Eurythenes plasticus Weston 2020, sp. nov.

Eurythenes plasticus sp. nov. Weston

(Figs. 4–8)

HOLOTYPE: Mature female, USNM 1615729, body length 48.1 mm.

PARATYPES: Mature male, USNM 1615732, GenBank (16S MT 021437), (COI MT 038070), body length 47.6 mm, Mariana Trench, Pacific Ocean (11.5911N, 144.84730E), cruise FK 141109, station LH14, depth 6010 m. Immature female, USNM 1615733 GenBank (16S MT 021438), (COI MT 038071), body length 38.6 mm, Mariana Trench, Pacific Ocean (11.6071N, 144.8331E), cruise FK 141109, station LH15, depth 6142 m. Juvenile, USNM XXXX3, body length 15.6 mm, same collection location as type locality

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PARAGENETYPE: Juvenile, GenBank (16S MT 021439), (COI MT 038072), body length 15.1 mm, same collection location as type locality.

NON-TYPE SPECIMENS: Three juveniles, body lengths 12.5, 13.5 & 15.7 mm, same collection location as type locality, USNM 1615731.

Type Locality. Mariana Trench, Pacific Ocean (12.64065N, 144.73796E), cruise FK141109, station WT02, depth 6865 m.

Etymology. The species names, plasticus, stems from Latin for plastic. This name speaks to the ubiquity of plastic pollution present in our oceans.

Diagnosis. Lateral cephalic lobe strongly produced, slightly triangular. Article 2 of mandibular palp narrow. Maxilliped inner plate with three to four apical protruding nodular setae. Gnathopod 1 subchelate, basis narrow (2.9x as long as wide), palm not protruding and weakly convex. Gnathopod 2 subchelate, coxa broad ventrally and weakly curved, palm convex. Pereopods 3 to 7 dactyli short. Pereopod 5 coxa bilobate and posterior lobe larger than anterior lobe. Epimeron 3 posteroventral corner subquadrate without small posteroventral tooth. Uropod 1 and 2 rami margins with spine-like setae. Dorsal carination with increasing degree on epimeron 1-3 and urosomite 1.

Description, based on holotype, female, USNM 1615729.

BODY (Figs. 4, 5, 6): surface smooth, without setae; urosomite 3 with an anterodorsal depression. Oostegites present on gnathopod 2 to pereopod 5, elongate but lacking setae. Coxa gills present on gnathopod 2 to pereopod 7. Colour pattern at time of recovery unknown.

HEAD (Fig. 5): rostrum absent; ventral corner of eye rounded and obliquely pointing backwards (Fig. 5C). Antenna 1 short, 0.1x as long as body length; accessory flagellum 12-articulate; primary flagellum 28-articulate; callynophore well-developed; calceoli absent (Fig. 5A). Antenna 2 medium length, 0.3x as long as body, 1.8x as long as antenna 1; flagellum 59-articulate; calceoli absent (Fig. 5B).

MOUTHPART BUNDLE (Fig. 5): Mandible left lacinia mobilis a long slender distally cuspidate robust seta; setal row left with 13 short, slender, robust setae; molar large, setose, vestigial distal triturating patch; palp article length ratio 1: 3.2: 2.6, article 2 posteriorly not expanded and distally not tapering, 3.4x as long as wide; article 3 blade-like (Fig. 5I). Maxilla 1 inner plate with nine apical and sub-apical plumose setae; outer plate with an 8/3 setal crown arrangement; palp longer than outer plate, 2-articulate, seven sub-apical and apical setae with one being a flag seta (Fig. 5H). Maxilla 2 inner and outer plates broad, inner plate 0.6x shorter than outer plate (Fig. 5G). Maxilliped inner plate large, sub-rectangular, four apical protruding nodular setae; outer plate subovate, with 12 apical setose setae; palp large and well-developed; dactylus well-developed, unguis present, six small apical setae (Fig. 5D, F).

PEREON (Figs. 6, 7): Gnathopod 1 coxa very weakly anteriorly concave, anteroventral margin with setae; palm crenulate, 0.4x as long as width of propodus, defined by one robust seta at base of palm and another robust seta at end of palm that is 2.6x longer; dactylus curved posteriorly, one long anterodistal seta, unguis present (Fig. 6A, B). Gnathopod 2 subchelate, coxa obovate, broad ventrally and weakly curved; propodus elongate, not expanded distally, 6.1x as long as wide; propodus 2.7x as long as wide, moderately expanded distally; palm crenulate, distal end defined by three robust setae; dactylus not reaching palmar corner, curved posteriorly, unguis present, one long anterodistal seta (Fig. 6A, B). Pereopod 3 coxa sub-rectangular, 2.0x as long as wide, setae on surface of coxa and along ventral and posterior margins; basis weakly expanded posteriorly, 2.7x as long as wide; merus expanded anteriorly, tuft of setae on anteroventral corner; propodus 4.8x as long as wide; dactylus short, 0.4x as long as propodus, unguis present (Fig. 6C). Pereopod 4 coxa broad, 1.2x as long as wide, 1.1x length of coxa 3, junction between anterior and ventral border bluntly angular (sub-rectangular), ventral border straight, posteroventral border straight and weakly oblique; leg almost identical with pereopod 3 (Fig. 6D). Pereopod 5 coxa bilobate, posterior lobe 1.3x longer and 1.6x wider than anterior lobe, ventral border of posterior lobe sub-triangular; basis expanded posteriorly, posterior margin smooth; merus broadly expanded posteriorly, 1.5x as long as wide, curved posterior margin; propodus slender, 6.2x as long as wide, seven groups of robust setae on the anterior margin; dactylus short, 0.4x as long as propodus, unguis present (Fig. 7A). Pereopod 6 coxa subquadrate, posterior margin weakly bilobate or weakly concave; basis expanded posteriorly, posterior margin distinctly crenate; merus broadly expanded posteriorly, 1.7x as long as wide, convex posterior margin; propodus slender, 5.9x as long as wide, eight groups of robust setae on the anterior margin; dactylus slender, short, 0.3x as long as propodus, unguis present (Fig. 7B). Pereopod 7 coxa sub-rectangular; basis with posterior border crenulate and strongly expanded, distal lobe moderately protruding; merus broadly expanded posteriorly, 1.6x as long as wide, convex posterior margin; propodus with normal stoutness, 5.6x as long as wide, eight groups of robust setae on the anterior margin; dactylus slender, short, 0.3x as long as propodus, unguis present (Fig. 7C).

PLEON AND UROSOME (Figs. 7, 8): Epimeron 1 anteroventral corner rounded with long slender setae; posteroventral corner produced into a small tooth. Epimeron 2 anteroventral margin lined with short fine setae; posteroventral corner produced into a strong tooth. Epimeron 3 ventral margin lined with long fine setae, weakly curved (Fig. 7D). Urosomite 1 with anterodorsal notch (Fig. 7D). Uropod 1 peduncle with one apicomedial setae; inner ramus subequal in length to outer ramus; outer ramus 0.85x as long as peduncle; outer ramus with 18 lateral and eight medial spine-like setae; inner ramus with 20 lateral and 11 medial spine-like setae (Fig. 8A). Uropod 2 peduncle with one apicomedial setae; inner ramus subequal in length (0.9x) to outer ramus; outer ramus subequal in length to peduncle outer ramus with 20 lateral and three medial spine-like setae; inner ramus with seven lateral and 16 medial spine-like setae (Fig. 8B). Uropod 3 inner ramus subequal in length to article 1 of outer ramus; article 2 of outer rami short, 0.05x length of article 1; setae of distolateral angle of peduncle of normal length and stoutness; medial margins of both rami with plumose setae (Fig. 8C). Telson 70% cleft, pair of apical setae on each lobe parallel with beginning of cleft, distal margin with a single apical seta on right lobe, distal end of left lob missing (Fig. 8D).

Variations. As with other species of Eurythenes, there appears to be very little sexual dimorphism. In part, this could be limited to having a single male specimen. The mature male paratype (USNM 1615732) has calceoli present on both antenna 1 and antenna 2. Both antennae are shorter than the holotype with antenna 1 accessory flagellum being 10-articulate, antenna 1 25-articulate, and antenna 2 54-articulate. Additionally, the maxilliped inner plate of the male paratype has three apical protruding nodular setae, specifically lacking the third setae present on the holotype (Fig. 5F). There were differences present in the juvenile paratype (USNM 1615730) that included typical cohort differences among Eurythenes, such as fewer setae on pereopods and uropods and reduced articulation on antennae (antenna 1 accessory flagellum 7-articulate, antenna 1 15-articulate, and antenna 2 38-articulate). In addition, the juvenile paratype had more pronounced and raised dorsal carination than on the adults (Fig. 7E). This difference was present among all the juvenile specimens observed.

Differential Diagnosis. As highlighted in d’Udekem d’Acoz & Havermans (2015), the morphological characteristics that separate and define the species within the gryllus -complex are hard to observe and should be used with caution. Eurythenes plasticus sp. nov. is a member of the gryllus -complex morphologically and genetically. Nevertheless, there is a combination of characters that are unique to E. plasticus sp. nov. and allow it to be distinguished from the morphologically similar species E. andhakarae, E. magellanicus, and E. aequilatus. The most distinctive characteristics are the robust, spine-like setae on rami of uropod 1 and 2 (Fig. 8A, B) and the lobes of pereopod 5 coxa (Fig. 7A), here being unequal, which is novel within Eurythenes. Eurythenes plasticus sp. nov. can be differentiated from E. andhakarae with article 2 of the mandible palp being narrow (instead of expanded), four protruding nodular spines on the inner plate of the maxilliped (versus three non-protruding), and straight ventral border of coxa 4 (opposed to curved). Eurythenes plasticus sp. nov. can be separated from E. magellanicus with a long gnathopod 1 palm (instead of short), a straight ventral border of coxa 4 (opposed to curved), a subquadrate posteroventral corner in epimeron 3 (instead of bearing a small tooth), and the rami of uropod 1 and 2 being subequal (opposed to uropod 2 outer ramus being shorter than inner ramus and uropod 1 outer ramus being longer than inner ramus). Eurythenes plasticus sp. nov. can also be distinguished from E. aequilatus by its eyes with a variable width (opposed to constant width), the outer plate of maxilla 1 with 8/3 crown arrangement (instead of 9/3 arrangement), and a long gnathopod 1 palm (instead of short).

Habitat, Distribution and Biology. Eurythenes plasticus sp. nov. was collected from the upper hadal depths of the Mariana Trench, between 6010 and 6949 m. Similar to sister species within the genus, E. plasticus sp. nov. is a benthic scavenger, as individuals of multiple cohorts entered the baited traps. Eurythenes plasticus sp. nov. is a member of a wider scavenging amphipod community comprised of A. gigantea, Bathycallisoma schellenbergi (Birstein & Vinogradov, 1958), Hirondellea dubia Dahl, 1959, H. gigas, Paralicella caperesca Shulenberger & Barnard, 1976, Paralicella tenuipe s Chevreux, 1908, and Valettietta anacantha (Birstein & Vinogradov, 1963), which were concurrently recovered in the traps (data unpublished).

Discussion

The salient finding of this study is the paired molecular and morphological identification approaches provided congruent evidence that E. plasticus sp. nov. represents an undescribed species within Eurythenes. Further, as a scavenger at upper hadal depths (6010 – 6949 m) in the Mariana Trench, E. plasticus sp. nov. is not exempt from ingesting microplastics that are bioavailable within the hadal zone.

In comparison to described Eurythenes species, E. plasticus sp. nov. was placed as part of the gryllus -complex and most closely related to the abyssal E. magellanicus (Fig. 2). The bPTP analysis of COI and both K2P analyses delineated E. plasticus sp. nov. to be a distinctive lineage, and these methods aligned with previous studies that detected cryptic speciation within the gryllus -complex (Havermans et al. 2013; Eustace et al. 2016; Narahara-Nakano et al. 2017). The 16S phylogeny specifically showed E. plasticus sp. nov. to be nearly identical to Eg7 (Fig. 2A; France & Kocher 1996; Havermans et al. 2013). This Eurythenes sp. was a singleton recovered from abyssal depths at the Horizon Guyot seamount, Pacific Ocean, and it was collected along with another Eurythenes sp. from the divergent Eg9 clade (Havermans et al. 2013). Confidence in the identification of Eg7 would be further strengthened with additional genetic or morphological data.

The morphological variation seen in E. plasticus sp. nov., such as an uneven coxa 5 lobe and lack of a tooth on the posteroventral corner of epimeron 3, supported the phylogenetic evidence as an undescribed lineage. Consistent with previous studies, these morphological characteristics should be used with caution, as some are difficult to discern objectively. Additional specimens, like from the Eg7 clade, may reveal phenotypic plasticity in the characteristics observed in this morphological study (d’Udekem d’Acoz & Havermans 2015). Continued application of a combined molecular and morphological approaches in future studies is likely to reveal further species diversity within the gryllus -complex.

The discovery of E. plasticus sp. nov. continues to align with the pattern Eurythenes that the geographic and bathymetric species distributions are complex (Havermans 2016). With the Eg7 singleton, the geographic range of E. plasticus sp. nov. thus far appears to be restricted to the Central Pacific Ocean. Across that ocean basin, E. plasticus sp. nov. has broad bathymetric range, ~ 3000 m. While it is common among Eurythenes to be found only in a single ocean basin and have a wide vertical distribution (Eustace et al. 2016; Havermans 2016), it is less common to span across the abyssal and hadal zones. Although, this is not unique, as it has been documented in other amphipods, such as A. gigantea (Jamieson et al. 2013). A species needs to be able to cope at the cellular, reproductive, and physiological levels in both the stable abyssal (Smith et al. 2008) and the dynamic hadal environments (Jamieson 2015; Downing et al. 2018). Yet, it was curious that during the present study, E. plasticus sp. nov. was only collected from upper hadal depths, despite amphipods being captured at shallower and deeper depths (43 additional deployments 4506 to 10545 m; data unpublished). This highlights that the distribution of E. plasticus sp. nov. is a patchwork. Further work and sampling will be required to understand the conditions that support the presence of this species.

The finding of a microplastic fibre in the hindgut of a juvenile was not unexpected. Deep-sea scavenging amphipods, as an adaption to their food limited environment, indiscriminately consume carrion (Blankenship & Levin 2007) and are known to inadvertently ingest microfibres present in the carrion and sediment (Jamieson et al. 2019). The detection of a microplastic adds to the number of hadal scavenging amphipods, including adult specimens of H. gigas from the Mariana Trench and Eurythenes sp. ‘hadal’ the Peru-Chile Trench (Jamieson et al. 2019), which have been found to have consumed plastic microfibers. Microplastic consumption by a juvenile indicates that scavenging amphipods are potentially ingesting microplastics throughout their life, which could pose acute and chronic health effects. While the ecotoxicological impacts of microplastic exposure has yet to be investigated on deep-sea amphipods, early work on other Malacostraca indicates that the ingestion of polypropylene fibres by the sand crab, Emerita analoga, increases adult mortality and decreases in retention of egg clutches (Horn et al. 2019).

This study adds to the growing body of literature on marine organisms ingesting plastic and microfibers (Besseling et al. 2015; Lusher et al. 2015; Bellas et al. 2016; Alomar & Deudero 2017). The microplastic found in the hindgut of E. plasticus sp. nov. was most similar to PET, which is one of the top five most prevalent synthetic plastic polymers produced and discarded globally (Geyer et al. 2017). Without substantial global changes to the life cycle of plastic, from reducing the rate of plastic production to improving waste management (Forrest et al. 2019), plastics and microfibres will continue to be transported to the deep sea and be ubiquitous in the hadal food chain for the foreseeable future.