This key is expanded from d’ Udekem d’ Acoz and Havermans (2015), and the caution of use remains. Character differences can be tough to objectively discern, and certain characteristics can be phenotypically variable between cohorts. Visual identification paired with DNA barcoding is strongly recommended.
1. Dactylus of pereopods 3–7 short (less than 0.3 of propodus)....................................................................................................2 Dactylus of pereopods 3–7 long (more than 0.6 of propodus).......................................... Eurythenes obesus (Chevreux, 1905)
2. The palm of gnathopod 2 minutely chelate or very protruding.................................................................................................3 The palm of gnathopod 2 subchelate or weakly protruding......................................................................................................4
3. The anterodorsal margin of the head forming an upturned ridge; posterodistal lobe of the basis of pereopod 7 very long...................................................................................................................... Eurythenes thurstoni Stoddart & Lowry, 2004 The anterodorsal margin of the head not forming an upturned ridge; palm of gnathopod 1 very short; posterodistal lobe of the basis of pereopod 7 short or fairly short................................................................................. Eurythenes atacamensis sp. nov.
4. Pereopods 6–7 and epimerons 1–3 not dorsally keeled to slightly keeled; pereopods 6–7 and epimerons 1–2 dorsally not sigmoid (without anterior concavity), epimeron 3 with distinct anterior concavity..................................................................5 Pereopods 6–7 and epimerons 1–3 dorsally strongly keeled and sigmoid (anteriorly slightly to distinctly concave)................................................................................... Eurythenes sigmiferus d’ Udekem d’ Acoz & Havermans, 2015
5. Eyes of variable width; the outer plate of maxilla 1 with 8/3 crown arrangement...................................................................6 Eyes of constant width; the outer plate of maxilla 1 with 9/3 crown arrangement.............................................................. Eurythenes aequilatus Narahara-Nakano, Nakano & Tomikawa, 2018
6. Article 2 of mandibular palp moderately to strongly expanded posteriorly..............................................................................7 Article 2 of mandibular palp not to weakly expanded posteriorly............................................................................................8
7. Maxilliped with 3 non-protruding nodular spines; pereopod 7 with basis posteriorly strongly expanded, with merus narrow..................................................................................... Eurythenes andhakarae d’ Udekem d’ Acoz & Havermans, 2015 Maxilliped with 8–9 non-protruding nodular spines; pereopod 7 with basis posterior border weakly expanded, with merus stout........................................................................................... Eurythenes maldoror d’ Udekem d’ Acoz & Havermans, 2015
8. Gnathopod 2 palm convex; uropod 1 and 2 rami subequal.......................................................................................................9 Gnathopod 2 palm straight; the outer ramus of uropod 1 and 2 are shorter than paired inner ramus......................................................................................................... Eurythenes magellanicus (H. Milne Edwards, 1848)
9. Ventral corner of eye rounded and obliquely pointing backward; maxilliped with 3–4 protruding nodular spines; gnathopod 1 palm convex....................................................................................................................… Eurythenes plasticus Weston, 2020a Ventral corner of eye sharp and pointing downward; maxilliped with 3–4 non-protruding nodular spines; gnathopod 1 palm straight…........................................................................................................ Eurythenes gryllus (Lichtenstein in Mandt, 1822) undescribed clade as those presented in Ritchie et al. (2015) and Eustace et al. (2016), namely, the Eurythenes sp. ‘PCT hadal’, with high support values (16S: BI =0.62, ML = 100; COI: BI = 0.94, ML = 75). This clade was repeatedly placed more basal in the phylogenies. In the 16 S topology, only E. thurstoni was basal to E. atacamensis sp. nov. The E. atacamensis sp. nov. clade in the 16S topology had two subclades; however, this distinction was not present in the COI phylogeny. Within the E. atacamensis sp. nov. clade, there was a lack of apparent patterns based on depth or station proximity to the trench axis.