Ryocalanus infelix, Tanaka, 1956

Ryocalanus infelix View in CoL described by Tanaka (1956). Park

(1986) recognized the family Spinocalanidae Vervoort, 1951 , previously included in Clausocalanoidea by Andronov (1974), to form a separate superfamily Spinocalanoidea based on the fact that the mostly bathypelagic Spinocalanoidea have less specialized features such as the presence of an outer seta on the maxilla and the swimming leg setation ( Park, 1986). Park also noted the similarity of mouthparts and swimming legs of Ryocalanoidea and Spinocalanoidea , but pointed out the marked difference between these taxa, based on the grasping right antennule of males.

Spinocalanoidea currently contain two families: Spinocalanidae View in CoL and Arctokonstantinidae View in CoL . The latter family was established by Markhaseva & Kosobokova (2001) andlatertreatedasasynonymforSpinocalanidae by Boxshall & Halsey (2004). Markhaseva (2008) and Markhaseva & Schulz (2008) gave a detailed analysis of Arctokonstantinidae View in CoL , concluding that, based on the derived morphology of the oral parts, as well as the basis and endopod of the first leg, this family represents a monophyletic group. Foxtonia Hulsemann & Grice, 1963 View in CoL and Sognocalanus Fosshagen, 1967 View in CoL , previously placed in Spinocalanidae View in CoL and Bathypontiidae View in CoL , were placed in Arctokonstantinidae (Markhaseva, 2008) View in CoL together with Arctokonstantinus Markhaseva & Kosobokova, 2001 View in CoL , Foxtosognus Markhaseva, 2008 View in CoL and Caudacalanus Markhaseva & Schulz, 2008 View in CoL .

In recent studies, genetic analyses of copepods have generated new insights into the origin and evolution of the Calanoida View in CoL (e.g. Ohtsuka & Nishida, 2017). Both, mitochondrial and nuclear molecular markers have been used for phylogenetic analyses to elucidate the evolutionary history of living organisms and have been proven to be useful in reconstructing copepod phylogenetic relationships (e.g. Blanco-Bercial et al., 2011; Laakmann et al., 2012; Bradford-Grieve et al., 2014, 2017). While species and population levels can be resolved based on mitochondrial gene fragments like cytochrome c oxidase subunit I (COI) (e.g. Bucklin et al., 2003; Goetze, 2003; Eyun et al., 2007; Aarbakke et al., 2014; Questel et al., 2016) and cytochrome b ( Provan et al., 2009; Milligan et al., 2011), nuclear 18S rDNA, 28S rDNA and internal transcribed spacer 2 (ITS2) (e.g. Braga et al., 1999; Bucklin et al., 2003; Laakmann et al., 2012) are more conserved and thus informative for phylogenetic analyses at intergeneric and higher taxonomic levels.

Multi-gene analyses of the calanoid superfamilies have been made to investigate the relationships within the Calanoida ( Blanco-Bercial et al., 2011; Bradford-Grieve et al., 2014) by using both mitochondrial and nuclear ribosomal gene regions. These analyses demonstrated a high support of the morphology-based phylogeny by Andronov (1974) with its amendments made by Bowman & Abele (1982) and Park (1986). These molecular-based phylogenetic studies did not, however, include representatives of the superfamilies Ryocalanoidea and Epacteriscidae , and refinements in the currently available phylogeny are expected when new data are added.

The discovery of two new species of ryocalanid copepods in the Kurile-Kamchatka trench belonging to the genera Ryocalanus Tanaka, 1956 and Yrocalanus Renz, Markhaseva & Schulz, 2012 and the description of the previously unknown female of Ryocalanus infelix , the type species of the Ryocalanidae , presented an opportunity to combine morphological and molecular data to further our knowledge on the phylogeny of the Calanoida .

Our morphological studies were complemented with molecular analyses of ryocalanoidean and spinocalanoidean copepod species using multi-gene approaches to gain insight into the relationship between the evolutionarily youngest calanoid copepod families from a molecular perspective. The phylogeny of the Ryocalanoidea is discussed, based on a combined morphological and molecular approach.