Stephonyx transversus, Sorrentino & Souza-Filho & Senna, 2018

Sorrentino, Rayane, Souza-Filho, Jesser F. & Senna, André R., 2018, A new species of Stephonyx (Amphipoda, Amphilochida, Lysianassoidea) from Brazil, Zootaxa 4388 (4), pp. 537-546 : 538-545

Stephonyx transversus sp. nov.

( Figures 1–5 )

Material examined. Holotype: adult female, 33.8 mm, Sergipe state, Brazil (12°01'49''S 37°24'61''W), 720 m depth ( MOUFPE 15324 ).

Type-locality. Sergipe state, Brazil (12°01'49''S –37°24'61''W), 720 m depth.

Etymology. Specific epithet corresponding to the transverse angle formed by the anterior margin and the palm of the propodus of gnathopod 2.

Diagnosis. Gnathopod 2, palm transverse. Maxilla 2, inner plate rounded, with one stout plumose subterminal seta and other smaller. Epimeral plate 3, posteroventral corner without spine. Telson 80% cleft, apical margin truncate.

Description. (Based on holotype) Head ( Figure 1A ), deeper than long, anterodorsal and anteroventral corner rounded, anterior margin concave medially; eyes (faded in alcohol), S inverted-shaped; ommatidia are not well defined. Antenna 1, almost half the size of antenna 2, article 1 twice the size of the article 2, dorsal margin slightly concave, article 3 short, half of the size of the second article; primary flagellum 26-articulate, article 1 with two well-developed fields of callynophores; accessory flagellum with two articles, the first is ¾ of the article 1 of primary flagellum, article 2 is ¼ of the article 1 of primary flagellum. Antenna 2, almost reaches the half of the body; flagellum 53-articulate.

Mouthpart: subquadrate bundle. Epistome and upper lip ( Figure 2D ) subtly separated; upper lip, anteroventral margin forming a setose acute angle. Mandible ( Figure 2B and E ) incisor smooth; lacinia mobilis simple, robust in both, 7 accessories setae; molar setose not triturative; palp 3-articulate, article 1, short, 1/3 of second article, article 2 and article 3, anterior margin with long and simple setae (article 2 up to half the margin with setae). Maxilla 1 ( Figure 3C ) inner plate rounded, small, about half the size of the outer plate; one stout subterminal seta and five smaller plumose setae,; outer plate, broad; setal-teeth with 7/4 crown arrangement: ST1, subconical, 3-cuspidate, ST2, subquadrate, short, half of the size of ST1, with two cusps, ST3 2-cuspidate, ST4 3-cuspidate, ST5-7 multicuspidate; STA at STD stout and multicuspidate; palp 2-articulate, article 1, subquadrate, about ¼ of the size of article 2, article 2, dorsal margin lightly geniculate; apical margin with 10 stout setae. Maxilla 2 ( Figure 2F ) plates slender; inner plate with thin setae proximally, lateral margin with long simple setae and setae half pectinate and half plumose; outer plate, apical margin with the same setae. Maxilliped ( Figure 2A ), inner plate narrow, half of the width of outer plate, lateral margin with plumose setae, apical margin with two robust and small setae; outer plate, rounded, 1.3 × longer than inner plate, lateral and apical margin with simple setae; palp 5-articulate, first article small, naked, article 2, margin elongated laterally, with simple setae; article 3, 1.5 × longer than article 2, laterals margins setose; article 4 with setae in laterals margins and apical margin; dactylus, surface with small setae, with nail.

Gnathopod 1 ( Figure 3A ) coxa reduced, anterior margin rounded, dorsal and posterior margins straight, ventral margin truncated; basis, tapering distally, 4.5 × longer than wide, anterior margin with setae, posteroventral margin with three setae; ischium elongated, 0.6 × the basis size, anterior and posterior margins with thin setae; merus short, half of the carpus width, posterior margin with a tuft of setae; carpus and propodus subequal in length, both have tufts of setae on posterior margin; propodus, anterior margin with a tuft of long setae, palm serrated, apical margin with a tuft of setae and two subapical stout setae; dactylus about 2 × longer than wide, inner margin with two cusps and one robust setae in apical margin. Gnathopod 2 ( Figure 3B ) coxa about twice longer than coxa 1, with facial setae; basis, approximately 5 × longer than wide, anterior margin with thin setae; ischium, anterior margin convex, with thin setae, posteroventral corner with a tuft of setae; merus small, about half of the length of carpus; carpus anterior margin with some plumose setae, posterior margin covered by plumose setae; propodus subrectangular, antero and posteroventral margins rounded, both covered by plumose setae, palm transverse, convex, proximally with fine setae and distally microserrated, bearing nine submarginal robust setae; dactylus chelate, curved posteriorly, anterior margin with one setae, posterior margin reaches all palm.

Pereopod 3 ( Figure 4A ) coxa, 2.6 × longer than wide, ventral margin with setae; basis 3.5 × longer than wide, anterior, posterior and posteroventral margin with setae, ischium 4 × smaller than basis, posterior margin setose; merus anteroventral corner expanded, with a tuft of setae, posterior margin with carpus 0.6 × the merus length, anteroventral corner with a tuft of setae, posterior margin setose; propodus with three setae in anterior margin, posterior margin with simple setae and a robust setae in posteroventral corner; dactylus, posterior margin with cusps proximally and concave medially. Pereopod 4 ( Figure 4B ) coxa expanded posteriorly, basis anterior and posterior with two tufts of setae, anteroventral and posteroventral corner with a tuft of setae; ischium small, 0.3 × the length of basis; merus anterior margin slightly convex proximally, posterior margin setose; carpus, about half of the length of the merus, anteroventral corner with a tuft of setae; posterior margin setose; propodus with some setae in anterior margin and posterior margin, posteroventral corner with a robust setae; dactylus with cusps and concave medially, with nail. Pereopod 5 ( Figure 4C ) coxa rounded; basis posteriorly expanded, posterior margin serrated, anterior margin with tufts of setae, posterior margin with one seta, posteroventral corner with tuft of setae; ischium anteroventral corner with tuft of setae; merus anterior margin with tufts of setae, posterior margin expanded, with tufts of setae, carpus, anterior margin with tufts of setae, posteroventral corner with one seta; propodus, anterior margin setose, posterior margin with three tufts of setae; dactylus anterior margin with cusps, with nail. Pereopod 6 ( Figure 4D ) coxa, anterior margin concave, posterior margin convex; basis expanded posteriorly, posterior margin serrated, anterior margin slightly concave, with setae, posterior margin with setae, ischium short, anterior margin with setae, merus, anterior margin with tufts of setae, posterior margin expanded with small setae; carpus with two tufts of setae in anterior margin, one on posteroventral corner; propodus anterior margin setose and posterior margin with three tufts of setae; dactylus with nail. Pereopod 7 ( Figure 5A ) coxa, anterior margin slightly concave, posterior margin convex; basis posterior expanded posteroventrally, margin convex, anterior margin concave medially, setose; ischium subquadrate, anterior margin with setae; merus, anterior and posterior margins with tufts of setae, posterior margin expanded; carpus anterior margin with tufts of setae, anteroventral and posteroventral corner with tuft of setae; propodus subrectangular, anterior margin with tufts of setae, posterior margin with three, posteroventral corner with a small plumose seta; dactylus with nail.

Epimeral plates 1 and 2 with triangular ventral process. Epimeral plate 1 posterior margin strongly convex, ventral margin slightly concave. Epimeral plate 2, posterior margin sinuous, posteroventral corner acute, ventral margin with row of setae. Epimeral plate 3, posterior margin slightly convex, posteroventral corner without spine ( Figure 4E ). Uropod 1 ( Figure 5B ) slightly shorter than uropod 2, peduncle and rami subequal in length, dorsal margins of peduncle and rami with robust setae. Uropod 2 ( Figure 5C ) peduncle and rami subequal in length, ventral margins of peduncle and rami with robust setae. Uropod 3 ( Figure 5D ) peduncle 1.7 × longer than wide, subconical, posterior margin concave proximally the dorsal and ventral margins, forming an acute rounded angle; rami subequal in length and 1.4 × longer than peduncle, inner ramus, longer than first article of outer ramus, dorsal margin with plumose setae, ventral margin with three subterminal setae, outer ramus 2-articulate, dorsal margin with plumose setae, ventral margin with three setae. Telson ( Figure 5E ) 80% cleft, apical margin truncate with cusp at the innerdistal corner, each lobe with one facial and two subterminal small stout setae.

Remarks. According to current papers ( Diffenthal & Horton 2007, Narahara et al. 2012, Lowry & Kilgallen 2014) Stephonyx biscayensis is a widely distributed species and has some divergences in intraspecific level. Some works ( Stephensen 1923, Schellenberg 1926, Chevreux 1927, Chevreux 1935, Griffiths 1975, K.H. Barnard 1940) do not have illustrations or have small and simple descriptions which causes great confusion.

With respect to the descriptions of S. biscayensis , in general, K.H. Barnard (1916) described a species from Cape Point ( South Africa) with a character divergent from than all other species: maxilla 1, outer plate with 8 setal teeth. All other species have 11 setal teeth on the outer plate. Comparing this species with the new species, the African species has a mandible with less setae on the borders of the molar; maxilla 1, palp with 5 robust setae, and maxilliped, inner plate with 3 robust apical setae, while the Brazilian species has a molar with slender setae; 10 setae in apex of palp of maxilla 1 and maxilliped with inner plate with 2 robust setae.

The material from off Kenya, Barnard, J.L (1961) described some differences between this specimen and that described by Chevreux (1908): “mandibular molar is more pointed and hairy, and lacks a triturating surface; the outer plates of the maxilliped have apical spines; the telson has an apical notch on each lobe; the third palp article of the mandible is slightly stouter; and the inner plate of maxilla 2 is slightly stouter”. The new species has a different number of articles in the antennae from the original description of S. biscayensis , with 26 articles in primary flagellum of antenna 1 and 53 in flagellum of antenna 2, and 19 articles in primary flagellum of antenna 1 and 33 articles in flagellum of antenna 2, respectively.

The setose molar of mandible also differs from specimen described by Ledoyer in 1986, in which the molar is less setose and conical. Nevertheless, the other characters are similar between the Stephonyx transversus sp. nov. and S. biscayensis sensu Ledoyer (1986) , including the transverse palmar angle of gnathopod 2 and the presence of robust setae in distal margin of propodus of pereopod 4 and the cusps in posterior margin of dactylus.

Comparing the illustration of the Caribbean specimen ( Lowry & Stoddart 1997), the new species has the head with anteroventral corner less convex, similar to head of that described by Narahara et al. (2012). These authors cite the presence of a rounded posteroventral corner of epimeral plate 2, not cited by any other work. This character is not shared by the Brazilian species.

Stephonyx transversus sp. nov. differs from the species described by Lowry & Stoddart (1989), from southwestern Ireland, the paper in which the genus Stephonyx is described, in having the following combination of characters ( S. biscayensis sensu Lowry & Stoddart, 1989 in parentheses): primary flagellum of antenna 1 with 26 articles (16 articles); flagellum of antenna 2 with 53 articles (31 articles); mandible molar with slender setae (molar with small and tiny setae); maxilla 1 inner plate large, one stout subterminal seta and five small setae, all plumose (inner plate narrow with three apically plumose setae); gnathopod 2 palm transverse, convex, margin microserrated (slightly acute, sinuous and smooth); pereopod 6 basis anterior margin with a convex prominence (almost straight); pereopod 7 coxa, anterior margin slightly concave (almost straight); epimeral plate 3, posterior margin without spine (weakly pointed); uropod 3 rami subequal in length (inner ramus reach half of the article 2 of outer ramus); telson cleft 80% (65% in the text – but according to figure 3 on page 8 the telson is cleft about 90% of its length). According to Lowry & Stoddart (1997), the description of the head in Lowry & Stoddart (1989) is incorrect, “the lobe is much broader and rounder and tends to curl in toward the body at the lower margin”, which is a character shared with the Stephonyx transversus sp. nov.

The new Brazilian species differs from S. uncinatus Senna & Serejo, 2007 , by the presence of the following characters states: anteroventral margin of upper lip forming a setose acute angle; mandible with a robust lacinia mobilis; gnathopod 1 propodus, palm with two subapical stout setae, dactylus, inner margin with two cusps; pereopods 3 and 4 dactyli with cusps on posterior margin; epimeral plate 3 ventral margin naked, posteroventral corner without spine; and telson, apical margin truncate with cusp, bearing two small stout setae. On the other hand, S. uncinatus has: the ventral margin of upper lip rounded; the mandible with the lacinia mobilis reduced and slightly setose; gnathopod 1, propodus, palm with three subapical curved stout setae, dactylus, inner margin without cusps; dactylus of pereopods 3 and 4, posterior margin smooth; epimeral plate 3, ventral margin anteriorly setose, posteroventral corner produced in a small subacute spine; and telson, apical margin rounded and smooth.

Because of the similarity among specimens, the authors cited above have described them as S. biscayensis . However, we have pointed out that there are differences between the descriptions, and revisions must be made to confirm that all specimens described since Lowry & Stoddart (1989) could be classified as S. biscayensis .

Figures

FIGURE 1. Stephonyx transversus sp. nov. Holotype, female, 33.8 mm. MOUFPE 15,324. 12°01'49''S 37°24'61''W, 720 m depth, Sergipe State, Brazil. Scale bar: 5 mm.

FIGURE 2. Stephonyx transversus sp. nov. Holotype, female, 33.8 mm. MOUFPE 15,324. 12°01’49’’S 37°24’61’’W, 720 m depth, Sergipe State, Brazil. A: maxilliped, B: mandible, C: lower lip, D: epistome, E: right mandible, F: maxilla 2. Scale bars: 0.5 mm for D, 1.0 mm for remains.

FIGURE 3. Stephonyx transversus sp. nov. Holotype, female, 33.8 mm. MOUFPE 15,324. 12°01’49’’S 37°24’61’’W, 720 m depth, Sergipe State, Brazil. A–B: gnathopods 1–2, C: maxilla 1. Scale bars: 2.0 mm for B, 1.0 mm for remains.

FIGURE 4. Stephonyx transversus sp. nov. Holotype, female, 33.8 mm. MOUFPE 15,324. 12°01’49’’S 37°24’61’’W, 720 m depth, Sergipe State, Brazil. A–D: pereopods 3–6, E: epimeral plates 1–3. Scale bars: 2.0 mm for B, 1.0 mm for remains.

FIGURE 5. Stephonyx transversus sp. nov. Holotype, female, 33.8 mm. MOUFPE 15,324. 12°01’49’’S 37°24’61’’W, 720 m depth, Sergipe State, Brazil. A: pereopod 7, B–D: uropods 1–3, E: telson. Scale bars: 1.0 mm.