward. However, phylogenetic analysis of molecular data obtained from these specimens and other members of Proneomeniidae indicate that the family is polyphyletic. We recovered representatives of two other families, Epimeniidae (Epimenia) and Strophomeniidae (Anamenia), nested within Proneomeniidae with strong support. Ancestral character state reconstruction indicates that characters commonly used in solenogaster taxonomy, such as the radula and foregut glands, may be more evolutionarily labile in this group than previously known. Therefore our work fills knowledge gaps regarding the diversity and distribution of members of this family but raises important questions about solenogaster taxonomy and systematics that should be further assessed with additional markers and broader taxon sampling. ZooBank: urn:lsid:zoobank.org:pub:BCADACD6-9AD0-442A-AD64-031BA8D88599 

So far, of the 292 known species of solenogasters (Mollusca, Aplacophora), 62 belong to the clade Pholidoskepia Salvini-Plawen, 1978. Of these, only two have an abyssal distribution (3500–6000 m depth). Among Pholidoskepia, Dondersiidae Simroth, 1893 is the most diverse family. This study contributes to the knowledge of this family with the description of one new genus and six new species from the abyssal South Atlantic Ocean: Dondersia ? foraminosa sp. n., Nematomenia divae sp. n., Nematomenia brasiliensis sp. n., Nematomenia ? guineana sp. n., Helluoherpia vieiralaneroi sp. n. and Inopinatamenia (gen. n.) calamitosa sp. n. Specimens were collected during DIVA (Latitudinal Gradients of Deep-Sea BioDIVersity in the Atlantic Ocean) expeditions in the Guinea (DIVA 2 Me 63/2, 2005) and Brazil (DIVA 3 Me 79/1, 2008) Basins. Specimens were characterized based primarily on the sclerites and internal anatomy, which was studied using histology. The importance of the radula and mantle sclerites for taxonomy is emphasized. Amended diagnoses for the family and some genera within this family are provided. This contribution increases the described diversity of Dondersiidae to ten genera and 38 species and highlights the need for more study of solenogasters in the deep sea.