Bathya brevicarpus gen. et sp. nov. (Amphipoda: Senticaudata: Calliopiidae), from hydrothermal vents, Okinawa Trough, North-west Pacific

One individual referable to Calliopiidae G.O. Sars, 1893 was collected from a chemically reduced habitat, the hydrothermal vent systems in Okinawa Trough, and was identified as a new genus and species belonging to this family after a morphological examination. A formal description of this new species and a discussion of the relationship of the new genus within Calliopiidae are presented.


Introduction
The calliopiids have been placed in the Eusiroidea Stebbing, 1888 (e.g., Bousfield & Hendrycks 1995, 1997De Broyer et al. 2007). A phylogenetic study based on morphology by Lowry & Myers (2013) showed that the Calliopiidae G.O. Sars, 1893 sit in the Hadziidira S. Karaman, 1943 and have a close relationship with Pontogeneiidae Stebbing, 1906. This family currently contains 27 genera and more than 100 species, most of which are described from arctic and subarctic regions (e.g., Ren & Huang 1991;Weisshappel 2001;d'Udekem d'Acoz 2007, 2010. The calliopiids reported from vent fields are represented by two species: Bouvierella curtirama Bellan-Santini & Thurston, 1996 collected at a depth of 1636 m from the mid-Atlantic Ridge and Leptamphopus fragilis Larsen & Krapp-Schickel, 2007 collected at a depth of 2656 m near Wuzza Bare Mount, northeast Pacific (Bellan-Santini & Thurston 1996;Larsen & Krapp-Schickel 2007). The Chinese research vessel "KEXUE", using the remote operated vehicle (ROV) "FAXIAN" to survey the biodiversity of hydrothermal vents in Okinawa Trough, collected one unusual specimen referable to Calliopiidae. A morphological examination shows that this individual is a new calliopiid. However, the new species does not belong to any of the described genera within this family based on the combination of the following morphological characters: a small rostrum; the absence of the antennae calceoli; the article 3 of antenna 1 not produced apicoventrally; the inner plate of maxilla 1 bearing only one apical seta; the gnathopod 2 not linear or greatly elongate and the carpus of the gnathopods 1-2 much shorter than the propodus; pereopod 5 slightly longer than pereopod 7; epimeron 3 smooth; the outer ramus of uropod 3 half-length of the inner ramus; the entire telson, longer than broad. Hence, a new genus, Bathya gen. nov., is erected to accomodate this new species herein. A key to all genera of Calliopiidae is also presented.

Material and methods
The present material was collected by ROV "FAXIAN", during expeditions to the Okinawa Trough hydrothermal vents by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) in April 2014. The type specimen examined is deposited in the Marine Biological Museum, Chinese Academy of Sciences (MBMCAS), Qingdao, China, and preserved in 75% ethanol. It was examined and dissected under a dissecting microscope (ZEISS Discovery V20). Line drawings were made with a tablet (Wacom Intuos Pro PTH-851) and Adobe Photoshop (ver. CS6). The length measurements are made along the outline of the animals, beginning from the anterior margin of head to the end of the telson.

Etymology
The generic name Bathya refers to the type species designated herein that was collected from bathyal waters.

Distribution
North-west Pacific, Okinawa Trough, the hydrothermal vents at a depth of 996.9 m.

Remarks
Actually, it is a little questionable that Bathya gen. nov. is placed under the Calliopiidae G.O. Sars, 1893 for having the pereopod 5 longer than pereopod 7. The phylogenetic study by Lowry & Myers (2013) based on morphology analyzed showed that the Calliopiidae and Pontogeneiidae Stebbing, 1906 cluster together and form a clade with Hornelliidae d'Udekem d 'Acoz, 2010 and Cheirocratidae d'Udekem d 'Acoz, 2010. The new genus is not listed under Cheirocratidae or Hornelliidae, which are also not having antennae calceoli, for having the following characteristics: the inner plate of maxilla 1 only bearing apical seta, maxilla 2 without oblique setal row, the rami of uropod 3 unequal in length, and the entire telson. The new genus is not suited to be placed in Pontogeneiidae for the propodus of gnathopod 2 having robust setae along the palmar margin. Moreover, Sanchoidae Lowry, 2006, which are known as endemic to Australia and associated with sponges (Lowry & Barnard 2001), have been listed under the Calliopiidae. However, the present genus cannot be incorporated in this family for the lack of a nonrecessed head and a dorsoventrally flattened urosome (Lowry 2006). Hence, it is more reasonable that this new genus is to be placed under Calliopiidae with an emendation of the diagnosis of this family.
Besides having the pereopod 5 longer than pereopod 7, Bathya gen. nov. can be distinguished its congeners of the Calliopiidae by having the combination of the following characters: eyes present but not well pigmented; antenna 1 longer than antenna 2; the absence of an accessory flagellum; the inner plate of maxilla 1 only bearing one apical seta; maxilla 2 without oblique setal row; gnathopod 1 similar in shape and size to gnathopod 2; pleonites 1-3 without dorsal carinae; the rami of uropod 3 unequal in length; the entire telson.

Diagnosis
As for the genus.

Etymology
The new species name contains the Latin 'brev' (= short), referring to the carpus of the gnathopods 1 and 2 being shorter than the propodus.
Head. Nearly subequal in length to pereonites 1 and 2 combined, rostrum small, anterior lobe rounded, lower margin with large and acute projecting tooth anterodistally; eyes present, but not pigmented, hardly visible in ethanol material.
uropods. Uropod 1 overreaching distal end of uropod 2, without interramal spur; peduncle shorter than rami, with 19 marginal and one large distal robust setae; outer ramus shorter than inner ramus, but subequal in broad to inner, with 9 marginal and three distal robust setae; inner ramus with 14 marginal and 3 distal robust setae. Uropod 2 peduncle distinctly shorter than rami, with 5 marginal and two distal robust setae; outer ramus 0.6 times shorter than inner ramus, with 10 marginal and three distal robust setae; inner ramus with 15 marginal and three distal robust setae. Uropod 3 shorter than uropod 2, peduncle shorter than rami, with one robust seta distally; outer ramus 0.55 times shorter than inner ramus, with 7 marginal robust setae; inner ramus with 15 marginal robust setae.
telson. Entire, longer than broad (length 1.7 times as long as width at base), distal margin convex.
Moreover, the morphological differences between several genera of Calliopiidae, such as Halirages Boeck, 1871, Haliragoides G.O. Sars, 1893 and Apherusa, are blurred (Ringvold & Tandberg 2014). The phylogenetic analyses based on a comparison of 18S and 28S rDNA sequences by Verheye et al. (2016) shows that the Calliopiidae are not monophyletic. Hence, the entire calliopiid family is in need of revision.

Identification key to genera of the Calliopiidae
Key based on original or amended descriptions of genera and adapted from previous keys given by Barnard (1964),  and Bousfield & Hendrycks (1997). All genera included in this key are according to Lowry & Myers (2013) and WoRMS (2020).