Eurythenes atacamensis Weston & Espinosa-Leal & Wainwright & Stewart & González & Linley & Reid & Hidalgo & Oliva & Ulloa & Wenzhöfer & Glud & Escribano & Jamieson 2021, sp. nov.

Eurythenes atacamensis sp. nov. Weston & Espinosa-Leal

(Figures 2–6)

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Eurythenes gryllus — Ingram and Hessler 1987: 1889.— Thurston et al. 2002: 205–210, figs. 1–7, table 1.— Jamieson et al. 2019: 1–9, fig. 1, table 1.

Eurythenes gryllus Peru-Chile (H)— Ritchie et al. 2015: 121–129, figs.2, 4, tables 1, 2.

Eurythenes sp. (Hadal Form)— Eustace et al. 2016: 91–97, fig. 1, fig. 2 (d)(e)(f), fig. 5, tables 2, 3.

Material Examined.

Holotype: Female, total body length 76.2 mm, Atacama Trench, eastern South Pacific Ocean (23° 22.774′ S, 71° 20.683′ W), expedition SO216, station 4, depth 8052 m, MNHNCL AMP-15816, genseq-1 16S (MW042884), COI (MW048996).

Paratypes: Female, total body length 70 mm, Atacama Trench, Pacific Ocean (23° 24.48′ S, 71° 19.91′ W), Atacamex Expedition, station 2, depth 8081 m, MZUC/UCCC 46674. Female, total body length 72 mm, Atacama Trench, Pacific Ocean (23° 24.48′ S, 71° 19.91′ W), Atacamex Expedition, station 2, depth 8081 m, MZUC/UCCC 46675, genseq-2 16S (MW290039), COI (MW288146). Male, total body length 50.8 mm, Atacama Trench, Pacific Ocean (23° 22.384′ S, 71° 23.577′ W), expedition SO216, station 4, depth 7204 m, MNHNCL AMP-15817. Female, type locality, MNHNCL AMP-15822. Intersex, total body length 58.8 mm, Atacama Trench, Pacific Ocean (24° 16.233′ S, 71° 25.386′ W), expedition SO216, station 6, depth 7834 m, MNHNCL AMP-15820, genseq-2 16S (MW042883). Juvenile, total body length 16.1 mm, Atacama Trench, Pacific Ocean (21° 44.497′ S, 71° 15.465′ W), expedition SO216, station 2, depth 6738 m, MNHNCL AMP-15819. Juvenile, total body length 38.4 mm, Atacama Trench, Pacific Ocean (21° 44.497′ S, 71° 15.465′ W), expedition SO216, station 2, depth 6714 m, MNHNCL AMP-15818. Juvenile, Atacama Trench, Pacific Ocean (22° 56.282′ S, 71° 40.686′ W), expedition SO216, station 7, depth 4974 m, MNHNCL AMP-15821.

Paragenetype: Juvenile, Atacama Trench, Pacific Ocean (22° 56.282′ S, 71° 40.686′ W), expedition SO216, station 7, depth 4974 m, genseq-2 16S (MW042880). Juvenile, Atacama Trench, Pacific Ocean (20° 20.608′ S, 71° 07.821′ W), expedition SO216, station 10, depth 5920 m, genseq-2 16S (MW042881), COI (MW048993). Female, Atacama Trench, Pacific Ocean (23° 02.998′ S, 71° 15.044′ W), expedition SO216, station 3, depth 7139 m, genseq-2 16S (MW042882), COI (MW048994).

Type Locality. Atacama Trench, eastern South Pacific Ocean (23° 22.774′ S, 71° 20.683′ W), expedition SO216, station 4, depth 8052 m.

Etymology. The species name, atacamensis, references the type locality, Atacama Trench, of this conspicuously abundant scavenging amphipod.

Diagnosis. Lateral cephalic lobe rounded and weakly pronounced. Ventral corner of the eye points linearly downwards. Article 2 of mandibular palp expanded posteriorly but not distally tapering. Maxilliped inner plate with three apical, non-protruding nodular setae. Gnathopod 1 subchelate; palm weakly formed, short. Gnathopod 2 minutely chelate; coxa sub-rectangular and posterior margin slightly rounded; palm obtusely angled. Pereopods 3 to 7 dactylus short. Epimeron 3 ventral margin rounded with a small tooth on the posteroventral corner. Uropod 2 inner ramus longer than outer ramus. Lack of dorsal carination or ridging, specifically at pereonite 3.

Description, based on holotype, female, MNHNCL AMP-15816.

Body (Fig. 2): surface smooth, without setae; urosomite 3 with an anterodorsal depression. Oostegites present on gnathopod 2 to pereopod 5, setae absent. Coxa gills present on gnathopod 2 to pereopod 7. Colour pattern before ethanol preservation unknown as the holotype was selected post-expedition.

Head (Fig. 3): rostrum absent; antennal sinus quadrate (Fig. 3d). Antenna 1 short, 0.13× as long as body length; accessory flagellum 14-articulate; primary flagellum 34- articulate; calceoli absent (Fig. 3a). Antenna 2 2.4× the length of antenna 1, 0.25× as long as body; article 4–5 with brush setae; flagellum 68-articulate with some brush setae; calceoli absent (Fig. 3b).

Mouthpart bundle (Fig. 3): Mandible left lacinia mobilis a long slender robust seta with smooth distal margin; incisor smooth and convex; setal row with 11 short, slender, robust setae; molar large, setose, small triturating surface; palp article-length ratio 1: 1.8: 1.6, article 3 sickle-shaped (Fig. 3c). Maxilla 1 inner plate with nine apical plumose setae; outer plate with an 8/3- crown arrangement; palp longer than the outer plate, 2- articulate, four apical and one apicolateral robust setae, with one subapical long setae (Fig. 3e–h). Maxilla 2 both plates broad, inner plate 0.6 × shorter than the outer plate (Fig. 3i). Maxilliped inner plate subrectangular, three apical, non-protruding nodular setae; outer plate subovate; palp 4-articulate, left and right are asymmetric with right palp exceeding past the outer plate, dactylus well-developed, unguis present (Figure 3j–l).

Pereon (Figs. 4 and 5): Gnathopod 1 coxa sub-quadrate, weakly concave on anterior and ventral margins; basis, long, length 2.2× breadth; palm weakly formed and short (0.1× as long as the posterior margin of propodus), crenulate with one robust seta at base of the palm and another at the end of palm (Fig. 4a–b). Gnathopod 2 coxa with setae along the posteroventral corner; basis elongate, length 6.9 times width, setae along posterior and ventral margins; posterior margin of merus expanded; propodus sub-rectangular, length 4.5 times width; palm with 2 robust setae on the posterodistal corner; dactylus not reaching palmar corner (Fig. 4c–d). Pereopod 3 coxa sub-quadrate, 1.5× as long as wide, setae on the surface of coxa and along ventral margin; basis expanded posteriorly, 2.3× as long as wide; merus expanded anteriorly, tuft of setae on the anteroventral corner; carpus stout, 0.6× as long as propodus; propodus 3.9× as long as wide; dactylus slender, short 0.3× as long as propodus, unguis present (Fig. 4e). Pereopod 4 coxa broad, 0.9× as long as wide, 1.1× length of coxa 3, the junction between anterior and ventral border bluntly angular (sub-rectangular), ventral border straight, posteroventral border weakly oblique; leg almost identical to pereopod 3 (Fig. 4f). Pereopod 5 coxa sub-rectangular, rounded on both the anterior and posterior margins; basis expanded posteriorly, posterior margin weakly crenulated; merus broadly expanded posteriorly, 1.5× as long as wide, posteroventral margin producing a point; carpus stout, 0.4× as long as propodus; propodus long and slender, 5.5× as long as wide, 11 groups robust setae along anterior margin; dactylus short, 0.4× as long as propodus, unguis present (Fig. 5a). Pereopod 6 coxa sub-rectangular, setae along the ventral margin, posterior margin straight; basis expanded posteriorly with posterior margin crenulated; merus expanded posteriorly, 1.5× as long as wide, convex posterior margin; propodus and dactylus nearly identical to pereopod 5 (Fig. 5b). Pereopod 7 coxa sub-rectangular; basis expanded posteriorly, posterior margin distinctly crenulated, distal lobe weakly protruding; merus broad and strongly expanded posteriorly, subequal length to width; propodus and dactylus nearly identical to pereopod 5 (Fig. 5c).

Pleon and urosome (Fig. 5): Epimeron 1 with setae along the anteroventral corner (Fig. 5d). Epimeron 2 with setae along the ventral margin, posteroventral corner produced into a strong tooth (Fig. 5d). Epimeron 3 ventral margin rounded with a small tooth on the posteroventral corner (Fig. 5d). Uropod 1 peduncle with 1 apicomedial seta, rami subequal, outer ramus 0.8× as long as peduncle (Fig. 5e). Uropod 2 peduncle with 2 apicomedial setae, outer ramus subequal in length to peduncle, inner ramus longer than outer ramus (1.2×; Fig. 5f). Uropod 3 setae of the distolateral angle of peduncle of normal length and stoutness; inner ramus subequal in length to article 1 of the outer ramus; outer rami article 2 0.8× the length of article 1, medial margins of both rami with plumose setae (Fig. 5g). Telson 77% cleft, distal margin of each lobe with one robust and one slender setae (Fig. 5h–i).

Variations. Prior to ethanol preservation, body colour of specimens ranged from white, pink, crimson, to dark red and the eye shape and colour were more defined (Fig. 6). This wide variation in body pigmentation is likely attributed to the moult/intermoult cycle (Baldwin and Smith 1987). Minor differences were observed between females and the male. The mature male paratype (MNHNCL AMP-15817) had calceoli present on both antennas 1 and 2. The primary flagellum of antenna 1 was 31-articulate with calceoli present between articles 8 and 20, and the accessory flagellum was 12-articulate. Antenna 2 was 65-articulate. The intersex paratype (MNHNCL AMP-15820) had protruding penile papillae that flexed towards each other but lacked calceoli on antenna 1 or 2. As with the holotype, the oostegites were present on pereopod 2–5; however, the flattened oostegites were not of full length relative to the total body length and lacked setae. Moderate differences were present between sexed and juvenile specimens, with fewer setae on pereopods and uropods and a reduction in articulation on antennae. Specifically, in the juvenile paratype (MNHNCL AMP-15818), the antenna 1 accessory flagellum was 10- articulate, antenna 1 was 26-articulate, and antenna 2 was 57-articulate. Further, the juvenile had more pronounced crenulation of the posterior margin of the basis on pereopods 5–7.

Feeding and distribution. This species is a benthopelagic scavenger, which is well documented by its rapid aggregation and feeding at baited the camera landers (Fig. 6a; Hessler et al. 1978). As with Eurythenes plasticus, individuals of E. atacamensis sp. nov. have been previously documented to ingest microplastics (Jamieson et al. 2019; Weston et al. 2020a). Eurythenes atacamensis sp. nov. has a wide bathymetric range (>3000 m) across abyssal to hadal depths (4974–8081 m), including the deepest point of the Atacama Trench. This species is considered to have a distribution localized to both sectors of the Peru-Chile Trench. Eurythenes atacamensis sp. nov. is a prominent member of a wider scavenging amphipod community (Fujii et al. 2013). This community is comprised of three species also endemic to the Peru-Chile Trench, Hirondellea thurstoni Kilgallen, 2015, Hirondellea sonne Kilgallen, 2015, and Hirondellea wagneri Kilgallen, 2015.

Differential diagnosis. In a genus with cryptic speciation (Havermans et al. 2013), Eurythenes atacamensis sp. nov. has distinct diagnostic features. These features include a smooth dorsal body, the palm of gnathopod 1 being very short, and the palm of gnathopod 2 being minutely chelate with an obtusely angled palm. Eurythenes atacamensis sp. nov. is the most similar morphologically to Eurythenes thurstoni Stoddart & Lowry, 2004, as they both have a minutely chelate gnathopod 2. Yet, E. atacamensis sp. nov. can be readily differentiated by the lack of an upturned ridge on the anterodorsal margin of head (present in E. thurstoni), uropod 2 inner ramus longer than outer ramus (opposed to subequal), and small tooth on the posteroventral corner of epimeron 3 (versus subquadrate). Eurythenes thurstoni is also smaller in total body size, most commonly not longer than 35 mm (Stoddart and Lowry 2004). Additionally, the two species have a disjunct vertical distribution, where E. thurstoni lives at bathyal depths (Stoddart and Lowry 2004; d’ Udekem d’ Acoz and Havermans 2015).