Yrocalanus kurilensis, Renz & Markhaseva & Laakmann, 2018

YROCALANUS KURILENSIS View in CoL SP. NOV.

( FIGS 11–14)

Type material

Holotype: Adult female, dissected, body length 1.75 mm, collection number SMF 37156 About SMF / 1–5 (one vial, four slides); Kurile-Kamchatka trench, 43.5666° N, 153.9666° E, station 5–10, project KuramBio, 11 August 2012, above the sea bed at a depth of 5376 m. GoogleMaps

Paratypes: One adult female, body length 1.75 mm, collection number SMF 37157 About SMF /1– 4 (one vial, three slides); Kurile-Kamchatka trench 42.2333° N, 151.7000° E, station 9–9, project KuramBio, 20 August 2012, above the sea bed at a depth of 5125 m GoogleMaps .

One adult male, body length 1.58 mm, collection number SMF 37158 About SMF /1–6 (one vial, five slides); Kurile-Kamchatka trench, 46.2333° N, 155.5333° E, station 2–9, project KuramBio, 3 August 2012, above the sea bed at a depth of 4863 m GoogleMaps .

One adult male, body length 1.53 mm, collection number SMF 37159 About SMF /1–5 (one vial, four slides); Kurile-Kamchatka trench, 46.9740° N, 157.3048° E, station 1–11, project KuramBio, 30 July 2012, above the sea bed at a depth of 5418 m GoogleMaps .

Etymology: The specific name is derived from the location of collection, the Kurile-Kamchatka trench.

Description: Based on female holotype unless otherwise stated.

Adult female: Total length 1.75 mm; prosome 4.6 times as long as urosome ( Fig. 11A, B). Rostrum ( Fig. 11A, C) two-pointed. Cephalosome and pediger 1 partly fused ( Fig. 11A, B), pedigers 4–5 separate; in lateral view posterolateral corners of prosome extended posteriorly into points, reaching distal margin of the genital double-somite. Urosome composed of genital double-somite and three articulated somites ( Fig. 11D, E). Genital double-somite symmetrical, with short spinules distolaterally on right side and ventromedial genital opening; in lateral view seminal receptacles elongated, turned upward. Dorsal posterior margins of genital double-somite to third urosomal somite each with row of spinules ( Fig. 11D). Caudal rami symmetrical, with row of spinules on inner margin and with two lateral setae (II and III), three terminal setae (IV–VI) and one dorsal seta (VII).

Antennule ( Fig. 11F) of 24 free segments and extending to distal border of pediger 2. In holotype armature as follows:

I – 2s + 1ae, II–IV – 4s + 3?, V – 2s + 1ae, VI – 2s, VII – 2s + 1ae, VIII – 2s, IX – 2s + 1ae; X–XI – 4s + 1ae?, XII – 1s + 1ae, XIII – 2s + 1ae, XIV – 2s + 1ae, XV – 1s + 1ae, XVI – 2s + 1ae, XVII – 2s + 1ae, XVIII – 2s + 1ae, XIX – 2s + 1ae, XX – 2s + 1ae, XXI – 2s + 1ae, XXII – 0s, XXIII – 1s, XXIV – 2s, XXV – 2s, XXVI – 2s, XXVII–XXVIII – 4s + 1ae.

Antenna ( Fig. 12A), coxa with 1, basis with 2 setae; endopod segment 1 with 2 setae, segment 2 with 17 setae; exopod 8-segmented, with 1, 3, 1, 1, 1, 1, 1, 3 setae.

Mandible ( Fig. 12B, C), gnathobase cutting edge with 8 unequal teeth plus ventral seta; basis with 3 setae; exopodal segments incompletely fused, with 6 setae; first endopod segment with 2 setae, second with 10 setae.

Maxillule ( Fig. 12D), praecoxal arthrite with 9 terminal spines, 4 posterior and 1 anterior setae; posterior and anterior surface of praecoxal arthrite with small spinules; coxal endite with 6 setae, coxal epipodite with 8 setae; proximal basal endite with 4 setae, distal basal endite with 5 setae; endopod with 14 setae; exopod with 11 setae.

Maxilla ( Fig. 12E), proximal praecoxal endite bearing 3 setae plus attenuation in holotype, 4 setae plus attenuation in paratype, distal praecoxal endite with 3 setae; coxal endites with 3 setae each; coxa with 1 outer seta; proximal basal endite with 3 setae; remaining endopod with 8 setae. All endites except for praecoxal endite with surface spinules.

Maxilliped ( Fig. 13A), syncoxa with 1 seta on proximal praecoxal endite, 2 setae on middle endite, and 3 setae on distal praecoxal endite; coxal endite with 3 setae; syncoxa with row of spinules. Basis with 3 distal setae; endopod with 2, 4, 4, 3, 3 + 1, and 4 setae and row of spinules at setae basis on segment 3 and 4.

Legs 1–4 biramous ( Fig. 13B–E), with 3-segmented exopods and 3-segmented endopods, except leg 1 endopod 1-segmented and leg 2 endopod 2-segmented. Leg 1–4 coxa with inner surface spinules. Legs 2–4 endopod segments with rows of spinules on posterior surface. Terminal spine on exopod segment 3 finely serrated. Seta and spine formula as in Table 5. Leg 1 ( Fig. 13B), basis with medial and lateral spinules; endopod lateral lobe poorly developed with spinules; exopod segment 2 with long lateral spine extending to distal end of exopod 3, and medial spinules, segment 3 terminal spine ca. 2.2 times as long as exopod.

Leg 3 ( Fig. 13D), exopod segments 1 and 2 with lateral spine (broken in segment 2 in paratype), segment 3 with three lateral spines (1 spine broken in paratype).

Leg 4 ( Fig. 13E), coxa with 3 strong distolateral spines and patches of spinules on posterior surface; basis with row of spinules on distal margin.

Adult male: Total length 1.53 and 1.58 mm ( Fig.14A, B). Rostrum ( Fig. 14A, C) two-pointed. Cephalosome and pediger 1 almost completely fused ( Fig. 14A, B), pedigers 4 and 5 separate. In lateral view posterolateral corners of prosome extended posteriorly into points,

slightly extending urosomite 1. Caudal rami ( Fig. 14D) symmetrical, with two lateral setae (II and III), three terminal setae (IV–VI) and one dorsal seta (VII)

Left antennule ( Fig. 14E) unmodified, of 24 free segments, extending to urosome; armature as follows: I – 1s + 1ae, II–IV – 6s + 4ae, V – 2s + 2ae, VI – 2s + 1ae, VII – 2s + 2ae, VIII – 2s + 1ae, IX – 2s + 2ae, X–XI – 4s + 3ae?, XII – 1s + 1ae; XIII – 2s + 1ae; XIV – 2s + 1ae, XV – 1s + 1ae, XVI – 2s + 1ae, XVII – 2s + 1ae, XVIII – 2s + 1ae, XIX – 2s + 1ae, XX – 2s + 1ae, XXI – 2s + 1ae, XXII – 0s XXIII – 1s + 1ae, XXIV – 2s + 1ae, XXV – 2s + 1ae, XXVI – 2s, XXVII–XXVIII – 3s + 1ae.

Right antennule ( Fig. 14F, G) strongly modified for grasping, of 24 free segments; segments XX–XXVI strongly enlarged; segment XX with 1 distal strong hook-like attenuation, segment XXI with a straight, strong, spine-like attenuation and a serrated strong spine, segments XXIII–XXIV with a large plate, segment XXVI with lateral lamella; hinges occurring between segments XVIII and XIX, XIX and XX, XX and XXI, and XXII and XXIII. Armature as follows: I – 1s + 1ae, II to III – 4s + 3ae, IV – 2s + 1ae, segments V-XI armature as in left antennule; XII – 1s + 1ae, XIII-XIX armature as in left antennule, XX – 1s + 1ae + spine like attenuation XXI – 1s + 2 spine like attenuations, XXII – 1s + 1ae, XXIII –XXIV – 2s +

1ae + 1?, XXV – 2s + 1ae, XXVI – 2s, XXVII-XVIII – 4s + 1ae.

Antenna as in female, except endopod segment 2 with 16 setae. Mandible similar to that of female, except basis with 2 setae. Maxillule as in female, but exopod with 10 setae. Maxilla similar to that of female, but proximal praecoxal endite bearing 3 setae. Maxilliped as in female.

Segmentation of legs 1–4 as in female. Coxa of leg 4 without strong distolateral spines. Endo- and exopods of leg 4 missing in paratypes. Leg 5 ( Fig. 14H) uniramous on both sides. Right leg 2-segmented, with small terminal spine, shorter than left leg. Left leg with 3-segmented exopod. Exopod segment 2 with row of spinules distally, exopod segment 3 with medial row of spinules.

R e m a rk s: T h e n e w s p e c i e s s h a r e s t h e m a i n morphological characters with species of the genus Yrocalanus ( Renz et al., 2013) . Both sexes are so far only known for Y. antarcticus Renz, Markhaseva & Schulz, 2012 , while Y. admirabilis Andronov, 1992 is only known from a male and Y. bicornis and Y. asymmetricus Markhaseva & Ferrari, 1996 only from female specimens.

Females of Yrocalanus kurilensis sp. nov. are easily distinguished from the remaining species of this genus by the shape of the rostrum, which is wider at its tips than in other Yrocalanus species, the shape of the posterolateral corners of the prosome with narrowed points, which is not found in other Yrocalanus species, the form of the genital double-somite, which is symmetrical in Yrocalanus kurilensis sp. nov, but asymmetrical in Y. asymmetricus , Y. bicornis ( Markhaseva & Ferrari, 1996) and Y. antarcticus ( Renz et al., 2012) and the 3 robust spines on the coxa of P4 (only 2 in other Yrocalanus females). At least two different types of setae could be observed on the antennule, with short, frayed setae occurring on segment V–VIII, XI, XIV–XVI, XVIII and XX–XXI.

Males of Yrocalanus kurilensis sp. nov. differ from the remaining species of this genus in the shape of ancestral segments of the right antennule. Segment XXI is equipped with a serrated spine reaching the lower third of compound segment XXIII–XXIV, while it reaches the distal border of segment XXIII–XXIV in Y. antarcticus and the distal border of segment XXV in Y. admirabilis ( Andronov, 1992) . Fused segments XXIII–XXIV are equipped with a large plate spanning the whole segment in Yrocalanus kurilensis sp. nov. This plate spans only half of the segment in Y. antarcticus and has the form of a small bulge in Y. admirabilis . Furthemore, the elongated projection on segment XXVI in Y. antarcticus and Y. admirabilis is absent in Y. kurilensis sp. nov. Differences can also be observed in the number and morphology of the spines and segments of leg 5, which is uniramous in Y. kurilensis sp. nov, but shows rudimentary endopods in Y. antarcticus and Y. asymmetricus . With the discovery of the new species, the maximum size of species within the genus has to be corrected from earlier descriptions ( Renz et al., 2013), with currently known members ranging between 0.95 mm ( Y. asymmetricus ) and 1.75 mm ( Y. kurilensis sp. nov.). With Y. kurilensis sp. nov. included into the analysis, the definition for the genus is: small copepods (<1.75 mm). The rostrum is bifid. The proximal inner part of the leg 1 endopod is smooth. The female and male left antennule segment XXII is without a seta. The male right antennule is modified for grasping and of highly complex structure, with the main hinge between segment XXII and fused segments XXIII– XXIV. The male P5 is uniramous or indistinctly biramous with small endopodal buds and with the distal exopod segments with or without spines.

MOLECULAR PHYLOGENY

To gain insights into the relationships among the evolutionarily youngest calanoid copepod groups, different species of Ryocalanoidea (genera Ryocalanus and Yrocalanus ) and Spinocalanoidea (genera Spinocalanus Giesbrecht, 1888 , Caudacalanus Markhaseva & Schulz, 2008 ) from the South Atlantic (Brazilian Basin), North Atlantic (Great Meteor Seamount ) and North Pacific (Kurile-Kamchatka trench) were analysed using multi-gene approaches. Sequencing of Cytb was successful only for one individual of Yrocalanus kurilensis sp. nov. from the Kurile-Kamchatka trench and failed completely for COI for this species. Amplification and/or sequencing of Cytb was furthermore not successful for three Ryocalanus individuals from the South Atlantic, and Spinocalanus cf. magnus Wolenden, 1904 and Paraeuchaeta parvula (Park, 1978) from the North Atlantic. ITS2 amplification/sequencing did not work for Spinocalanus abyssalis Giesbrecht, 1888 and S. aspinosus Park, 1970 . Sequencing was, however, successful for nuclear genes 18S and 28S (see accession numbers in Table 1 for successfully sequenced genes).

The integration of our data into the sequence dataset of Bradford-Grieve et al. (2014) resulted in support for the same calanoid superfamily phylogeny as the original analyses by Blanco-Bercial et al. (2011) and Bradford-Grieve et al. (2014).

Adding these new data resulted in a well-supported relationship between ryocalanoidean, spinocalanoidean and clausocalanoidean copepods [Bootstrap Support (BS): 87/96, Bayesian Posterior Probability (BPP): 1/1, without and with regard of the three different codon positions for the mitochondrial genes COI and Cytb, Fig. 15].

Withinthisclade,RyocalanoideaandSpinocalanoidea form a highly supported clade (BS:99/100; BPP:1/1). The species of Ryocalanidae ( Ryocalanus and Yrocalanus ) did not form a monophyletic group, as only species of Ryocalanus were found in one clade (BS: 86/90; BPP: 1/1), while Yrocalanus was located in a supported clade with species of Spinocalanidae ( Spinocalanus ) and Arctokonstantinidae ( Caudacalanus and Foxtonia Hulsemann & Grice, 1963 ) (BS: 60/75; BPP: 0.97/1).

The Arctoconstantinidae were not supported as a monophyletic group in both analyses. Individuals of the same species (i.e. Yrocalanus kurilensis sp. nov. and Ryocalanus infelix ) and species within the same genera (i.e. Ryocalanus , Spinocalanus and Caudacalanus ) were highly supported as close relatives (BS: 100/100; BPP: 1/1, respectively).

The close relationship of Ryocalanidae , Spinocalanidae and Arctokonstantinidae and the position of Ryocalanidae in the system of Calanoida was investigated using longer fragments of 18S, 28S and COI, and Cytb together with ITS2 ( Fig. 16). This analysis revealed a high support of species within the same genus (BS: ≥99/100 and BPP: 1/1, respectively, as well as high support of the close relationship of the superfamilies Ryocalanoidea and Spinocalanoidea (BS: 100/100; BPP: 1/1). Again, Ryocalanidae could not be supported as a monophyletic clade as Yrocalanus appeared in a highly supported clade together with all Spinocalanidae and Arctokonstantinidae (BS: 96/94; BPP: 0.95/0.95).

Compared to the multiple-gene analyses, the single-gene analysis of 28S resulted in similar results with high support of a Ryocalanus clade and one clade comprising Spinocalanus , Caudacalanus and Yrocalanus (results not shown).

DNA SEQUENCE VARIATION

There was a considerable variation in the rate of molecular evolution between the three gene loci, 18S, 28S and ITS2, with highest interspecific levels of divergence within ITS2 (0.059 –0.539) and lowest levels in 18S (0.001 –0.029; Table 6). Uncorrected genetic p-distances between the genera Yrocalanus and Ryocalanus within the Ryocalanoidea were found to be higher (18S: 0.016 –0.022; 28S: 0.096–0.12; ITS2: 0.462 –0.516) than between the genera Caudacalanus and Spinocalanus within the Spinocalanoidea (18S: 0.012 –0.016; 28S: 0.056 –0.069; ITS2: 0.339 –0.368). Highest distances were found between Spinocalanus and Yrocalanus for 18S, Yrocalanus and Ryocalanus for 28S and Spinocalanus and Yrocalanus for ITS2. Within the ribosomal locus 18S, sequence distances were low between Caudacalanus and most Ryocalanus species compared to differences between species of other genera. Intraspecific genetic divergences for Yrocalanus kurilensis sp. nov. and Ryocalanus infelix were 0 for 18S and 28S, and 0–0.005 for ITS2.