﻿One new species of Stegocephalus Krøyer, 1842 (Amphipoda, Stegocephalidae) described from a seamount of the Caroline Plate, NW Pacific

﻿Abstract A new species of the subfamily Stegocephalinae, Stegocephaluscarolussp. nov., is described from a seamount in the Caroline Plate. Two related species, S.cascadiensis (Moore, 1992) and S.longicornis (Gurjanova, 1962), were previously reported in the North Pacific. Important morphological characters which differentiate S.carolussp. nov. from S.cascadiensis are found in antenna 1, the mouthparts, pereopod 7 and the length of rami of uropods 2 and 3. The new species differs from S.longicornis by characters of antenna 1, the mouthparts and the shape of epimeral plate 3. Additionally, the morphological differences between the new species and the remaining seven species of Stegocephalus are also presented.


Introduction
The family Stegocephalidae Dana, 1852 was revised by Berge and Vader (2001), and divided into five subfamilies: Andaniexinae Berge & Vader, 2001, Andaniopsinae Berge & Vader, 2001, Bathystegocephalinae Berge & Vader, 2001, Parandaniinae Berge & Vader, 2001and Stegocephalinae Dana, 1852.Of these, Stegocephalinae is the largest, comprising 14 genera with 49 species (Horton et al. 2023), characterized by the presence of gaping and geniculate outer plate of maxilla 2, the flagellum of antenna 1 composed of not more than 10 articles and a telson that is longer than broad (Berge and Vader 2001).As the type genus of the Stegocephalinae, Stegocephalus Krøyer, 1842 can be distinguished from the other 13 genera by having the flagellum article 1 of antenna 1 not longer than the peduncle, the flagellum of antenna 2 with more than 10 articles and the palp of the maxilla 1 two-articulate (Barnard and Karaman 1991;Berge and Vader 2001).There are nine valid species of Stegocephalus, which are found in the Arctic, Southern Ocean (Antarctica), Atlantic, southern Mediterranean Sea and North Pacific (Berge and Vader 2001).

Materials and methods
The present material was collected by ROV FAXIAN, during expeditions to seamounts (Fig. 1) on the Caroline Plate by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) during June and July 2019.The material was sorted on board and fixed in 96% ethanol, then transferred to 75% ethanol in the laboratory.The specimens are deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China.The specimens were examined and dissected with a dissecting microscope (ZEISS Discovery V20).Line drawings were completed using the software Adobe Photoshop CS6 with a graphics tablet.Length measurement was made along the outline of the animal, beginning from the rostrum to the posterior margin of the telson; total length of the specimens ranged from 3.4 to 8.9 mm.Additionally, COI (PP188368), 16S rDNA (PP187329) and 18S rDNA (PP187328) sequences of Stegocephalus carolus sp.nov.were obtained from its mitochondrial genome by homologous alignment.Barnard and Karaman 1991;Berge and Vader 2001).Description.First pereon segment longer than the rest; pleonite 1-3 dorsally smooth; epimeral plate 1 with posteroventral corner broadly rounded; epimeral plate 2 with posteroventral corner subacute; epimeral plate 3 without minute serration at posteroventral corner, posteroventral corner strongly produced, triangular.Head partially covered by pereonite 1 and coxa 1; much deeper than long.Rostrum absent.Eyes not apparent in ethanol-fixed material.Antenna 1 with callynophore well developed; peduncle about twice as long as first primary flagellar article; article 1 slightly longer than articles 1-2 combined; primary flagellum 7-articulate, article 1 longest; accessory flagellum 1-articulate, beyond article 1 of primary flagellum, bearing long setae apically.Antenna 2 slightly longer than antenna 1, peduncle (articles 3-5) slightly shorter than flagellum, article 4 longer than article 5, not setose; flagellum 12 articles, article 1 longer than rest of articles.

Stegocephalus carolus
Mouthparts.Epistomal plate absent.Upper lip broader than long, bilobate, slightly asymmetrical.Lower lip with the narrow lobes bearing row of simple setae.Right Mandible incisor with 9 teeth; incisor of left mandible with 7 teeth, left lacinia mobilis with 9 teeth.Maxilla 1 palp 2-articulate, rectangular, apex not reaching above the apex of outer plate, apically bearing 4 plumose setae; outer plate with 9 setal-teeth arranged in a pseudocrown; inner plate with a well-developed shoulder, setae pappose.Maxilla 2 gaping and geniculate; outer plate with about 10 apical setae bearing distal hooks; inner plate with marginal and submarginal setae pappose.Maxilliped with inner plate subrectangular, not exceeding base of palp article 2, distal margin slightly concave, setose, medial setae-row present; outer plate only with inner margin bearing about 7 small robust setae; palp 4-articulate, article 2 distinctly shorter than article 1, inner margin produced distally, dactylus short, about 1/5 length of article 3.
Uropods and telson.Uropod 1 peduncle 1.4 times longer than rami, outer margin densely setose, inner margin with only 2 robust setae; rami lanceolate, subequal in length, both rami with inner and outer margins setose.Uropod 2 shorter than uropod 1, peduncle subequal in length to rami, both margins setose; rami subequal in length, both rami with inner and outer margins setose.Uropod 3 shortest, peduncle distinctly shorter than rami, not setose; outer ramus 1-articulate, shorter than inner ramus, both rami with inner and outer margins setose.Telson longer than broad, without setae, cleft about 0.8 of total length, distally acute.Etymology.The new species is named after its type locality, the Caroline Plate.
Distribution.Presently known only from a seamount of the Caroline Plate, at a depth of 813-1221 m.
Remarks.According to the revision of the family Stegocephalidae Dana, 1852 by Berge and Vader (2001), Stegocephalus carolus sp.nov.belongs to the subfamily Stegocephalinae Dana, 1852.The new species shares distinctive characteristics with other species in the genus Stegocephalus Krøyer, 1842, including a two-articulate palp of maxilla 1, a produced palp article 2 of the maxilliped, a conspicuously expanded basis of pereopod 6, a 1-articulate outer ramus of uropod 3, and a cleft telson.Two Stegocephalus species, S. cascadiensis (Moore, 1992) and S. longicornis (Gurjanova, 1962), have been previously reported in the North Pacific (Moore 1992;Berge and Vader 2001).
Stegocephalus cascadiensis (Moore, 1992) was originally reported from the Cascadia Abyssal Plain at a depth of 2740-2818 m (Moore 1992).Stegocephalus carolus sp.nov.differs from S. cascadiensis by the peduncle of antenna 1 longer than the first flagellar article, and the accessory flagellum extending beyond the distal margin of the first flagellar article.Additionally, the apical setae on the outer plate of maxilla 2 are hooked in the new species, whereas, they are unhooked in S. cascadiensis.In S. carolus sp.nov., the first palp article of the maxilliped is subequal in length to the second article, whereas in S. cascadiensis, the first palp article is much longer than the second.The inner plate of the maxilliped is subrectangular with a concave distal margin in S. carolus sp.nov., whereas it is subtriangular in S. cascadiensis.The second palp article of the maxilliped in S. carolus sp.nov.has a produced inner margin, and the posterior margin of pereopod 7 is slightly serrate instead of smooth.Furthermore, the rami of uropod 2 are subequal in S. carolus sp.nov., whereas the inner ramus is longer than the outer ramus in S. cascadiensis.Lastly, the outer ramus of uropod 3 in S. carolus sp.nov. is shorter than the inner ramus, while in S. cascadiensis, it is longer than the inner ramus (Moore 1992, fig. 7).
Stegocephalus longicornis (Gurjanova, 1962) was originally reported from the Bering Sea at a depth of over 2440 m.The new species differs from S. longicornis by: the shorter antenna 1, the 7-articulated primary flagellum and the 1-articulated accessory flagellum, while the primary flagellum is 11-articulate and the accessory flagellum is 3-articulate in S. longicornis; the distal margin of the inner plate of the maxilliped is concave rather than convex, and the inner margin of palp article 2 is produced, versus not produced in S. longicornis; and the posteroventral corner of epimeral plate 3 is strongly produced and triangular, while epimeral plate 3 is only produced in a small triangular angle in S. longicornis (Gurjanova 1962, fig. 130).
The remaining species of Stegocephalus are found in the Arctic, North Atlantic, or the Southern Ocean (Berge and Vader 2001).Stegocephalus carolus sp.nov.can be easily distinguished from S. abyssicola (Oldevig, 1959) by its strongly projected posteroventral corner of the third epimeral plate (Oldevig 1959).It is also different from S. ampulla (Phipps, 1774), S. inflatus Krøyer, 1842, S. rostrata KH Barnard, 1932, andS. similis Sars, 1891 by the absence of a rostrum (Barnard 1932;Steele 1967;Berge and Vader 2001).Especially, it differs from S. inflatus by the pale of maxillae 1 being 2-articulated, while it is 1-articulated in S. inflatus (Steele 1967, fig. 1B); epimeral plates 2 and 3 have the posteroventral corner broadly rounded, while epimeral plates 2 and 3 are produced into a sharp tooth in S. inflatus (Steele 1967, fig. 1A).Additionally, the new species can be distinguished from S. kergueleni (Schellenberg, 1926) and S. nipoma (J.L. Barnard, 1961) by the presence of a toothed left lacinia mobilis, the posterior margin of gnathopod 1 is not densely setose; and the subacute posteroventral of epimeral 2 is not broadly rounded (Schellenberg 1926;Barnard 1961).Lastly, it also differs from S. similis by the shape of the third epimeral plate, which is neither serrated nor notched (Oldevig 1959).
ZooKeys 1195: 121-130 (2024), DOI: 10.3897/zookeys.1195.114209Yanrong Wang et al.: A new species of Stegocephalus from seamount During a biodiversity survey of seamounts on the Caroline Plate in the northwest Pacific in 2019, conducted by the Chinese research vessel KEXUE, several individuals referable to Stegocephalus were collected.These specimens exhibit some distinctive characteristics differentiating them from the other nine described Stegocephalus species, and they are described as a new species herein.

Figure 1 .
Figure 1.A map showing the location of the sampling site (red triangle) in the NW Pacific.