New mud dragons from Svalbard: three new species of Cristaphyes and the first Arctic species of Pycnophyes (Kinorhyncha: Allomalorhagida: Pycnophyidae)

Background Kinorhynchs are marine, microscopic invertebrates inhabiting the seafloors. Their segmented trunk equipped with spines and processes has inspired scientists to give them the common name “mud dragons.” Even though kinorhynchs have been known since the 19th century, less than 300 species are known to science, and it is still considered a largely understudied animal group—in particular in the Arctic, from which only 23 species are known so far. Methods Samples were collected at eight stations around Svalbard and in the fjords of Spitsbergen. Meiofauna was extracted from the sediment cores with LUDOX centrifugation method, and kinorhynchs were picked up and mounted for light- and scanning electron microscopy. Results Four new species of the kinorhynch family Pycnophyidae are described from Svalbard: Cristaphyes dordaidelosensis sp. nov., C. glaurung sp. nov., C. scatha sp. nov., and Pycnophyes ancalagon sp. nov. The new species are generally recognized by their distribution of setae along the trunk segments. Discussion After the discovery of the new species, Pycnophyidae becomes with 14 species the most diverse kinorhynch genus in the Arctic, closely followed by Echinoderidae with 13 species. So far, these are the only kinorhynch families with an Arctic distribution.


INTRODUCTION
The kinorhynch fauna of Svalbard has recently been explored during annual meiofauna samplings in the fjords and surrounding waters, through a period from 2013 to 2017. The studies have so far resulted in the description of four new species of Echinoderes (Grzelak & Sørensen, 2018, in press), a redescription of Echinoderes arlis Higgins, 1966 (Grzelak & Sørensen, in press), and the finding of four species of Echinoderes that otherwise are only known from West Greenland and Eastern Canada (Grzelak & Sørensen, 2018, in press).
All meiofaunal organisms were counted and classified at higher taxonomic levels under a Nikon SMZ1500 stereomicroscope after staining with Bengal Rose to facilitate sorting process. After sorting, kinorhynchs were picked out and stored in a 4% formaldehyde solution.
Specimens for light microscopy (LM), were dehydrated through a graded series of water and glycerin and mounted in Fluoromount-G. The specimens were examined using an Olympus BX51 microscope (University of Copenhagen) and a Nikon E600 (Institute of Oceanology, Sopot) microscope, both equipped with differential interference contrast optics. The microphotographic documentation was done using an Olympus DP27 camera, and measurements were made with Cell^D software. All obtained dimensions reported in the tables are based on mounted LM specimens. Specimens for scanning electron microscopy (SEM) were dehydrated through a graded alcohol-acetone series and critical point dried. Dried specimens were mounted on aluminum stubs, sputter coated with platinum-palladium mix and examined with a JEOL JSM-6335F Field Emission scanning electron microscope. Line art figures were made with Adobe Illustrator CS6, based on imported LM micrographs, and supplemented with information obtained with SEM. The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid: zoobank.org:pub:72D489B2-E8B6-499B-A0C1-10BCCF5E8A29. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.  Diagnosis Cristaphyes with middorsal processes on segments 1-10, with the process of segment 10 projecting well beyond the terminal segment. Setae present in: subdorsal positions of segment 8, laterodorsal positions of segments 3-7 and 9, lateroventral positions of segments 2-10 inclusive one additional set of lateroventral setae on segment 10, ventrolateral positions of segments 5 and 10, ventromedial positions on segment 9, and paraventral positions of segments 3, 7, and 9; position of ventral setae on segment 8 vary between paraventral and ventromedial. Males with ventromedial tubes on segment 2; female morphology unknown. Posterolateral processes of segment 10 acute. Lateral terminal spines present.

Etymology
The species name dordaidelosensis, meaning "living in Dor Daidelos," is inspired by the book Silmarillion by JRR Tolkien. According to the book, Dor Daidelos is "The Region of Everlasting Cold" and the northernmost region of Middle Earth in the First Age.

Material examined
Holotype  Table 1 for detailed station data.

Description
Adults with head, neck, and eleven trunk segments (Figs. 2,3A,4A and 4H). The trunk is nearly parallel sided from segments 1 to 8. The terminal segment is almost completely covered by segment 10. Segment 1 consists of a tergal, two episternal and a midsternal plate (Figs. 2B,3C and 4C), whereas the following ten segments consist of a tergal and two sternal plates. Lateral terminal spines are present, and about the same length as segments 8-10. Only male morphology is known. For complete overview of measures and dimensions, see Table 2. Distribution of cuticular structures, that is, sensory spots, tubes, and setae, is summarized in Table 3.    The head was fully retracted in the three examined specimens, hence no information on head morphology is available. The neck has four dorsal and two ventral placids; all placids are rectangular, and the most dorsal pair appears to be broadest, but exact measures of their width could not be obtained.
Middorsal processes are present on segments 1-10 ( Fig. 2A); processes on segments 1-7 project only slightly beyond the posterior segment margins, but they become gradually longer at the more posterior segments (Fig. 4K). The strong middorsal processes of segments 8 and 9 project well beyond the posterior segment margins, and the relatively long but thinner middorsal process of segment 10 projects beyond the trunk (Figs. 2A, 3H-3I, 4K and 4M). Rounded to oval glandular cell outlets are present in series on the dorsal and ventral sides, in subdorsal positions on segment 1 and 8-10 (Figs. 2A, 3B and 3I), in laterodorsal positions in segments 2-7 (Figs. 2A, 3D and 3F), and in ventromedial positions on segments 1-10, located on the episternal plates, and hence more laterally displaced on segment 1 (Figs. 2B, 3C, 3E and 3G).
Smooth, hairless areas (muscle scars) marking subcuticular muscle attachment sites are present anteriorly on the segments, in laterodorsal and ventromedial positions on segments 2-9 (Figs. 2, 4E and 4F). Segment 1 is basically smooth, whereas segments 2-10 are covered with very minute acicular hairs. Secondary fringes, formed by one to two bands, are present on segments 2-10. Pachycycli, and rounded to oval peg-and-socket joints are present on segments 2-10 ( Fig. 4B). Paraventral apodemes are absent on all segments. Segment 1 with middorsal process, rising medially on segment, and projecting slightly beyond the posterior segment margin; ridge of process is covered by densely set hairs (Fig. 4B). Midsternal plate trapezoid, but with lateral sides close to being parallel (Figs. 3C and 4C). Anterior segment margin with narrow reticulated area along the margins of the tergal and midsternal plates, and larger reticulated areas at the anteroventral corners of the episternal plates. The segmental plates terminate posteriorly in free flaps, with finely serrated margins. Sensory spots present in paradorsal positions at posterior segment margin near projecting part of middorsal process; and as two pairs in subdorsal positions, one pair medially on segment and the other more anterior (Figs. 2A and 4B); both pairs appear to be located in the anterior ends of elongated depressed areas in the cuticle. Sensory spots furthermore present in laterodorsal (Fig. 4B) and ventromedial ( Fig. 4C) positions, more posteriorly on segment.
Segment 2 with middorsal process and paradorsal sensory spots as on preceding segment, but with ridge of middorsal process expanding from the most anterior part of the segment (Figs. 2A,4B and 4D). Tergal plate furthermore with two pairs of laterodorsal sensory spots (Fig. 4D), flanking the muscle scar, and a pair of lateroventral setae. Sternal plates with ventromedial tubes (Figs. 2B, 3C and 4C), putatively representing a sexually dimorphic male character, and ventromedial sensory spots, located posterior to the tubes (Fig. 3C). Posterior segment margin as on preceding segment. Segment 3 with middorsal process and paradorsal sensory spots as on preceding segment (Figs. 2A,3D and 4D). Tergal plate furthermore with subdorsal and laterodorsal sensory spots, and laterodorsal and lateroventral setae; laterodorsal setae are located more lateral than the sensory spots in same position (Fig. 4D). Sternal plates with paraventral setae (Figs. 2B,3E and 4E).
Segment 6 with tergal plate as on preceding segment, but with slightly longer middorsal process, and with laterodorsal sensory spots and setae in switched positions, so that the sensory spots now are more lateral (Figs. 2A,3F and 4F). Sternal plates as on segment 4 (Figs. 2B).
Segment 8 with longer middorsal process, flanked by paradorsal sensory spots; sensory spots are not as close to the posterior segment margin as those on preceding segments. Tergal plate otherwise with subdorsal sensory spots (located slightly closer to the middorsal process than those on preceding segments), subdorsal setae, laterodorsal sensory spots, and lateroventral setae (Figs. 2A,3I,4I and 4K). Sternal plates with setae in paraventral areas in one specimen (Fig. 3G), or in ventromedial positions, but very close to the paraventral areas in another specimen (Fig. 4J). Sensory spots present more lateral, also in ventrolateral positions (Figs. 2B and 4J).
Segment 9 with even longer middorsal process, flanked by paradorsal sensory spots as on segment 8. Tergal plate otherwise with two pairs of subdorsal sensory spots, laterodorsal setae, laterodorsal sensory spots, and lateroventral setae (Figs. 2A, 3I, 4K and 4M). Sternal plates with ventrolateral and paraventral sensory spots, and ventromedial and paraventral setae, with the latter pair being located very close to the midsternal junction (Figs. 2B and 3G).
Segment 10 with middorsal process without conspicuous middorsal ridge, expanding from the posterior segment margin, and projecting well beyond the terminal end of the trunk (Figs. 2A, 3H, 4K and 4M). Tergal plate otherwise with two pairs of sensory spots in subdorsal positions (near posterior segment margin), and two pairs of setae in lateroventral positions; the dorsalmost pair is perfectly aligned with the lateroventral setae on the preceding segments, whereas the additional pair is more ventral, and appears to attach directed in the densely haired area near the tergosternal junction ( Remarks for Cristaphyes dordaidelosensis sp. nov. Of the additional 19 species accommodated in Cristaphyes, C. dordaidelosensis sp. nov. is easily distinguished from the eight species that do not have lateral terminal spines. Of the eleven remaining species, five of them have none or only very short middorsal process of segment 10 that does not project beyond the posterior margin of the segment. These include C. carinatus (Zelinka, 1928), C. chilensis (Lang, 1953), C. cryopygus, C. longicornis (Higgins, 1983), and C. odhneri (Lang, 1949). C. dordaidelosensis sp. nov. is easily distinguished from these by its long projecting middorsal process of segment 10 (see Zelinka, 1928;Lang, 1949Lang, , 1953Higgins, 1983;Higgins & Kristensen, 1988).
In summary, C. dordaidelosensis sp. nov. is distinguished by its combination of lateral terminal spines, middorsal processes on segments 1-10, with the process on segment 10 clearly projecting beyond the terminal segment, and by its relatively few ventral setae, mostly in paraventral positions.

Etymology
The species is named glaurung, after Glaurung-the father of the dragons, bread by Morgoth in the dungeons of Angband-in the book Silmarillion by JRR Tolkien.  Table 1 for detailed station data.
Full-size  DOI: 10.7717/peerj.5653/ fig-7 and that introvert sectors 3-5 are identical with sectors 7-9, respectively. The mouth cone has nine outer oral styles (Fig. 8C), arranged as one anterior to each introvert sector, expect for the middorsal sector 6 ( Fig. 6). Each outer oral style consists of a single, rather flexible unit (Fig. 8C). Proximally they attach to the mouth cone via a fringed sheath. An additional fringed structure is present externally on this sheath, and every second of these structures (anterior to even-numbered introvert sectors) carry a long, spike-like median fringe tips (Fig. 8C).   Note: LTS, lateral terminal spine; MDP10, middorsal process on segment 10; MSW-7, maximum sternal width, measured on segment 7 in this species; S, segment lengths; SW-10, standard width, always measured on segment 10; TL, trunk length.
The introvert is equipped with spinoscalids, arranged in transverse rings and longitudinally in ten sectors, defined by the primary spinoscalids of ring 01 (Fig. 6). The primary spinoscalids consist of a stout proximal unit with a median fringe, and a long, slender end piece (Fig. 8D). Spinoscalids of rings 02 and 03 have more narrow proximal sheaths, with rather long median fringes. End pieces are long, and more slender than those in ring 01. Spinoscalids of the remaining rings also consist of a proximal sheath and an end piece, but they are shorter, and the conspicuous median fringe is missing. Instead, they appear to have a smaller, transverse fringe near the attachment points. Ring-wise arrangement of spinoscalids is as follows: Ring 01-10 primary spinoscalids; Ring 02-10 spinoscalids, one medially in each sector; Ring 03-20 spinoscalids, one pair in each sector; Ring 04-five spinoscalids, one medially but in uneven-numbered sectors only; Ring 05-14 spinoscalids, one pair in uneven-numbered sectors, and one medially in even-numbered sectors, except the middorsal sector 6; Ring 06-14 spinoscalids, one medially in uneven-numbered sectors and sector 6, and one pair in remaining sectors (Figs. 6 and 8D). Described sector-wise, all uneven-numbered sectors have seven spinoscalids, arranged as a double diamond. Even-numbered sectors, except sector 6, have spinoscalids forming two chevrons (i.e., a single spinoscalid anterior to a pair in each chevron), with a blank ring separating the two chevrons. Sector 6 has a chevron in Rings 02-03, and then a single, medial spinoscalid in Ring 06. A total of 14 trichoscalids are present posterior to the spinoscalid rings. They are located as single trichoscalids in even-numbered sectors, and in sector 1, and as pairs in the remaining uneven-numbered sectors (Fig. 6).
The neck has four dorsal and two ventral placids; all placids are rectangular and measures in width: 33 mm (subdorsal pair) and 26 mm (laterodorsal and ventral pairs), respectively. Middorsal processes are present on segments 1-10; processes on segments 1-7 project only slightly beyond the posterior segment margins, but they become gradually longer at the more posterior segments (Figs. 5A, 7D, 7H, 8A, 8E, 8J and 8M). The strong middorsal processes of segments 8 and 9 project well beyond the posterior segment margins, and the relatively long but thinner middorsal process of segment 10 projects beyond the trunk. Segment 2 with middorsal process and paradorsal sensory spots as on preceding segment, but with ridge of middorsal process expanding from the most anterior part of the segment (Fig. 8E). Tergal plate furthermore with two pairs of laterodorsal sensory spots, flanking the muscle scar, and a pair of subdorsal and lateroventral setae (Figs. 5A, 7E, 8E and 8J). Sternal plates with ventromedial sensory spots in both sexes located lateral to glandular cell outlets; females furthermore with ventromedial setae, located lateral to the sensory spots (Figs. 5B, 7E, 8G and 8I), and males with ventromedial tubes (Figs. 5C and 8H). Posterior segment margin as on preceding segment. Segment 3 with middorsal process and paradorsal sensory spots as on preceding segment. Tergal plate furthermore with subdorsal and laterodorsal sensory spots, and subdorsal and lateroventral setae; subdorsal setae are located more dorsal than those on segment 2 (Figs. 5A, 7D and 8E). Sternal plates with ventromedial sensory spots and paraventral setae (Figs. 5B, 7E, 8G, 8J and 8L).
Segment 5 with tergal plate as on preceding segment, except for the slightly longer middorsal process, and setae that vary in positions between subdorsal and laterodorsal, but most commonly occur in laterodorsal positions (Figs. 5A,7F and 8K); the variation appears to be due to random intraspecific variation rather than sexual dimorphism. Sternal plates with ventrolateral and paraventral setae and ventromedial sensory spots (Figs. 5B and 8L).
Segment 7 with tergal plate as on preceding segment, but with slightly longer middorsal process (Figs. 5A and 8K). Sternal plates with ventromedial sensory spots in both sexes; females furthermore with setae in ventromedial positions (Figs. 7G and 8P), and males with setae in paraventral positions.
Segment 10 with middorsal process without conspicuous middorsal ridge and flanking sensory spots, expanding from the posterior segment margin, and projecting well beyond the terminal end of the trunk (Figs. 5A, 7I and 8M). Tergal plate otherwise with two pairs of sensory spots in subdorsal and one pair in laterodorsal positions (all near posterior segment margin), and pair of lateroventral setae. Sternal plates with ventrolateral setae and ventromedial sensory spots, near posterior segment margin (Figs. 5B, 5D, 7J and 8O-8P). Posterior margin of tergal plate is straight as on all preceding segment, whereas the margins of the sternal plates are deeply concave in males (Figs. 5D and 8O), but only slightly concave in females (Fig. 5B). Posterolateral corners of tergosternal junctions form nearly right-angled caudal projections (Figs. 7J and 8O).
Segment 11 hardly projecting beyond segment 10. Lateral terminal spines present (Figs. 5A-5B, 7A and 8M). Tergal plate with pair of slightly projecting type 3 sensory spots in subdorsal positions. Margin of sternal plates with pair of pointed, horn-like projections. Two pairs of penile spines present in males (Fig. 5D).
Cristaphyes glaurung sp. nov. is also easily distinguished from the eight congeners without lateral terminal spines, and the five additional ones with no or only very short middorsal process of segment 10. The remaining species with lateral terminal spines, and slightly or conspicuously projecting middorsal process of segment 10 include: C. arctous, C. chukchiensis, C. cristatus, C. furugelmi, C. nubilis, C. abyssorum, and the species described above, C. dordaidelosensis sp. nov. C. glaurung sp. nov. is fairly easily distinguished from these by its paraventral setae on segments 3-9 in males, and 3-6 plus 9 in females. C. abyssorum and C. nubilis also have sternal setae on most segments, but they are never paraventral, and their ventromedial setae are quite lateral (Adrianov & Maiorova, 2015;Sánchez, Pardos & Sørensen, 2014). Males of C. furugelmi have ventral setae on segment 7 that get close to the paraventral positions, but on its other segments, and in females, it has either quite laterally displaced ventromedial setae, or no sternal setae at all (Adrianov & Malakhov, 1999). As pointed out above, C. arctous apparently lacks sternal setae completely, and has conspicuously short and obtuse middorsal processes on segments 1-8 (Adrianov & Malakhov, 1999). Oppositely, C. glaurung sp. nov. has spinose and projecting middorsal processes on all segments, from segment 1 to 10. Also C. cristatus is easily distinguished from C. glaurung sp. nov. due to its lack sternal setae on segments 3 to 6 (Sánchez et al., 2013). In terms of seta distribution patterns, C. chukchiensis is closest to C. glaurung sp. nov. It has lateroventral setae on segments 2-10 (same in C. glaurung sp. nov.), laterodorsal setae on segments 2-9 (same segment distribution in C. glaurung sp. nov., but mostly in subdorsal positions), paraventral setae on segment 5 (same in C. glaurung sp. nov.), and ventromedial setae on segments 2-9 (same segment distribution in C. glaurung sp. nov., but mostly in paraventral positions) (Higgins, 1991). The two species can be distinguished by the subtle differences in positions of setae, but two better differential characters are the lack of a middorsal process on segment 1 in C. chukchiensis, and its broad trapezoid midsternal plate of segment 1 that differs from the Erlenmeyer flask-shaped midsternal plate in C. glaurung sp. nov. C. dordaidelosensis sp. nov. clearly differs from C. glaurung sp. nov. by its lack of sternal setae on segments 4 to 6. An even easier differential character, that was used during the initial identification of the species from Svalbard, is the shape of the posterolateral processes of segment 10. In C. dordaidelosensis sp. nov. the process is pointed and shaped by sides that meet in an acute angle, whereas the processes in C. glaurung sp. nov. have sides that forms a nearly right angle (compare processes shown on Fig. 3J with Fig. 7J). After seeing these differently shaped processes, it was very easy, even at low magnification, to distinguish the two species.
In summary, C. glaurung sp. nov. is distinguished by its combination of lateral terminal spines, middorsal processes on segments 1-10, with the process on segment 10 clearly projecting beyond the terminal segment, and by its ventral setae in paraventral positions, at least on segments 3-6 and 9.
The arrangement of spinoscalids on the introvert is identical with the pattern in Pycnophyes chalgap (Sánchez et al., 2013) and C. cristatus (see Sánchez et al., 2013).
Cristaphyes scatha sp. nov.   Diagnosis Cristaphyes with middorsal processes on segments 1-10, with the process of segment 10 very short, not projecting beyond the terminal segment. Setae present in: paradorsal position on segments 4 and 6 (unpaired, single setae on both segments), laterodorsal positions of segments 2-8 (but alternating between very laterally displaced setae on uneven numbered segments, and setae close to the subdorsal region on even numbered segments), paralateral positions of segment 1, lateroventral positions of segments 2, 4, 6, 8, and 10, ventrolateral positions on segment 5, and ventromedial positions on segments 2-9 in females and 3-9 in males (setae on segment 9 displaced and very close to the paraventral region). Males with ventromedial tubes on segment 2. Lateral terminal spines present.

Description
Adults with head, neck, and eleven trunk segments (Figs. 9A-9B, 10, 11A and 12A-12B). The trunk is nearly parallel sided from segments 1 to 9. The terminal segment is almost completely covered by segment 10. Segment 1 consists of a tergal, two episternal and a midsternal plate (Figs. 9B, 11C and 12F), whereas the following ten segments consist of a tergal and two sternal plates. Lateral terminal spines are present, and about the same length as segments 8-10. For complete overview of measures and dimensions, see  Table 6. Distribution of cuticular structures, that is, sensory spots, tubes and setae, is summarized in Table 7.
The mouth cone has nine outer oral styles, arranged as one anterior to each introvert sector, expect for the middorsal sector 6 (Fig. 10). Each outer oral style consists of a single flexible unit. Proximally they attach to the mouth cone, but it was difficult to visualize details in the distal part of the mouth cone because this part of the single specimen available for SEM was slightly collapsed.
The introvert is equipped with spinoscalids, arranged in transverse rings and longitudinally in 10 sectors, defined by the primary spinoscalids of ring 01 (Figs. 10 and 12C). The primary spinoscalids consist of a stout proximal unit with a weakly developed median fringe, and a long, slender, end piece (Fig. 12C). Spinoscalids of rings 02 and 03 have more narrow proximal sheaths, with short fringes along their proximal margins. End pieces are long, and more slender than those in ring 01. Spinoscalids of the remaining rings also consist of a proximal sheath and an end piece, but they are shorter. Ring-wise arrangement of spinoscalids is as follows: Ring 01-10 primary spinoscalids; Ring 02-10 spinoscalids, one medially in each sector; Ring 03-20 spinoscalids, one pair in each sector; Ring 04-five spinoscalids, one medially but in uneven-numbered sectors only; Ring 05-15 spinoscalids, one pair in uneven-numbered sectors, and one medially in even-numbered sectors; Ring 06-15 spinoscalids, one medially in uneven-numbered sectors, and a pair in even-numbered sectors (Figs. 10 and 12C). Described sector-wise, all uneven-numbered sectors have seven spinoscalids, arranged as a double diamond. Even-numbered sectors have spinoscalids forming two chevrons (i.e., a single and a pair of spinoscalids in each chevron), with a blank ring separating the two chevrons. A total of 14 trichoscalids are present posterior to the spinoscalid rings. They are located as single trichoscalids in even-numbered sectors, and in sector 1, and as pairs in the remaining uneven-numbered sectors (Fig. 10).
The neck has four dorsal and two ventral placids (Figs. 9 and 12D); all placids are rectangular and measures in width: 42 mm (subdorsal pair) and 33 mm (laterodorsal and ventral pairs), respectively.
Middorsal processes are present on segments 1-10; processes on segments 1-9 project only slightly beyond the posterior segment margins, whereas the process on segment 10 is even shorter (Figs. 9A, 11D, 11I, 12A, 12D-12E, 12G, 12I and 12L-12M). Rounded to oval glandular cell outlets are present in series on the dorsal and ventral sides, in subdorsal positions on segments 1 and 10, in laterodorsal positions in segments 2-9, in ventromedial positions on segments 1 and 3-10, and in paraventral positions on segment 2 (Figs. 9A-9B, 11B-11C and 11J). Smooth, hairless areas (muscle scars) marking subcuticular muscle attachment sites are present anteriorly on the segments, in laterodorsal and ventromedial positions (Fig. 12K) on segments 2-9 (Figs. 9A and 9B). All segments, except anterior 3/4 of segment 1, are covered with very minute acicular hairs. Secondary fringes, formed by one or two wavy bands, are present in segments 2-10. Pachycycli, and peg-and-socket joints are present on segments 2-10. Paraventral apodemes are absent on all segments. Segment 1 with middorsal process, arising on posterior 1/3 of segment, and projecting slightly beyond the posterior segment margin; ridge of process is covered by densely set hairs . Midsternal plate trapezoid (Figs. 9B,11C and 12F). Anterior segment margin with narrow reticulated area along the margins of the tergal plate, and slightly larger reticulated areas along margins of episternal plates. The segmental plates terminate posteriorly in free flaps, with finely serrated margins. Setae present in paralateral positions (Figs. 9A, 11B inset and 12E). Sensory spots present in paradorsal positions at posterior segment margin near projecting part of middorsal process; and as two pairs in subdorsal positions, one pair medially on segment and the other more anterior; anterior pair appears to be located in the anterior ends of elongated depressed areas in the cuticle (Figs. 9A and 12E). Sensory spots furthermore present in laterodorsal and ventromedial positions, more posteriorly on segment (Figs. 9A-9B, 11C and 12E-12F). All sensory spots, on this and following segments, appear conspicuously large and distinct in this species.
Segment 2 with middorsal process and paradorsal sensory spots as on preceding segment, but with ridge of middorsal process expanding from a more anterior position on the segment (Figs. 9A and 12G). Tergal plate furthermore with setae in laterodorsal (close to subdorsal) and lateroventral positions, one pair of sensory spots in subdorsal positions, and two pairs in laterodorsal positions, flanking the muscle scar and the laterodorsal setae (Figs. 9A, 11B and 12G). Sternal plates with ventromedial sensory spots in both sexes; females furthermore with ventromedial (very close to ventrolateral) setae (Figs. 9C and 12F), and males with ventromedial tubes (Figs. 9B and 11C inset). Posterior segment margin as on preceding segment. Segment 3 with middorsal process and paradorsal sensory spots as on preceding segment. Tergal plate furthermore with subdorsal and laterodorsal sensory spots, and laterodorsal setae located much more lateral than those on segment 2 (Figs. 9A, 11B and 12G). Sternal plates with ventromedial setae and sensory spots (Figs. 9B and 12H).
Segment 4 with middorsal process and paradorsal sensory spots as on preceding segment. Tergal plate furthermore with unpaired seta in paradorsal position, paired laterodorsal, and lateroventral setae (laterodorsal ones located more dorsal than those on preceding segment), and sensory spots in subdorsal and laterodorsal positions (Fig. 9A). Sternal plates as on preceding segment (Figs. 9B and 12H).
Segment 8 similar to segments 4 and 6, except for the absence of paradorsal setae (Figs. 9A-9B, 11F and 12K-12L). Setae on the sternal plates are ventromedial, as on preceding segments, but laterally displaced and very close to ventrolateral positions.
Segment 9 with middorsal process, flanked by paradorsal sensory spots. Tergal plate otherwise with sensory spots in subdorsal and laterodorsal positions (Figs. 9A, 11F and 12L). Sternal plates with ventrolateral sensory spots, and ventromedial setae and sensory spots, but with the setae located very close to the paraventral region, closer to the midventral line than the sensory spots (Figs. 9B, 9D, 11G and 12N).
Segment 10 with very short middorsal process without conspicuous middorsal ridge and flanking sensory spots, not projecting beyond the terminal segment (Figs. 9A, 11J,  12M and 12N). Tergal plate otherwise with lateroventral setae, and two pairs of sensory spots in subdorsal positions and one pair in laterodorsal positions (all near posterior segment margin) (Figs. 9A, 11I and 12M). Sternal plates with ventromedial sensory spots, near posterior segment margin (Figs. 9B, 9D, 11J and 12N). Posterior margin of tergal plate is straight as on all preceding segment, whereas the margins of the sternal plates are concave, and similar in both sexes. Posterolateral corners of tergosternal junctions form slightly pointed caudal projections.
Cristaphyes scatha sp. nov. is easily distinguished from the eight congeners without lateral terminal spines. The species' very short middorsal process on segment 10 also distinguishes it from congeners with a longer process that projects beyond the terminal segment. These include C. abyssorum, C. arctous, C. chukchiensis, C. cristatus, C. dordaidelosensis sp. nov., C. furugelmi, C. glaurung sp. nov., and C. nubilis (Higgins, 1991;Adrianov & Malakhov, 1999;Sánchez et al., 2013;Sánchez, Pardos & Sørensen, 2014;Adrianov & Maiorova, 2015). The remaining five congeners without or only with very short, non-projecting middorsal process on segment 10 include C. carinatus, C. chilensis, C. cryopygus, C. longicornis, and C. odhneri. C. carinatus is quite easily distinguished from its congeners by its anteriorly extended midsternal plate on segment 1 that projects beyond the anterior margins of its episternal plates (Zelinka, 1928). The description of C. chilensis does not provide any consistent information about distribution of setae, but the species is easily distinguished from most of congeners by the broadly pointed posterior tergal margin of segment 10 (Lang, 1953). Segment 10 of C. scatha sp. nov. also has a small, middorsal, posterior process, but the sides of the process only expand from the paradorsal positions. The edges of the middorsal posterior point or process in S. chilensis starts expanding almost from the laterodorsal positions.
The remaining three species are mainly distinguished by their distribution of setae. Information on seta distribution in C. odhneri provided by Lang (1949) is probably not complete (no dorsal setae are reported), but the illustrations of Lang (1949) clearly indicate that setae on the sternal plates are located very medial on each plate, or closest to the midventral line. In C. scatha sp. nov. the ventromedial setae are much more laterally displaced, and are located very close to the ventrolateral areas. Both C. cryopygus and C. longicornis are distinguished from C. scatha sp. nov. by the distribution pattern of their setae on the tergal plates. C. cryopygus has longitudinally aligned laterodorsal setae on segments 2, 4, and 6-10, lateroventral setae on segments 2 and 10 (Higgins & Kristensen, 1988), and no paralateral setae on segment 1. On the opposite, C. scatha sp. nov. has laterodorsal setae on segments 2-8 that alternate between more dorsal (even numbered segments) and more lateral (uneven-numbered segments) positions, lateroventral setae in even-numbered segments only, and presence of paralateral setae on segment 1. C. scatha sp. nov. shows the greatest resemblance with C. longicornis, described from Belize (Higgins, 1983). Both species have paralateral setae on segment 1, laterodorsal setae on segments 2-8, and lateroventral setae on even numbered segments only. However, C. longicornis has laterodorsal setae on segment 9 also (missing in C. scatha sp. nov.), and its laterodorsal setae are all aligned, opposed to the laterodorsal setae in C. scatha sp. nov. that shifts positions. The episternal plates of C. longicornis furthermore have a seta each, whereas such setae are missing in C. scatha sp. nov., and finally males of C. longicornis do not have ventromedial tubes on segment 2.

Material examined
Full  mounted for SEM and stored in the first author's personal reference collection. See Fig. 1 for localities and Table 1 for detailed station data.

Description
Adults with head, neck, and eleven trunk segments (Figs. 13A-13B, 14A and 15A-15B). The trunk is nearly parallel sided from segments 1 to 9. Segment 1 consists of a tergal, two episternal and a midsternal plate (Figs. 13B-13C, 14C and 15F), whereas the following   10 segments consist of a tergal and two sternal plates. Lateral terminal spines are present, and about the same length as segments 8-10. For complete overview of measures and dimensions, see Table 8. Distribution of cuticular structures, that is, sensory spots, tubes, and setae, is summarized in Table 9.
The mouth cone has nine outer oral styles, arranged as one anterior to each introvert sector, except for the middorsal sector 6. Each outer oral style consists of a single, rather flexible unit (Fig. 15C). Proximally they attach to a smooth mouth cone without any other conspicuous structures.
The introvert is equipped with spinoscalids, arranged in transverse rings and longitudinally in 10 sectors, defined by the primary spinoscalids of ring 01. The primary spinoscalids consist of a stout proximal unit with a median fringe, and a long, slender, end piece (Fig. 15D). Spinoscalids of rings 02 and 03 have proximal sheaths similar to those on the primary spinoscalids, but thinner and shorter end pieces. Spinoscalids of the remaining rings also consist of a proximal sheath and an end piece, but they are even shorter, and without a median fringe. Ring-wise arrangement of spinoscalids is as follows (identical with the arrangement in C. scatha sp, nov., hence see Fig. 10): Ring 01-10 primary spinoscalids; Ring 02-10 spinoscalids, one medially in each sector; Ring 03-20 spinoscalids, one pair in each sector; Ring 04-five spinoscalids, one medially but in uneven-numbered sectors only; Ring 05-14 spinoscalids, one pair in uneven-numbered sectors, and one medially in even-numbered sectors, except the middorsal sector 6; Ring 06-14 spinoscalids, one medially in uneven-numbered sectors and sector 6, and one pair in remaining sectors. Described sector-wise, all uneven-numbered sectors have seven spinoscalids, arranged as a double diamond. Even-numbered sectors, except sector 6, have spinoscalids forming two chevrons (i.e., a single and a pair of spinoscalids in each chevron), with a blank ring separating the two chevrons. Sector 6 has a chevron in Rings 02-03, and then a single, medial spinoscalid in Ring 06 (Figs. 10 and 15D). Fourteen trichoscalids are present posterior to the spinoscalid rings. They are located as single trichoscalids in even-numbered sectors, and in sector 1, and as pairs in the remaining uneven-numbered sectors. The neck has four dorsal and two ventral placids; all placids are rectangular and measures in width: 50 mm (subdorsal pair), 24 mm (laterodorsal pair), and 30 mm (ventral pair).
Middorsal elevations with intracuticular atria are present on segments 1-9 (Figs. 13A, 15A, 15I and 15L); segment 10 has a weak elevation only, but no intracuticular structures. The cuticle appears rather thin, which makes it harder to visualize intracuticular structures in LM. Glandular cell outlets are present in series on the ventral side, in ventromedial positions on segments 1 and 2, and in ventrolateral positions on segments 3-10 (Figs. 13A-13B, 14C and 15K); glandular cell outlets on segment 1 are located medially on episternal plates, whereas those on segments 2-10 are located anteriorly on segment, at the rim of the secondary fringe (Fig. 15K). On the dorsal side, glandular cell outlets were only identified in subdorsal positions on segment 1 (Fig. 14B). Muscle attachment sites (muscular scars) are likewise weakly defined, and were not identified. Paired paraventral apodemes are present on segments 8-10 (Figs. 13B and 14F-14J), and in two specimens on segment 7 as well; apodemes are largest on the most posterior segments. Segment 1 is nearly completely smooth (Fig. 15E), whereas segments 2-10 have minute, scale-like cuticular hairs. Secondary fringes, formed by one to two bands, are present in segments 2-10; the secondary fringe is broad and lobed on the ventral side of segment 2, but narrower dorsally, and on the remaining segments. Pachycycli, and peg-and-socket joints are present on segments 2-10.
Segment 1 with trapezoid midsternal plate with short pointed midventral process (Fig. 15F). Anterior segment margin with narrow reticulated area along the margins of the tergal plate, and slightly larger depressed and reticulated areas along margins of the midsternal and episternal plates (Figs. 15E and 15F). The segmental plates terminate posteriorly in free flaps, with finely serrated margins. Setae are absent. Sensory spots present on posterior part of tergal plate in paradorsal and laterodorsal positions, and on anterior part of plate in subdorsal and laterodorsal positions (Figs. 13A and 15E). Episternal plates with one pair of ventromedial sensory spots on anterior parts of plates (Figs. 13B, 14C and 15F).
Segment 2 with tergal setae in laterodorsal and lateroventral positions, one pair of sensory spots in paradorsal positions, two pairs in subdorsal positions, and one pair in laterodorsal positions, more lateral than the setae (Figs. 13A, 14B and 15G). Sternal plates with ventromedial sensory spots in both sexes; females furthermore with ventromedial setae (Figs. 13B, 14C and 15H), and males with ventromedial tubes (Figs. 13C, 14E and  15K). Posterior segment margin as on preceding segment. Segment 3 with tergal setae in laterodorsal positions, and sensory spots in paradorsal, subdorsal and laterodorsal positions (Fig. 13A). Sternal plates with ventromedial setae and sensory spots (Figs. 9B and 15H); setae are most commonly closest to the midventral line, but in one specimen they had switched positions. Segment 4 with tergal plate similar to segment 3, except for the addition of a pair of lateroventral setae (Fig. 13A). Sternal plates as on preceding segment, but with setae always appearing closest to the midventral line (Figs. 13B and 15J).
Segment 5 with tergal similar to that of segment 3, but with laterodorsal setae and sensory spots switching position in some specimens (Figs. 13A, 14D and 15I). Sternal plates identical with segment 4, but with the addition of a pair of ventrolateral setae (Figs. 13B and 15J).
Segment 6 with unpaired seta in paradorsal position (Figs. 13A and 15I). Segment otherwise similar to segment 4. Segment 7 with tergal plate similar to segments 3 and 5 (Figs. 13A, 14F and 15L), and sternal plates similar to with preceding segment (Figs. 13B and 15M); the exact position of the ventromedial setae differs between the specimens (independent of sex), and appear in some specimens very close to the paraventral areas.
Segment 9 with tergal setae in laterodorsal positions, and one pair of sensory spots in paradorsal and subdorsal positions, and two pairs in laterodorsal positions (Fig. 13A); in most specimens both laterodorsal sensory spots are located lateral to the setae, but in a single specimen, one pair of sensory spots is more dorsal, located very close to the subdorsal positions. Sternal plates with setae in ventromedial positions, and sensory spots in ventrolateral and ventromedial positions (Figs. 13B, 13D and 15N).
Segment 10 with two pairs of very closely set lateroventral setae (Figs. 13A-13B, 13D, 14H, 13J, 15O-15Q); no other setae are present on the segment. Sensory spots are present as two pairs in subdorsal positions, one pair in laterodorsal positions, and one pair in ventrolateral positions, all close to the posterior segment margin (Figs. 13A-13B, 13D, 15O and 15Q). Posterior margin of tergal plate is slightly convex, whereas the margins of the sternal plates are concave with narrow extensions near the midventral articulation. Posterolateral corners of tergosternal junctions form slightly pointed caudal projections.
Segment 11 projecting beyond segment 10. Lateral terminal spines present (Figs. 13A-13B and 14A). Tergal plate with pair of type 3 sensory spots in subdorsal positions (Fig. 14I). Margin of sternal plates with pair of short, truncate projections. Males apparently only with a single pair of penile spines, lateral to a tuft of long fringe-like extensions (Figs. 13D, 14H and 15P).

Epibiontic growth
All examined specimens carried one to numerous unidentified loricate, protist (most likely ciliate) epibionts (Figs. 14C, 15A-15B, 15M and 15P). The colonization is noteworthy because none of the examined species of Cristaphyes carried any epibionts. This suggests that the epibionts show a genus-or species preference when they infect.
The presence of epibiontic growth on kinorhynchs is not uncommon, and scattered information can be found in the literature (Adrianov & Higgins, 1996;Ostmann, Nordhaus & Sørensen, 2012;Sørensen & Landers, 2017Herranz, Yangel & Leander, 2018), even though there have been few attempts to address the phenomenon in general. Amongst species of Pycnophyidae, Neuhaus (2013) reports that sessile epibionts are common on P. communis Zelinka, 1928, P. parasanjuanensis Adrianov & Higgins, 1996, Setaphyes dentatus (Reinhard, 1881, and Setaphyes kielensis (Zelinka, 1928). The first author furthermore has unpublished photos of K. greenlandica, Leiocanthus pardosi (Sánchez et al., 2013), and P. tubuliferus Adrianov, 1989 with growth of similar epibionts attached. Since the available reports of epibionts often come without proper identification, it is still way too premature to draw any conclusions about host-symbiont preferences, but the frequent reports of such epibionts suggests that the topic should be addressed in future studies.
The new species fits the emended genus diagnosis of Pycnophyes (see Sánchez et al., 2016): middorsal elevations are present on segments 2-9 (excludes Leiocanthus), middorsal processes are not present (excludes Cristaphyes), paradorsal setae are present on segments 6 and 8 only (excludes Krakenella and Setaphyes), lateroventral setae are present on even numbered segments (excludes Setaphyes and also Higginsium), and ventrolateral setae on segment 5 only (excludes Fujuriphyes, Higginsium, and Setaphyes). Sánchez et al. (2016) suggest that the length of the lateral terminal spines never exceeds 20% of the total trunk length in species of Pycnophyes. With spine/trunk length ratios of 18.5-25.2%, and an average of 21.5%, P. ancalagon sp. nov. is in the upper part of the range, but we still feel it can be justified to assign the new species to this genus.
Besides the new species, Pycnophyes currently accommodates 25 species. Sánchez et al. (2016) provide a nearly complete species list for the genus (the questionable species P. echinoderoides Zelinka, 1928 is omitted though, but according to Neuhaus (2013) the species is still valid), and only P. alexandroi Pardos, Sánchez & Herranz, 2016 has been added to the genus since then. Six of the species are easily distinguished from P. ancalagon sp. nov. because they do not have lateral terminal spines. Of the remaining ones, the Mediterranean species P. echinoderoides Zelinka, 1928 can also be excluded from the comparison since its description is based on juvenile specimens exclusively, and adult morphology is unknown (Zelinka, 1928). Also the North Atlantic P. calmani Southern, 1914 is rather poorly described, but one of the few known characteristics for the species is its short lateral terminal spines, measuring 80-100 mm in length only (Southern, 1914), which distinguishes it from P. ancalagon sp. nov. that has 50-100% longer spines.
The single character that distinguishes P. ancalagon sp. nov. from its remaining 17 congeners is the presence of unpaired paradorsal setae on segments 6 and 8. This character appears so far to be unique for the new species. But even with this character sp. nov. that has no outer middorsal structures on segment 10, the specimen from St. H1 has short, pointy process that extends beyond the posterior margin of the segment. This could be interpreted as a late juvenile trait, but it also puts the identity of the specimen in question. Based on the presence of this process, together with our disability of examining cuticular structures on the ventral side, and the fact that it occurs on a different locality than the other P. ancalagon sp. nov., we choose not to assign a species name to this particular specimen.

DISCUSSION
The descriptions of four new kinorhynch species bring the number of pycnophyid species from Svalbard up to seven, and the total number of Arctic pycnophyids up to 14. A complete summary of all records of pycnophyids in the Arctic, inclusive some previously unpublished records, is presented in Table 10 (see also Fig. 16). This shows that Pycnophyidae is just as diverse in the Arctic as Echinoderidae, which currently is represented with 13 species in the Figure 16 Map showing the distribution of Pycnophyidae in Arctic region. Numbers refer to species names summarized in Table 10.
Full-size  DOI: 10.7717/peerj.5653/ fig-16 Arctic (Grzelak & Sørensen, in press). Interestingly, Pycnophyidae and Echinoderidae are so far the only two kinorhynch families that have been recorded from the Arctic Region. Amongst the pycnophyid genera, Cristaphyes and Krakenella are the most diverse in the Arctic, with, respectively, six and seven species present. In fact, besides these genera, P. ancalagon sp. nov. is the only Arctic species of Pycnophyidae that does not belong to either Cristaphyes or Krakenella. Considering that the Arctic oceans still remain unexplored to a great degree, it is still way too premature to conclude that Cristaphyes and Krakenella are more adapted to colonize or specify in Arctic waters, but our current information clearly indicates that this option should be explored further. Grzelak & Sørensen (in press) suggested that at least some species of Arctic Echinoderes appeared to show a circumpolar distribution. A similar distribution pattern is not really clear for any of the recorded pycnophyids. K. greenlandica is so far the Arctic pycnophyid that shows the greatest distribution (Table 10), but the species still appears to be restricted to waters around Greenland. Even though species of Pycnophyidae are just as diverse in the Arctic as Echinoderidae, the pycnophyids are usually much less abundant, and it requires a denser and more exhaustive sampling to reveal their true distribution.

CONCLUSIONS
Discovery of four new Arctic mud dragons species from Svalbard demonstrates that the European Arctic represents a region where richness of Kinorhyncha is probably significantly greater than is already known. With the high degree of probability, we can assume that this is also true for other Arctic sectors. An improved sampling and more exhaustive search for Arctic pycnophyids in the future, will hopefully demonstrate their distribution patterns, and show if the species are regionalized or can be found widespread through the Arctic.