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The Antarctic seasonal ice zone (SIZ) is an ecologically important region of the Southern Ocean. However, the abundance and vertical distribution of micro- and mesozooplankton in this zone, in particular the contribution of nauplii of smaller copepod taxa, are poorly understood. We investigated the vertical distribution, abundance, and species composition of micro- and mesozooplankton in the SIZ of Luetzow-Holm Bay, East Antarctica, in January 2009. Zooplankton samples were collected using a 60- mu m mesh closing net (appropriate to estimate the quantitative abundance of copepod nauplii) vertically hauled from three depth layers (0-100, 100-200, 200-500 m). Small calanoid copepods, Oithonidae, and Oncaeidae were dominant among the zooplankton communities in this region. Cluster analysis revealed that the communities were separated by depth into epipelagic (0-200 m) and mesopelagic (>200 m) groups. Distribution patterns in the 0- to 100-m layer changed at the sea ice edge, but showed no clear relationship with frontal structures. Nauplii of copepods including Oithona spp., Oncaea spp., and Calanoida were major contributors to total zooplankton abundance. The abundance of these nauplii in the surface layer was high in open water and the marginal ice zone but low in the pack ice zone where chlorophyll a concentrations were high. Low water temperature associated with sea ice, rather than food availability, may have caused the low copepod nauplii abundance observed in the surface layer within the pack ice zone. These findings provide valuable background information to monitor variations in zooplankton community structure in response to changing environmental conditions.

Since the discovery of the first oncaeid copepod described by Philippi in 1843 as Oncaea venusta, great progress has been achieved regarding the morphological/descriptive taxonomy of the microcopepod family Oncaeidae, occurring in all great oceans and all depth layers of the ocean. The species diversity of this family is still underestimated and the ecological role of oncaeids within the marine ecosystem is not yet well understood, but the life strategy appears to be fundamentally different from most other pelagic microcopepod families. The present paper aims at a comprehensive review of the current state of knowledge of this microcopepod family, including taxonomic and phylogenetic issues, questions of species identification, specific morphological and molecular genetic characteristics, information on regional and vertical distribution and abundance, motion behaviour, feeding and food relationships, reproduction aspects, biomass and elemental composition, respiration and metabolic rates. Relevant open questions are highlighted, and examples are given of shortcomings and high uncertainties in results of current attempts to include oncaeid copepods in various aspects of global marine ecosystem studies. It is concluded that continued support of taxonomic research is required for Oncaeidae and other small copepod species, based on an integrated approach of morphological and molecular genetic methods and user-friendly regional identification keys, to allow an adequate consideration of oncaeids in advanced ecological studies and to achieve a better understanding of the ecological role of this abundant microcopepod family in marine ecosystems.

The Oncaeidae is an important family of small copepods in marine ecosystems and is characterized by high diversity and abundance. The family was established by Giesbrecht in the late 19 th century. Since then, 115 oncaeid species have been discovered and described. However, Oncaeidae species are small-sized and show considerable morphological similarity. The family includes a great number of sibling species and intraspecific form variants. Consequently, there are large knowledge gaps regarding their taxonomy and diversity, and their importance in marine ecosystems may have been underestimated. In this study, we have charted the progress of oncaeid taxonomy and diversity studies worldwide. We have identified the major challenges faced by the researchers and have described the marine science techniques. We also provide an overview of the distribution and ecology of the family Oncaeidae. Progress has been made toward understanding the distribution, habitat preference, and feeding behavior of oncaeids. The knowledge of oncaeids taxonomy has also been considerably increased by improvements in the research methodology, such as the use of molecular biology techniques and interdisciplinary approaches in tandem with the traditional taxonomic methods.

Three species of Spinoncaea Böttger-Schnack, 2003 are newly recorded in three locations of the equatorial and temperate Pacific Ocean collected by using a net of 60 μm mesh size. For all three species, morphological characters and patterns of ornamentation were analyzed in detail and illustrations of both sexes, also including form variants of the females, are provided. For the first time, information about the variability of various continuous (morphometric) characters are given, such as the spine lengths on the rami of the swimming legs or the proportions of urosomites. The complementary morphological descriptions of the Pacific specimens focus on similarities or modifications of characters as compared to earlier descriptions of these species from the type locality and various other localities. For S. ivlevi (Shmeleva, 1966), originally but insufficiently described from the Adriatic Sea, the Pacific material is similar in most aspects to the comprehensive redescription of the species from the Red Sea and from the type locality, except for a difference in the morphometry of the distal endopod segment on the antenna, which is discussed here. For S. tenuis Böttger-Schnack, 2003, and S. humesi Böttger-Schnack, 2003, the Pacific material mostly coincides with the characteristic features as described in the original account from the Red Sea. For all three species, differences and/or additions in ornamentation details were found in Pacific specimens (e.g., on the intercoxal sclerite of the first swimming leg or on the genital somite of the male) and females with aberrant morphology were detected. Genetic analyses based on 12S srRNA revealed for two species, S. ivlevi and S. humesi , little or no differences in genetic sequences between Pacific specimens and those recorded from the Mediterranean Sea, thus demonstrating that specimens from both locations are conspecific. For S. tenuis , for which no comparable genetic data are available, 12S srRNA amplification was unsuccessful as was the amplification of mitochondrial COI (barcoding) for all three species. The applicability of using COI amplification for barcoding of oncaeid copepods is discussed.

The pelagic marine copepod family Oncaeidae is highly diversified (over 100 species worldwide) and includes a great number of sibling species, which are difficult to identify morphologically, because of their very small size (0.18–1.2 mm total length as adults). Global investigations of oncaeid biodiversity are severely hampered by insufficient taxonomic knowledge, in particular for species which have first been described from the European Mediterranean Sea ( type locality ). Many of these species have been reported as key taxa of small-sized copepod communities in very distant oceanic regions. However, due to the taxonomic uncertainties it cannot be excluded that at least some of these allegedly cosmopolitan species in reality represent a complex of distinct, yet closely related, species. To improve the basis for the identification of Oncaeidae of Mediterranean origin, new diagnostic characters in combination with traditional methods were applied in the present study. Copepods were sampled with fine nets of 0.1 mm mesh size down to a maximum depth of 1,000 m on a west-east-transect in the Mediterranean Sea. Oncaeid species and form variants were predefined morphologically and the genetic identity of the morphospecies was analysed by about 650 and 500 bp region of the mitochondrial COI and 12S srRNA gene sequence, respectively (barcoding). A total of 67 individuals from 24 oncaeid species and forms were successfully analysed, including 12 species and one form of Mediterranean origin. For Oncaeidae, the 12S amplification turned out to be more successful (23 species) than the COI amplification (13 species and 1 form). Together, the morphological and molecular results are discussed with respect to three topics: (1) confirmation of a genetic distinction of three Triconia species, which have been interpreted as sibling species by morphological characters, (2) genetic distance of species within the ovalis -complex of oncaeids and (3) the taxonomic status of two form variants of Oncaea mediterranea (Claus).

An interactive (multi-access) global identification key ( OncIdent ) has been developed for the pelagic marine microcopepod family Oncaeidae and made accessible online. Details of the general approach and development of the key are given in Böttger-Schnack and Schnack (J Nat Hist 49:2727–2741, 2015 ). After beta-testing, new additions include illustrations for all species and feature attributes considered, plus a textual summary of each species’ feature states in the key. Additional taxonomic notes are given where required, highlighting morphological or molecular genetic peculiarities or problems, with links to large data bases leading directly to more comprehensive information about each species. The present paper briefly reviews the taxonomic background for key construction, summarizes the opportunities and limitations of the current online version OncIdent2.0 , and provides guidance for its practical use.

Species composition and vertical distribution of oncaeid copepods, which are potentially important prey for juvenile fish, were investigated in the Kuroshio Extension region, the NW Pacific, in April, August, November 1998 and February 2001. Samples were collected from 8 discrete layers in the epipelagic zone (0–200 m depth) using MOCNESS (0.064 mm mesh) during both day and night. Thirty-five oncaeid species were identified. ‘Oncaea’ (s.l.) zernovi and Spinoncaea ivlevi were numerically the dominant species comprising 20.0–48.2% and 15.2–26.8%, respectively, of adult oncaeid copepods in the epipelagic zone. Cluster analysis on all samples revealed that these were separated into three groups with discrete vertical ranges; the first one appearing in the 0–50 m depth surface layer in April and August and consisting mainly of Oncaea (s. str.), the second one located in the deepest layer and composed mostly of ‘O.’ zernovi and S. ivlevi with some mesopelagic species, and the third one located above the second one and having intermediate species composition. Species-specific vertical distributions indicate that most oncaeid populations shifted downward from August to November, when the thermocline remarkably descended. However, most Oncaea spp. did not show a downward shift with the thermocline, and were positively correlated to appendicularian abundances, suggesting that appendicularian houses, known to be oncaeid habitats and to provide food, were a possible factor affecting their vertical distribution. Niche partitioning, allowing coexistence of congeners, might be explained by differences in body size and distribution layers in Oncaea and by differences in distribution layer in Triconia.

Two new species of Triconia in Oncaeidae, including both sexes of Triconia constricta n. sp. and females of Triconia pararedacta n. sp., are described from south of Jeju Island in the East China Sea. Triconia constricta belongs to the dentipes-subgroup of Triconia characterized by the absence of integumental pockets on the anterior surface of the labrum. It is distinguished from the closely related species of this subgroup, T. dentipes (Giesbrecht, 1891), T. elongata Böttger-Schnack, 1999, and T. giesbrechti Böttger-Schnack, 1999, by the following combination of morphological features in females: 1) lateral margins of genital double-somite in dorsal view slightly constricted at midlength; 2) P5 with very long outer basal seta, reaching beyond paired secretory pores on posterior part of genital double-somite, as well as distinctive length ratios of exopodal setae; in both sexes; 3) length ratios of caudal setae, and 4) relative spine lengths on distal endopodal segments of swimming legs 2 to 4. Triconia pararedacta is a member of the conifera-subgroup characterized by a dorsal projection on the second pedigerous somite in the female. It differs from females of other species of this subgroup in the following: 1) very small-sized dorsal projection on second pedigerous somite, 2) different length to width ratio of P5 exopod, 3) relative lengths of outer basal seta and exopodal setae of P5, and 4) different length ratio of outer distal spine to distal spine on endopods of P2-P4. Additional character states are proposed for defining the dentipes-subgroup within Triconia and for recognizing three sets of species within the conifera-subgroup.

The species diversity of the pelagic microcopepod family Oncaeidae collected with nets of 0.1-mm mesh size was studied at 6 stations along a west-to-east transect in the Mediterranean Sea down to a maximum depth of 1,000 m. A total of 27 species and two form variants have been identified, including three new records for the Mediterranean. In addition, about 20, as yet undescribed, new morphospecies were found (mainly from the genera Epicalymma and Triconia) which need to be examined further. The total number of identified oncaeid species was similar in the Western and Eastern Basins, but for some cooccurring sibling species, the estimated numerical dominance changed. The deep-sea fauna of Oncaeidae, studied at selected depth layers between 400 m and the near-bottom layer at >4,200 m depth in the eastern Mediterranean (Levantine Sea), showed rather constant species numbers down to -3,000 m depth. In the near-bottom layers, the diversity of oncaeids declined and species of Epicalymma strongly increased in numerical importance. The taxonomic status of all oncaeid species recorded earlier in the Mediterranean Sea is evaluated: 19 out of the 46 known valid oncaeid species are insufficiently described, and most of the taxonomically unresolved species (13 species) have originally been described from this area (type locality). The deficiencies in the species identification of oncaeids cast into doubt the allegedly cosmopolitan distribution of some species, in particular those of Mediterranean origin. The existing identification problems even of well-described oncaeid species are exemplified for the Oncaea mediacomplex, including O. media Giesbrecht, O. scottodicarloi Heron and Bradford-Grieve, and O. waldemari Bersano and Boxshall, which are often erroneously identified as a single species (0. media). The inadequacy in the species identification of Oncaeidae, in particular those from the Atlantic and Mediterranean, is mainly due to the lack of reliable identification keys for Oncaeidae in warm-temperate and/or tropical seas. Future efforts should be directed to the construction of identification keys that can be updated according to the latest taxonomic findings, which can be used by the non-expert as well as by the specialist. The adequate consideration of the numerous, as yet undescribed, microcopepod species in the world oceans, in particular the Oncaeidae, is a challenge for the study of the structure and function of plankton communities as well as for global biodiversity estimates.

Community structure and vertical distribution of oncaeid copepods were investigated at an offshore site in Tosa Bay, southern Japan. Samples were collected with a 0.063 mm closing type net from eight discrete layers between the surface and 500 m depth in August and November 2005. A total of 45 species and three form variants belonging to the genera Triconia, Oncaea, Spinoncaea, Conaea and Epicalymma were identified. The greatest number of species was consistently found in the lower epipelagic zone, at 50–100 m. The vertical distribution patterns of oncaeid copepods were similar between August and November, with the highest abundances in the upper epipelagic zone above the thermocline. The oncaeid maxima corresponded with the depth where appendicularians occurred in high numbers. Dominant species in the water column (0–500 m) were O. venusta small form, O. media, O. scottodicarloi, O. waldemari, O. zernovi, O. tregoubovi, S. ivlevi, S. tenuis and T. conifera. The vertical succession of species composition was almost the same between August and November, although several species showed a downward shift of their depth-range to some extent in November. The oncaeid copepod community could be differentiated into three distinct groups according to the depth layers, each corresponding to different hydrographic conditions in the water column.