LYSIANASSOIDEA Dana, 1849

Winfield, Ignacio, Hendrickx, Michel E. & Ortiz, Manuel, 2017, Stephonyx californiensis sp. nov. (Amphipoda: Lysianassoidea: Uristidae), a new bathyal scavenger species from the Central Gulf of California, Mexico, and comments on the bathymetric and geographic distribution of the Stephonyx species group, Journal of Natural History (J. Nat. Hist.) 51 (47 - 48), pp. 2793-2807 : 2795-2803

Superfamily LYSIANASSOIDEA Dana

Family URISTIDAE Hurley

Genus Stephonyx Lowry and Stoddart

Stephonyx californiensis sp. nov.

( Figures 1–7 )

Material examined

Holotype. Adult male (used for drawings), 22.3 mm total length, Gulf of California (24°16 ʹ N, 108°24 ʹ W), Mexico, 2 April 2002, 1150 m, baited trap (ICML-EMU-11,794).

Type locality

Gulf of California (24°16 ʹ N, 108°24 ʹ W), Mexico ( Figure 1 ) .

Diagnosis

Body smooth; eyes absent; lateral cephalic lobes medially developed, acute; antennae subequal in length; gnathopod 1 chelate, dactylus simple with three distal stout setae, inner margin sinuous with minute setae; gnathopod 2 subchelate, carpus with ventral margin crenulate, propodus subovate, palm deeply excavate, dactylus slightly shorter than palm; inner plate of maxilliped lanceolate, large, with seven marginal, nodular robust setae, distally; lobes of telson each with two robust, dorsal setae, distal margin truncated, with one penicillate, two simple setae, and two short robust setae.

Description. Holotype, adult male (ICML-EMU-11,794), body length 22.3 mm.

Body ( Figure 2 ). Body smooth, not carinate. Head ( Figures 2 and 3 (c)) deeper than long; lateral cephalic lobe middle developed, acute; rostrum and eyes absent. Epimeral plates 1–3 ( Figure 2 ) smooth; plate 1 antero- and posteroventral corners rounded; plates 2–3 anteroventral corners convex, posteroventral corners pointed.

Antennae ( Figure 3 ). Antenna 1 ( Figure 3 (a)) 0.33× body length, slightly shorter than antenna 2; article 1 of peduncle longest, length approximately 2× length of articles 2 and 3 combined, articles 2–3 shorter than 1; primary flagellum 24-articulate, article 1 longest, callynophore well developed, 2-field, calceoli absent; accessory flagellum 8- articulate, article 1 longest. Antenna 2 ( Figure 3 (b)) peduncular articles 4 and 5 longest, not swollen; flagellum 35-articulate, calceoli absent.

Gnathopods ( Figure 4 ). Gnathopod 1 ( Figure 4 (a)) chelate, coxa reduced, subtriangular; basis large and wide; merus short; ischium linear, subequal in length to ischium; propodus linear ( Figure 4 (b)), elongate, two brushes of facial medial setae; palmar margin convex, anterodistal corner with a robust seta; dactylus well developed, inner margin with minute robust and simple setae, a row of minute submarginal robust setae, several facial simple and robust setae, unguis absent, three large teeth distally. Gnathopod 2 ( Figure 4 (c)) subchelate, coxa large, subrectangular, slightly shorter than coxa 2, basis elongate; merus with numerous minute, simple setae marginally, larger submarginal setae; carpus subequal in length to ischium, ventral margin crenulate, numerous marginal and facial simple setae; propodus subovate ( Figure 4 (d)), short, about 0.5× length of carpus, numerous facial and submarginal large setae, posterodistal corner with five short and one large stout seta, palm deeply excavate anteriorly; dactylus elongate, a plumose seta and three short cuticular robust setae.

Mouthparts ( Figure 5 , 6 ). Mouthpart bundle subquadrate. Maxilla 1 ( Figure 5 (a)) inner plate short, about 0.33× outer plate length, four apical plumose setae; outer plate wide,

with numerous marginal and facial simple setae, several subdistal and distal minute robust setae, 11 spine teeth in a 7/4 crown arrangement ( Figure 5 (b)): ST1 stout, 3-denticulate; ST2 stout, 4-denticulate; ST3 slender, 4-denticulate; ST4 slender, 5-denticulate; ST5 stout, 6-denticulate; ST6 slender, 8-denticulate; ST7 slender, 7-denticulate; STA stout, 3-denticulate; STB slender, 5-denticulate; STC slender, 7-denticulate; STD stout, 7-denticulate; palp large, 2-articulate, six terminal short stout setae, right with an additional flag seta and a row of subdistal simple setae ( Figure 5 (a)). Mandible ( Figure 5 (c)) incisor well developed, smooth; left lacinia mobilis a smooth peg, 5-denticulate (right lacinia mobilis reduced, 2- denticulate); left accessory spine row with four small robust setae (right with three small robust setae); molar strong, setose, distal half triturative; mandibular palp 3-articulate, attached midway and upper to molar, article 1 short, article 2 elongate, slender, with 24 short and 11 large A2 setae, and 23 D2 setae, article 3 falcate, a large A3 seta, two rows of short facial and large submarginal B3 setae, and five large-plumose E3 setae. Maxilla 2 ( Figure 6 (d)) inner plate narrow, slightly shorter than outer, margin setose, medial margin with at least 17 plumose setae, three larger subdistal, plumose setae; outer plate narrow distally, with numerous marginal and submarginal distal plumose setae. Epistome and upper lip ( Figure 5 (e)) separated; epistome short, slightly bilobate; upper lip acute, deeper than epistoma. Lower lip ( Figure 5 (f)) inner and outer lobes well developed, both with numerous marginal and submarginal simple setae, mandibular processes lanceolate. Maxilliped ( Figure 6 (a)) inner plate large, lanceolate, six nodular spines on inner corner, another near outer corner, oblique setal row with 14 marginal plumose setae, five distal plumose setae ( Figure 6 (b)); outer plate large, subovate, 17 small medial facial, stout setae, 10 submarginal plumose setae distally, 10 distal robust setae, six pappose apical setae, other 26 pappose marginal setae, and nine large proximal facial setae; palp large, article 2 largest, unguis and terminal setae present.

Uropods ( Figure 6 ). Uropod 1 ( Figure 6 (f)) peduncle with dorsal marginal setae, rami subequal in length, inner ramus with minute marginal, simple setae, outer ramus smooth; uropod 2 ( Figure 6 (e)) with dorsal marginal setae, inner ramus 1.3× length of outer ramus; uropod 3 ( Figure 6 (d)) peduncle 0.6× length of rami, rami subequal in length, lanceolate, both dorsal margin setose, outer ramus 2-articulate. Telson ( Figure 6 (c)) deeply cleft, rami conical, each ramus with two robust dorsal setae, distal margin truncated, with one penicillate seta, two robust setae and two simple setae.

Pereopods ( Figure 7 ). Pereopods dactyli short, stocky. Pereopod 3 ( Figure 7 (a)) simple, coxa large, subtriangular, posterodorsal corner with minute robust setae. Pereopod 4 ( Figure 7 (b)) simple, coxa large, posteroventral lobe well developed. Pereopod 5 ( Figure 7 (c)) coxa bilobate, wider than deep, posterior lobe slightly produced ventrally, basis expanded, posterior margin rounded, with five minute robust setae. Pereopod 6 ( Figure 7 (d)) coxa shorter than basis, basis with posterior margin convex, merus expanded posteriorly. Pereopod 7 ( Figure 7 (e)) coxa short, about 0.5× basis, ventral margin convex, basis posterior margin partially convex, merus expanded posteriorly.

Habitat

At 1150 m depth. Environmental parameters were not measured during the PANDAL survey. However, repetitive sampling in 2000 ‒ 2001 in the same locality at similar depths indicates that the trap was deployed on muddy bottom (98 ‒ 99% mud), with % Corg 4.1 ‒ 6.8, and with dissolved oxygen concentrations close to bottom between 0.43 and 0.73 mL/L. Stephonyx californiensis sp. nov. is therefore associated with a depth fringe located below the lower boundary of the oxygen minimum zone that characterises the Gulf of California’ s central and southern portions ( Hendrickx and Serrano 2010).

Etymology

The new species is named after the Gulf of California where it was collected.

Distribution

Stephonyx californiensis sp. nov. is only known from the type locality.

Remarks

The material examined belongs to the genus Stephonyx and is distinguished from other genera of Uristidae for its chelate first gnathopods, a significantly reduced coxa in gnathopod 1, and fully developed 7/4 crown of setal teeth on the outer plate of maxilla 1 ( Lowry and Kilgallen 2014). Stephonyx californiensis sp. nov. is morphologically close to S. arabiensis , S talismani , S. laqueus and S. perexcavatus with which it shares the following characters: body smooth, not carinate dorsally; carpus of the gnathopod 1 more than 0.5× length of propodus; carpus of the gnathopod 2 not elongate, 2× length of propodus; article 1 of the mandibular palp short, smaller than 0.5× length of article 2; gnathopod 2 palm excavate. Stephonyx californiensis sp. nov., however, can be easily distinguished from these four species based on the following characters ( Table 1): (1) eyes absent in S. californiensis sp. nov. vs eyes present (either poorly or well developed) in the other four species; (2) antennae 1 and 2 subequal in length in S. californiensis sp. nov. vs antenna 1 shorter than 2; (3) dactylus of gnathopod 1 simple, with three distal robust setae in S. californiensis sp. nov. vs dactylus either distally bifid or simple without robust teeth; (4) carpus of gnathopod 2 ventral margin crenulate in S. californiensis sp. nov. vs ventral margin smooth; (5) inner plate of maxilliped subtriangular, with seven marginal, nodular robust setae in S. californiensis sp. nov. vs subrectangular or subtriangular with only three marginal, nodular robust setae.

Figures

Figure 1. Map of western Mexico, East Pacific, showing the sampling locality.

Figure 2. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): habitus. Setae on antennae and uropods not included. Scale bar = 4.0 mm.

Figure 3. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): (a) antenna 1; (b) antenna 2; (c) ventral view of head. Scale bar: a = 0.5 mm; b = 0.7 mm.

Figure 4. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): (a) gnathopod 1; (b) gnathopod 1 propodus and dactylus (tip enlarged); (c) gnathopod 2; (d) gnathopod 2 propodus (tip enlarged) and dactylus. Scale bar = 1.8 mm.

Figure 5. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): (a) left maxilla 1; (b) spine teeth of maxilla 1 outer plate (enlarged), a distal margin of right palp mandibular (enlarged); (c) left mandible; (d) maxilla 2; (e) upper lip; (f) lower lip. Scale bar: a, c = 0.9 mm; d = 0.7 mm; e = 0.2 mm; f = 1.2 mm.

Figure 6. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): (a) maxilliped; (b) inner plate of same (tip without setae); (c) telson (tip enlarged); (d) uropod 3; (e) uropod 2; (f) uropod 1. Scale bar: a = 0.8 mm; c = 1.2 mm; d = 1.6 mm; e, f = 2.0 mm.

Figure 7. Stephonyx californiensis sp. nov., holotype male (22.3 mm length): (a) pereopod 3; (b) pereopod 4; (c) pereopod 5; (d) pereopod 6; (e) pereopod 7. Scale bar: a = 2.4 mm; b, c = 3.1 mm; d, e = 3.2 mm.