Mucrosenia kendalli Gee & Huys 1994

Mucrosenia kendalli Gee & Huys, 1994

(Fig. 2)

Specimens examined. Male dissected on four slides (collection number IORAS-Har204–207), collected from 27 m depth at location 73°14’044”N 156°25’957”E (East Siberian Sea); and two additional males dissected on four slides each (collection numbers IORAS-Har208–211 and IORAS-Har212–215), collected at same location. Specimens were collected in September 2017, leg. A. Udalov. Additional material: two males collected from the Kara Sea in September 2015 (at 125 and 301 m depth); seven males and four females from East Siberian Sea in September 2017 (at 27, 45 and 58 m depth).

Description of male. Total body length measured from tip of rostrum to posterior margin of caudal rami: 330 μm.

Antennule (Fig. 2A–E) six-segmented, sub-chirocer. Segment 1 (Fig. 2A) with three rows of spinules on dorsal surface and a pinnate seta at anterior distal corner. Segment 2 (Fig. 2 A) short with pinnate seta at anterior distal corner. Segment 3 (Fig. 2B) almost triangular with ten setae: two strongly pinnate setae at posterior margin, one pinnate seta at anterior proximal corner, seven (three of which are pinnate) setae medially on anterior margin. Segment 4 (Fig. 2C) short and rectangular with eight setae at anterior distal corner (one of them long and pinnate). Segment 5 (Fig. 2D) moderately swollen with convoluted anterior face bearing two rows of denticles; ten setae and on anterior distal corner, five setae pinnate, one short and four long simple setae, and an aesthetasc at distal corner. Segment 6 (Fig. 2E) convoluted; with row of spinules and three pinnate setae at proximal margin, three pinnate and three plane setae and an aesthetasc at distal margin.

P2 (Fig. 2F). Intercoxal sclerite almost square and unadorned. Coxa with row of small spinules in centre of anterior part and tree rows of spinules on distal margin. Basis with outer pinnate seta and row of spinules at base of rami. Both rami three-segmented, almost equal length. Exopod-1 with rows of spinules on outer and distal margin, and one outer seta; exopod-2 with rows of spinules on outer and distal margin, one outer seta and one inner seta; exopod-3 with patches of spinules on outer and distal margin, and seven pinnate setae (223). Endopod-1 with two rows of spinules on outer margin and one small inner seta; endopod-2 modified into mucroniform process reaching middle of endopod-3. Endopod-3 modified with two inner setae (one pinnate and one simple) and one seta in center of anterior face, inner margin bearing three dentate elements.

P5 (Fig. 2G). Elements fused medially, exopod not fused to baseoendopod. Baseoendopod with patches of spinules at distal outer margin, outer seta long and pinnate; endopodal lobe reduced, bearing two setae of different length; exopod with four setae, inner and outer setae inserted at a distance from terminal two.

P6 (Fig. 2H). A single plate fused to somite bearing tree setae and several spinules on each side.

Variability. No variable features or asymmetries were observed among the twelve males and four females examined, and most differences between male and female specimens are all part of the typical sexual dimorphism expressed in this family.

Remarks. The genus Mucrosenia (type species M. kendalli) was established on the basis of a single female. The second member of the genus, M. kliei (Smirnov, 1946), was originally described as Psammis kliei by Smirnov (1946), also from a single female taken at 60 m depth from the East Siberian Sea. Our specimens exhibit autapomorphies of the genus Mucrosenia (as defined by Gee & Huys 1994) which can be seen in both sexes, viz. P2 endopod-2 with a mucroniform process reaching almost to the end of endopod-3 (this sexually dimorphic character is more pronounced in the males but also present in females); P2 endopod-2 without inner seta; posterior displacement of seta II on the caudal ramus; presence of tuft of long setules at inner distal corner of caudal ramus; P2 endopod shorter than exopod; and absence of inner seta on exopod-1 of P2–P4.

Furthermore, some particular features (morphology of anal somite and pseudoperculum; P1 endopod of female distinctly longer than exopod; separation of P5 exopod and baseoendopod marked by distinct suture on dorsal surface; P2 endopod-2 with prominent mucroniform process) displayed by the females from the Kara and East Siberian Seas conform fully to the description of M. kendalli but apparently differ from M. kliei. The single female of the latter species was collected from the same region and depth as some of our specimens (Smirnov 1946). The discrepancies between Smirnov’s illustrations and text description have been expounded in detail by Gee & Huys (1994). Since the male is still unknown for M. kliei, the comparison of males for both species is impossible. Therefore, the validity and definite status of the specimen described by Smirnov (1946) is still a matter of conjecture.

Notes about ecology and distribution. Both above-mentioned species were originally discovered in muddy sediments off the western coast of Spitsbergen, Greenland Sea, at depths of 30 m (D. spitsbergensis) and 95 m (M. kendalli) (Gee & Huys 1994). Our specimens were found in similar habitats but in rather distant locations. Danielssenia spitsbergensis was found in silt in the Kara Sea (Voronin Trench) at 108 and 335 m depth. Mucrosenia kendalli was collected from silty sediments in the Kara Sea (Voronin Trench) at 125 and 301 m depth, and in the East Siberian Sea (inn silt at 27–58 m depth). These findings substantially extend the distribution ranges for both species, both geographically (eastward) and bathymetrically (from the sublittoral zone down to the lower continental shelf). This suggests a wide circum-Arctic distribution for both species, just like for some other genera in the Danielseniinae, e.g. Danielssenia, Archisenia Huys & Gee, 1993 and Paradanielssenia Soyer, 1970.

Harpacticoids of the Siberian Arctic Seas are poorly studied. Not more than a dozen Pseudotachidiidae species have been reported before from this region (Smirnov 1946). In particular, the subfamily Danielsseniinae includes 63 definitive species from 19 genera. Taking the recent data (Garlitska & Azovsky 2016; Chertoprud et al. 2018; Garlitska et al. 2019) into account, 18 of these species (ten genera) have been reported from the Arctic seas. So, this subfamily is equally represented in polar waters and in the tropics (19 species, 12 genera).