﻿A new species of Orchomenella (Amphipoda, Tryphosidae) described from hydrothermal vent in the Okinawa Trough, Northwest Pacific

﻿Abstract A new species of the family Tryphosidae, Orchomenellacompressasp. nov., is described from hydrothermal vents in the Okinawa Trough. This is the first known Orchomenella species found in vent fields. Important morphological characters that differentiate O.compressasp. nov. from its congeners are the absence of eyes, the compressed distal three articles of gnathopod 2, the shape of the posterior margin of epimerons 2 and 3, and the number of dorsal spines on the telson. The genetic divergence of the analyzed COI gene clearly supports this new taxon.


Introduction
The family Tryphosidae was established as a subfamily in family Lysianassidae Dana, 1849 by Lowry and Stoddart (1997), separate from Lysianassinae based on following characters: molar of mandible asetose, apical margin of the outer plate of the maxilliped bearing robust setae, gnathopod 1 subchelate, and telson cleft.The genus Orchomenella Sars, 1890 contains 34 species (WoRMS 2023), which are found in various habitats ranging from shallow water to deep sea (Barnard 1969;De Broyer 1984, 1985;Jung et al. 2017).Molecular analysis indicates that Orchomenella is probably not monophyletic (Havermans et al. 2010), but the genus has been retained awaiting further evidence (Jung et al. 2017).
During a biodiversity survey of hydrothermal vents in the Okinawa Trough in the western Pacific conducted by the Chinese research vessel Kexue in 2014, several individuals referable to Orchomenella were collected.After careful examination, these specimens exhibited some distinctive characters differentiating them from known Orchomenella species.The present study describes this new species.

Materials and methods
The present materials were collected by ROV Faxian suction sampler, together with Iheyaspira lequios Okutani, Sasaki & Tsuchida, 2000 and Probathylepas faxian Ren & Sha, 2015, during expeditions to hydrothermal vents on the Okinawa Trough by the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) in 2014.The specimens are deposited in the Marine Biological Museum, Chinese Academy of Sciences (MBMCAS), Qingdao, China.Specimens were examined and dissected using a dissecting microscope (Zeiss Discovery V20).Line drawings were made with a tablet (Wacom Intuos Pro PTH-851) and Adobe Photoshop CS6 (v.13).Length measurement is made along the outline of the animal, beginning from the anterior margin of head to the end of the urosomite 3.
The COI sequence of Orchomenella compressa sp.nov.(657 bp) was obtained from its mitochondrial genome by homologous alignment, and the mitochondrial genome of the new species was obtained by Illumina HiSeq sequencing.For Illumina pair-end sequencing of each strain, at least 3 μg of genomic DNA was used for sequencing library construction.Paired-end libraries with insert sizes of ~400 bp were prepared following Illumina's standard genomic DNA library preparation procedure.Purified genomic DNA was sheared into smaller fragments with a desired size by Covaris, and blunt ends were generated by using T4 DNA polymerase.After adding an "A" base to the 3' end of the blunt phosphorylated DNA fragments, adapters were ligated to the ends of the DNA fragments.The desired fragments were purified through gel-electrophoresis, then selectively enriched and amplified by PCR.The index tag was introduced into the adapter at the PCR stage, as appropriate, and we did a library quality test.Finally, the qualified Illumina pair-end library was used for Illumina Nova-Seq 6000 sequencing (150 bp*2, Shanghai Biozeron Co., Ltd).The raw paired end reads were trimmed and quality controlled by Trimmomatic with parameters (SLIDINGWINDOW:4:15 MINLEN:75) (v.0.36 http://www.usadellab.org/cms/uploads/supplementary/Trimmomatic).Clean data obtained by above quality control processes were used to do further analysis.

Systematics
Mouthparts.Upper and lower lip typical for the genus.Mandible incisors symmetrical, smooth, subtriangular; left lacinia mobilis a long, slender peg; accessory setal row without distal setal tuft, both left and right with 3 short, slender, simple setae; molar well developed, columnar; palp 3-articulate, attached lower than molar, article 1 shortest, article 3 blade-like, shorter than article 2, fringed with long simple setae.Maxilla 1 with inner plate narrow with 2 pappose apical setae; outer plate with 11 setal-teeth in 7/4 crown arrangement; palp large, 2-articulate, with 9 terminal short robust setae and 1 long, plumose seta.Maxilla 2 inner plate narrower and slightly shorter than outer plate.Maxilliped with inner plate rectangular, distal margin with teeth and 1 or 2 robust setae; outer plate does not extend to distal margin of palp article 3, with 2 apical, robust setae; palp 4-articulate, dactylus nearly as long as article 3, unguis present.
Etymology.From the Latin compressa (= compressed), referring to the compressed distal 3 articles of the gnathopod 2.
Distribution.Presently known only from Okinawa Trough, at a depth of 1243 m.
Remarks.This is the first time that a species of Orchomenella has been reported from hydrothermal vents.The new species can be distinguished from other Orchomenella species in lacking of eyes and having the distal three articles of gnathopod 2 compressed.The new species is morphologically most similar to O. tabasco (Barnard, 1967), which was collected from the Cedros Trench at 1720-1728 m.However, O. compressa sp.nov.differs from O. tabasco by fol-   lowing characters: carpus of the gnathopod 1 shorter than propodus in the new species, rather than subequal in length in O. tabasco, and telson cleft more than 50% in the new species, rather than cleft only 40% in O. tabasco (Barnard 1967).
The ML tree inferred from partial COI sequences from 10 species of Orchomenella, including the new species, is shown in Fig. 5. Orchomenella compressa sp.nov. is clustered with O. pinguis and O. minuta with high bootstrap support (100%).Interspecific genetic divergence (K2P) among these 10 species is summarized in Table 2.The pairwise distance was 0.09%-0.21%.The new species is closest to O. pinguis and O. minuta genetically (0.096 and 0.092, respectively), although morphological characters do not support this close relationship.It is a pity that the COI sequence of O. tabasco, which is morphologically most similar to O. compressa sp.nov., is still unavailable.Of the species analyzed, O. rotundifrons (Barnard, 1932), is most genetically divergent from the new species (0.215) (Havermans et al. 2010;Hupalo et al. 2022).

Figure 5 .
Figure 5. Phylogenetic relationships among Orchomenella compressa sp.nov.and nine species of Orchomenella registered in GenBank, analyzed by the maximum-likelihood (ML) method with two Orchomenyx species as outgroup taxa.Bootstrap values of ML (>50) are indicated above branches of clades.

Table 1 .
Details of specimens and GenBank accession numbers used in this study.

Table 2 .
Genetic divergence of the mitochondrial cytochrome c oxidase subunit I gene among the 10 species of Orchomenella calculated from Kimura 2-parameter corrected calculations.