Amphipod crustaceans were collected at all 55 stations sampled with an epibenthic sledge during two IceAGE expeditions (Icelandic marine Animals: Genetics and Ecology) in 2011 and 2013. In total, 34 amphipod families and three superfamilies were recorded in the samples. Distribution maps are presented for each taxon along with a summary of the regional taxonomy for the group. Statistical analyses based on presence/absence data revealed a pattern of family distributions that correlated with sampling depth. Clustering according to the geographic location of the stations (northernmost North Atlantic Sea and Arctic Ocean) can also be observed. IceAGE data for the Amphilochidae and Oedicerotidae were analysed on species level; in case of the Amphilochidae they were compared to the findings from a previous Icelandic benthic survey, BIOICE (Benthic Invertebrates of Icelandic waters), which also identified a high abundance of amphipod fauna.

Amphipods constitute an abundant part of Icelandic deep-sea zoobenthos yet knowledge of the diversity of this fauna, particularly at the molecular level, is scarce. The present work aims to use molecular methods to investigate genetic variation of the Amphipoda sampled during two IceAGE collecting expeditions. The mitochondrial cytochrome oxidase subunit 1 (COI) of 167 individuals originally assigned to 75 morphospecies was analysed. These targeted morhospecies were readily identifiable by experts using light microscopy and representative of families where there is current ongoing taxonomic research. The study resulted in 81 Barcode Identity Numbers (BINs) (of which >90% were published for the first time), while Automatic Barcode Gap Discovery revealed the existence of 78 to 83 Molecular Operational Taxonomic Units (MOTUs). Six nominal species (Rhachotropis helleri, Arrhis phyllonyx, Deflexilodes tenuirostratus, Paroediceros propinquus, Metopa boeckii, Astyra abyssi) appeared to have a molecular variation higher than the 0.03 threshold of both p-distance and K2P usually used for amphipod species delineation. Conversely, two Oedicerotidae regarded as separate morphospecies clustered together with divergences in the order of intraspecific variation. The incongruence between the BINs associated with presently identified species and the publicly available data of the same taxa was observed in case of Paramphithoe hystrix and Amphilochus manudens. The findings from this research project highlight the necessity of supporting molecular studies with thorough morphology species analyses.

As one of the oldest branches of biology, taxonomy deals with the identification, classification and naming of living organisms, using a variety of tools to explore traits at the morphological and molecular level. In the deep sea, particular challenges are posed to the taxonomic differentiation of species. Relatively limited sampling effort coupled with apparent high diversity, compared to many other marine environments, means that many species sampled are undescribed, and few specimens are available for each putative species. The resulting scarce knowledge of intraspecific variation makes it difficult to recognize species boundaries and thus to assess the actual diversity and distribution of species. In this review article, we highlight some of these challenges in deep-sea taxonomy using the example of peracarid crustaceans. Specifically, we offer a detailed overview of traditional as well as modern methods that are used in the taxonomic analysis of deep-sea Peracarida. Furthermore, methods are presented that have not yet been used in peracarid taxonomy, but have potential for the analysis of internal and external structures in the future. The focus of this compilation is on morphological methods for the identification, delimitation and description of species, with references to molecular analysis included where relevant, as these methods are an indispensable part of an integrative taxonomic approach. The taxonomic impediment, i.e. the shortage of taxonomists in view of a high undescribed biodiversity, is discussed in the context of the existing large taxonomic knowledge gaps in connection with the increasing threat to deep-sea ecosystems. Whilst peracarid crustaceans are used here as an exemplary taxon, the methodology described has broad relevance to many other deep-sea taxa, and thus will support broader research into deep-sea biodiversity and ecology more widely.