The history of the systematics of isopod suborders is summarised. Several authors have suggested that the traditional suborder Flabellifera is paraphyletic and includes one or more of the suborders Gnathiidea, Epicaridea and Anthuridea. Two suborders, Cymothoida and Sphaeromatidea, have been proposed as replacement taxa for the Flabellifera, but it has not been possible on the basis of phylogenetic analyses to elucidate significant relationships between the suborders and families. Morphological characters are used to explore relationships between 35 genus-, family- and suborder-level taxa of flabelliferan Isopoda in a cladistic analysis (using Phreatoicidea and Asellota as outgroups) and to derive a new classification. The analysis did not find a synapomorphy for 'Flabellifera' sensu lato , but recognises two diverging clades of 'long-tailed' isopods. Members of the Oniscidea are not part of either clade. Nor are the Tainisopidea, a new suborder erected for members of the family Tainisopidae. The Tainisopidea has many synapomorphies and plesiomorphic features, but does not share characters with either clade. The first clade comprises Phoratopidea (for Phoratopus remex ) and sister-taxa Cymothoida and Limnoriidea. Representatives of these suborders have uropodal rami ventral to the pleotelson and articulating from side-to-side inside the branchial space. The new suborder, Phoratopidea, is for one species with unique, broad articles of pereopods 3 and 4 with reduced dactyls. It lacks the synapomorphies of the following two suborders. In members of the suborder Cymothoida, the mandibular molar is either a flat triangular blade, reduced to a conical process, or absent, and the maxillipedal endite is rarely longer than palp article 1 (or is absent), distally tapering and has few setae. The suborder Limnoriidea is diagnosed as lacking the mandibular molar, and the non-tapering, slender (except in Keuphylia ) maxillipedal endite reaches to at least the distal margin of palp article 4. Members of the second clade share a vaulted pleotelson enclosing a branchial chamber defined by ventrolateral ridges and uropods lateral to the pleotelson margin that fold down alongside the branchial space. It comprises two suborders. Members of the Sphaeromatidea have pleonite 1 much narrower than pleonite 2 and a reduced (or absent) right lacinia mobilis fused to the spine row. They lack operculiform uropods, which characterise Valvifera. The suborder Anthuridea is reduced to superfamily rank and Epicaridea is reduced to two superfamilies within Cymothoida. Unambiguous relationships between most families are resolved, but Sphaeromatidae is suspected to be paraphyletic, Paravireia is placed as the most plesiomorphic of the Sphaeromatoidea and a new family, Basserolidae, is proposed. The Tainisopidea includes freshwater taxa in a relictual environment. The sole species of Phoratopidea is marine, rare, and its ecology is unknown. The Cymothoida are most diverse in tropical regions. Members of the most plesiomorphic family, the Cirolanidae, are mobile predators or scavengers and the more derived families are ectoparasites on fishes and other crustaceans. Members of the Limnoriidea are mainly tropical and at least one family is herbivorous. The Valvifera and Sphaeromatidea are benthic, with respiratory pleopods in a branchial chamber. They are most diverse in the temperate southern hemisphere, and most are detritivores.

Our knowledge of the biodiversity of the Southern Ocean (SO) deep benthos is scarce. In this review, we describe the general biodiversity patterns of meio-, macro- and megafaunal taxa, based on historical and recent expeditions, and against the background of the geological events and phylogenetic relationships that have influenced the biodiversity and evolution of the investigated taxa. The relationship of the fauna to environmental parameters, such as water depth, sediment type, food availability and carbonate solubility, as well as species interrelationships, probably have shaped present-day biodiversity patterns as much as evolution. However, different taxa exhibit different large-scale biodiversity and biogeographic patterns. Moreover, there is rarely any clear relationship of biodiversity pattern with depth, latitude or environmental parameters, such as sediment composition or grain size. Similarities and differences between the SO biodiversity and biodiversity of global oceans are outlined. The high percentage (often more than 90%) of new species in almost all taxa, as well as the high degree of endemism of many groups, may reflect undersampling of the area, and it is likely to decrease as more information is gathered about SO deep-sea biodiversity by future expeditions. Indeed, among certain taxa such as the Foraminifera, close links at the species level are already apparent between deep Weddell Sea faunas and those from similar depths in the North Atlantic and Arctic. With regard to the vertical zonation from the shelf edge into deep water, biodiversity patterns among some taxa in the SO might differ from those in other deep-sea areas, due to the deep Antarctic shelf and the evolution of eurybathy in many species, as well as to deep-water production that can fuel the SO deep sea with freshly produced organic matter derived not only from phytoplankton, but also from ice algae.

Remote deep-ocean sediment (DOS) ecosystems are among the least explored biomes on Earth. Genomic assessments of their biodiversity have failed to separate indigenous benthic organisms from sinking plankton. Here, we compare global-scale eukaryotic DNA metabarcoding datasets (18 S -V9) from abyssal and lower bathyal surficial sediments and euphotic and aphotic ocean pelagic layers to distinguish plankton from benthic diversity in sediment material. Based on 1685 samples collected throughout the world ocean, we show that DOS diversity is at least threefold that in pelagic realms, with nearly two-thirds represented by abundant yet unknown eukaryotes. These benthic communities are spatially structured by ocean basins and particulate organic carbon (POC) flux from the upper ocean. Plankton DNA reaching the DOS originates from abundant species, with maximal deposition at high latitudes. Its seafloor DNA signature predicts variations in POC export from the surface and reveals previously overlooked taxa that may drive the biological carbon pump.

Discoveries of new species often depend on one or a few specimens, leading to delays as researchers wait for additional context, sometimes for decades. There is currently little professional incentive for a single expert to publish a stand-alone species description. Additionally, while many journals accept taxonomic descriptions, even specialist journals expect insights beyond the descriptive work itself. The combination of these factors exacerbates the issue that only a small fraction of marine species are known and new discoveries are described at a slow pace, while they face increasing threats from accelerating global change. To tackle this challenge, this first compilation of Ocean Species Discoveries (OSD) presents a new collaborative framework to accelerate the description and naming of marine invertebrate taxa that can be extended across all phyla. Through a mode of publication that can be speedy, taxonomy-focused and generate higher citation rates, OSD aims to create an attractive home for single species descriptions. This Senckenberg Ocean Species Alliance (SOSA) approach emphasises thorough, but compact species descriptions and diagnoses, with supporting illustrations and with molecular data when available. Even basic species descriptions carry key data for distributions and ecological interactions (e.g., host-parasite relationships) besides universally valid species names; these are essential for downstream uses, such as conservation assessments and communicating biodiversity to the broader public. This paper presents thirteen marine invertebrate taxa, comprising one new genus, eleven new species and one re-description and reinstatement, covering wide taxonomic, geographic, bathymetric and ecological ranges. The taxa addressed herein span three phyla (Mollusca, Arthropoda, Echinodermata), five classes, eight orders and twelve families. Apart from the new genus, an updated generic diagnosis is provided for four other genera. The newly-described species of the phylum Mollusca are Placiphorella methanophila Vončina, sp. nov. (Polyplacophora, Mopaliidae), Lepetodrilus marianae Chen, Watanabe & Tsuda, sp. nov. (Gastropoda, Lepetodrilidae), Shinkailepas gigas Chen, Watanabe & Tsuda, sp. nov. (Gastropoda, Phenacolepadidae) and Lyonsiella illaesa Machado & Sigwart, sp. nov. (Bivalvia, Lyonsiellidae). The new taxa of the phylum Arthropoda are all members of the subphylum Crustacea: Lepechinella naces Lörz & Engel, sp. nov. (Amphipoda, Lepechinellidae), Cuniculomaera grata Tandberg & Jażdżewska, gen. et sp. nov. (Amphipoda, Maeridae), Pseudionella pumulaensis Williams & Landschoff, sp. nov. (Isopoda, Bopyridae), Mastigoniscus minimus Wenz, Knauber & Riehl, sp. nov. (Isopoda, Haploniscidae), Macrostylis papandreas Jonannsen, Riehl & Brandt, sp. nov. (Isopoda, Macrostylidae), Austroniscus indobathyasellus Kaiser, Kniesz & Kihara, sp. nov. (Isopoda, Nannoniscidae) and Apseudopsis daria Esquete & Tato, sp. nov. (Tanaidacea, Apseudidae). In the phylum Echinodermata, the reinstated species is Psychropotes buglossa E. Perrier, 1886 (Holothuroidea, Psychropotidae). The study areas span the North and Central Atlantic Ocean, the Indian Ocean and the North, East and West Pacific Ocean and depths from 5.2 m to 7081 m. Specimens of eleven free-living and one parasite species were collected from habitats ranging from an estuary to deep-sea trenches. The species were illustrated with photographs, line drawings, micro-computed tomography, confocal laser scanning microscopy and scanning electron microscopy images. Molecular data are included for nine species and four species include a molecular diagnosis in addition to their morphological diagnosis. The five new geographic and bathymetric distribution records comprise Lepechinella naces Lörz & Engel, sp. nov., Cuniculomaera grata Tandberg & Jażdżewska, sp. nov., Pseudionella pumulaensis Williams & Landschoff, sp. nov., Austroniscus indobathyasellus Kaiser, Kniesz & Kihara, sp. nov. and Psychropotes buglossa E. Perrier, 1886, with the novelty spanning from the species to the family level. The new parasite record is Pseudionella pumulaensis Williams & Landschoff, sp. nov., found in association with the hermit crab Pagurus fraserorum Landschoff & Komai, 2018.

The role of geomorphological features as drivers for benthic deep-sea biodiversity remains poorly understood. By disentangling the putative Haploniscus belyaevi Birstein, 1963a species complex from the abysso-hadal Kuril-Kamchatka Trench (KKT) region in the North-west Pacific Ocean, we aim to shed light on deep-sea differentiation and how it is related to potential bathymetric barriers such as the KKT and the Kuril-Island Ridge (KIR). Our integrative taxonomic approach featured morphological and molecular delimitation methods, also considering the post-marsupial development due to pronounced sexual dimorphism. Mitochondrial 16S and COI markers were sequenced and several molecular species delimitation methods were applied. By combining the different results we were able to delineate six distinct species within the belyaevi complex, including several morphologically cryptic species, and found hints of three additional species groups in the complex. Even though several of these species were distributed across the KKT and/or KIR, limited gene flow and depth-differentiation were indicated supporting previous notions that these geomorphological features play a role in deep-sea benthos speciation.