Epimeria liui sp. nov., a new calcified amphipod (Amphipoda, Amphilochidea, Epimeriidae) from a seamount of the Caroline Plate, NW Pacific

Abstract A calcified individual of Epimeria Costa, 1851 collected from an unnamed seamount of the Caroline Plate, NW Pacific, is recognized as new to science herein. This increases the number of known Epimeria species of the North Pacific to nine. Epimeria liuisp. nov. differs from its similar congeners by having a rostrum hardly reaching to the end margin of first peduncular article of antenna 1, the presence of large pyriform eyes, the size-increasing mid-dorsal teeth starting from pereonite 6 to pleonite 2, the projection on coxa 5 not extending to epimeral plate 1, and by having a nearly quadrate telson notched medially. To facilitate identification the new species is included in a key to Pacific species of Epimeria.


Introduction
The genus Epimeria Costa in Hope, 1851 currently contains nine subgenera and 85 described species (WoRMS 2019). This almost cosmopolitan genus is particularly diverse in the Southern Ocean (59 species), and has been recorded from the intertidal down to 5695 m depth (Stephensen 1947;Shimomura and Tomikawa 2016;d'Udekem d'Acoz and Verheye 2017). When the Chinese research vessel KEXUE surveyed the biodiversity of seamounts on the Caroline Plate, NW Pacific during 2019, one individual referable to Epimeria was collected. The specimen exhibits some distinctive characters differentiating it from other described Epimeria species, so it is identified as new to science herein. This new species is described, and morphologically compared to other very similar species are presented, and a key to all Pacific Epimeria species is also provided.

Material and methods
The present material was collected by ROV FAXIAN, during expeditions to seamounts on the Caroline Plate by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) during June to July 2019. The specimen is deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China. The individual was 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 telson. Coxae 1-4 progressively longer, coxae 1-3 narrow, coxa 4 five-sided; coxae 5-6 with or without tooth or process projecting laterally. Gnathopods weak; gnathopod 2 longer than gnathopod 1. Pereiopod 6 > pereiopod 5 > pereiopod 7; basis of that with longitudinal carina on both sides. Coxal gill from gnathopod 2 to pereiopod 7. Oostegite large, from gnathopod 2 to pereiopod 6. Uropods well developed. Urosomite 1 always with a rounded or tooth-like process. Telson incised or cleft, rarely emarginate or entire. Diagnosis. Rostrum hardly reaching to distal margin of first peduncular article of antenna 1; eyes present, pigmented, pyriform. Maxilliped palp article 4 with more than two teeth in internal margin. Coxa 5 with posterodistal corner produced. Pereonites 6, 7 and pleonites 1, 2 with size-increasing mid-dorsal teeth, the one on pereonite 6 blunt and small.

Systematics
Description. Body calcified. Head. Rostrum nearly as long as head, not reaching to distal margin of first peduncular article of antenna 1; anterior cephalic margin with a small lobe medially, lateral cephalic slightly produced; eyes bulging on head, pigmented, pyriform. Antenna 1 with peduncular article 1 about twice as long as article 2, 3 times as long as article 3, without distal tooth; accessory flagellum scale-like, hardly reaching to half-length of first flagellar article; primary flagellum with 26 articles, sparsely setose. Antenna 2 nearly as long as antenna 1, peduncular article 4 slightly longer than article 5; flagellum with 29 articles.
Mouthparts. Mandible with incisor and lacinia mobilis strongly dentate; molar triturative; palp article 3 densely setose medially, with two long setae distally. Maxilla 1 with inner plate subtriangular, obliquely convex inner margin with 10 stout plumose setae; outer plate distal margin oblique, with 11 lobate robust setae; palp exceeding outer plate; palp 2-articulate, article 2 with 3 robust setae and 5 long setae distally, inner margin bearing row of dense setae. Maxilla 2 with long, slender setae distally on lateral and medial plates. Maxilliped with outer plate broadly rounded distally, bearing short setae, hardly reaching to distal margin of palp article 3; inner plate with row of short setae medially and anteriorly; palp medial margin strongly setose, article 3 with groups of long setae reaching distal end of dactylus, dactylus with serrate medial margin.
Uropods and telson. Uropod 1 peduncle subequal in length to rami, outer margin setose; rami subequal in length, margins bearing small robust setae. Uropod 2 peduncle subequal to outer ramus, outer margin setose; outer ramus shorter than inner ramus, both rami outer and inner margins setose. Uropod 3 peduncle much shorter than rami, inner margin with robust setae; rami subequal in length, inner and outer margins of both rami bearing short robust setae. Telson nearly as long as wide, posterior margin notched medially.
Coloration. Freshly captured specimen of Epimeria liui sp. nov. show distinct orange eyes and rose-to ivory-colored body.
Etymology. The species is named in honor of the late Prof. Dr. Ruiyu Liu (J.Y. Liu), the Institute of Oceanology, Chinese Academy of Sciences, for his great contribution to the carcinology of China. Remarks. Eight Epimeria species have been reported from the northern Pacific, including E. abyssalis Shimomura & Tomikawa, 2016, E. cora J.L. Barnard, 1971, E. morronei Winfield et al., 2012, E. ortizi Varela & García-Gómez, 2015, E. pacifica Gurjanova, 1955, E. pelagica Birstein & Vinogradov, 1958, E. subcarinata Nagata, 1963and E. yaquinae McCain, 1971. Epimeria liui sp. nov. can be distinguished from above species by the following characters: rostrum hardly reaching to the distal margin of first peduncular article of antenna 1; the presence of pyriform pigmented eyes; the projection of coxa 5 not reaching to epimeral plate 1. Actually, E. liui sp. nov. more closely resembles E. bruuni Barnard, 1961 andE. horsti Lörz, 2008, which occur in the southern Pacific, by the produced mid-dorsal carinae starting from pereonite 5 or 6 and having the process on coxa 5 not extending to pleonite 1. The new species differs from E. bruuni by the mid-dorsal teeth starting on pereonite 6 and the pleonite 3 not having a large acute mid-dorsal tooth. Epimeria liui sp. nov. is especially similar to E. horsti for the coloration of the animal body. But it morphologically differs from E. horsti by the rostrum not extending to the distal margin of first peduncular article of antenna 1, the anterior cephalic margin having a semicircular lobe, the coxa 5 having a ridge whereas this part in E. horsti appears to be smooth (Lörz 2008, figs 1, 5), the middorsal blunt tooth of pereonite 6 not forming a triangular acute tooth as in E. horsti    (Lörz 2008, fig. 1), the pleonite 3 not having a mid-dorsal tooth, the posteroventral angle of the epimeron 3 not being produced, and by the telson being notched medially.
The key to the species of Epimeria based on Lörz and Coleman (2014) and Shimomura and Tomikawa (2016) is presented below.