Eusirus bonnieri Othaitz & Sorbe 2020, sp. nov.

Eusirus bonnieri sp. nov.

(Figs 2–8)

Material examined. Holotype. 1 male, 7.5 mm BL, MNHN-IU-2013-5702. Capbreton Canyon. RV Côtes de la Manche, OXYBENT 9 cruise, 23 June 1999, Roscoff suprabenthic sledge, sample code: OB9-TS09, 43º36.05’N, 1º46.97’W, 786– 785 m, muddy bottom.

Allotype. 1 mature female (telson lost), ca. 7.0 mm BL, MNHN-IU-2013-5703. Data as for the holotype.

Paratypes. 1 male, 5.9 mm BL, MNHN-IU-2013-5711; 1 mature female, MNHN-IU-2013-5712; 1 male, 6.2 mm BL, MNHN-IU-2013-5713 (posterior part of body); 1 male, 6.3 mm BL, MNHN-IU-2013-5714 (posterior part of body); 2 mature females, MNHN-IU-2013-5715; 1 immature female, MNHN-IU-2013-5716; 5 mature females, MNHN-IU-2013-5717; 17 males, MNHN-IU-2013-5718; 1 mature female, 7.3 mm BL, MNHN-IU-2013-5719 (posterior part of body); 5 mature females, MNHN-IU-2013-5720; 5 males, MNHN-IU-2013-5721; 1 immature female, MNHN-IU-2013-5722. Data as for the holotype.

1 male, 6.5 mm BL, MNHN-IU-2013-5706 (posterior part of body). Capbreton Canyon. RV Côtes de la Manche, SEDICAN 1 cruise, 25 June 2001, Roscoff suprabenthic sledge, sample code SC1-TS26, 43º35.96’N, 1º47.85’W, 761 m, muddy bottom.

Additional material. 1 mature female, MNHN-IU-2013-5707 (body fragments); 1 male, MNHN-IU-2013- 5708 (body fragments); 1 mature female, 6.5 mm BL, MNHN-IU-2013-5709 (body fragments); 1 mature female, 6.5 mm BL, MNHN-IU-2013-5710 (body fragments). Data as for the holotype.

1 male, MNHN-IU-2013-5704 (body fragments); 1 male, 7.0 mm BL, MNHN-IU-2013-5705 (body fragments). Data as for specimen MNHN-IU-2013-5706.

1 mature female, MNCN-20.04/12001. Capbreton Canyon. RV Côtes de la Manche, OXYBENT 6 cruise, 7 December 1998, Roscoff suprabenthic sledge, sample code OB6-TS09, 43º35.79’N, 1º47.33’W, 795-797 m, muddy bottom.

2 mature females, MNCN-20.04/12002. Capbreton Canyon. RV Côtes de la Manche, OXYBENT 7 cruise, 24 January 1999, Roscoff suprabenthic sledge, sample code OB7-TS09, 43º39.53’N, 1º39.01’W, 370-379 m, muddy bottom.

1 mature female, MNCN-20.04/12003. Capbreton Canyon. RV Côtes de la Manche, OXYBENT 10 cruise, 2 May 2000, Roscoff suprabenthic sledge, sample code OB10-TS19, 43º37.78’N, 1º42.95’W, 645- 642 m, muddy bottom.

3 males, MNCN-20.04/12004; 1 mature female, MNCN-20.04/12005. Capbreton Canyon. RV Côtes de la Manche, SEDICAN 1 cruise, 25 June 2001, Roscoff suprabenthic sledge, sample code SC1-TS26, 43º35.96’N, 1º47.85’W, 761 m, muddy bottom.

2 juveniles, 1 male, 1 indeterminate, MNCN-20.04/12006; 1 immature female, 2 mature females, MNCN- 20.04/12007; 13 juveniles, MNCN-20.04/12008. Capbreton Canyon. RV Côtes de la Manche, SEDICAN 1 cruise, 25 June 2001, Roscoff suprabenthic sledge, sample code SC1-TS30, 43º36.37’N, 1º45.41’W, 780 m, muddy bottom.

Etymology. The species is named after Jules Bonnier (1859–1908; Station Zoologique de Wimereux) in recognition of his pioneering work on bathyal peracarids from the Bay of Biscay. His study was published in 1896, only one year after the ‘Caudan’ campaign. Among many new bathyal species belonging to amphipods, cumaceans, isopods and tanaids (Bonnier, 1896), he also described Eusirus biscayensis from one female sampled on muddy bottoms off Arcachon Bay (940 m depth).

Diagnosis. Rostrum highly deflexed, nearly extending to half of article 1 of antenna 1 peduncle. Eyes lacking. Pereonite 7 with a weak dorsal longitudinal carina, ending with a small tooth. Accessory flagellum of antenna 1 slightly longer than 4 proximal articles of main flagellum. Left and right mandibles dissymmetrical: left lacinia mobilis unilamellate, right lacinia mobilis bilamellate, in both males and females. Gnathopod 2 propodus broad and rounded. Merus of pereopods 5–7 with posterodistal process greatly produced (≥ 27% of article maximum length). Epimeron 3 with serrate posterior margin.

Description of male. Body (Fig. 2). Pereonite 7 dorsally with a weak longitudinal carina, ending with a small tooth. Pleonites 1–3 dorsally with a weak longitudinal carina, ending with a small tooth located in a notch of the posterior margin of each somite. One short seta below each tooth. Epimeron 1 smaller than subsequent ones, ventrally rounded. Epimeron 2 roundly produced anterodistally and toothed posterodistally. Epimeron 3 with distal margin convex, posterodistal corner rounded and hind margin serrated.

Head (Fig. 2) about as long as pereonites 1 and 2 combined. Rostrum highly deflexed (90º), 0.5 times length of article 1 of antenna 1 peduncle. Lateral lobes rounded. Eyes absent. Epistome unproduced.

Antenna 1 (Figs. 2, 4G) longer than antenna 2, 0,45x BL, without aesthetascs or calceoli. Peduncle articles 1–3 decreasing in length (ratios 4.0: 1.3: 1, respectively), distomesial margin of article 1 with hooked process, distolateral margin of articles 2 and 3 with acute process, article 3 shorter than first article of flagellum. Flagellum with at least 28 articles, about 3.6x as long as peduncle, 10 proximal articles with mesial margin heavily setose, the subsequent ones each with two small setae. Accessory flagellum uniarticulate and elongate, slightly longer than four proximal articles of flagellum, with 3 tufts of small setae along mesial margin and four apical ones.

Antenna 2 (Figs. 2, 4F), without aesthetascs or calceoli. Peduncle article 4 longer than article 5 (length ratio: 1.4: 1). Flagellum slightly longer than peduncle article 5, with at least 14 articles.

Upper lip (Fig. 3C) entire, shorter than broad, slightly more prominent than straight epistome and separated by incision, distal margin rounded and heavily setose.

Mandible (Figs. 3D, G, 7 A–F) incisor of left and right mandible (Md) broad and smooth. Lacinia mobilis of left Md large, unilamellate, with six blunt teeth. Lacinia mobilis of right Md smaller, bilamellate and flabellate; main lamella with one big lateral tooth followed by series of tiny teeth; accessory lamella smaller, with many tiny teeth on distal margin and smooth inner face. Setal row of left Md with four stout serrulate setae and three slender pappose setae arranged in two parallel lines; setal row of right Md with three stout serrulate setae and three slender pappose setae, arranged in two parallel lines. Palp 3-articulate; article 1 short and bare, article 2 0.9x article 3, distally constricted, with three simple setae along half distal margin; article 3 straight with simple setae all along ventral margin and three long setae on its distal part. Molar of left and right mandibles similar in shape, triturative; grinding surface with marginal ring of long blades, apically multipointed.

Lower lip (Fig. 3H) inner lobes small, outer lobes gaping and with small apical and lateral setae, mandibular lobes weakly produced and apically rounded.

Maxilla 1 (Fig. 3F) inner plate relatively small, apically rounded and naked. Outer plate with eight distal denticulate robust setae. Palp 2-articulate, reaching well beyond apex of outer plate, basal article 0.6x distal article, article 2 only apically setose.

Maxilla 2 (Fig. 3B) plates subequal in length, apically rounded. Inner plate broader than outer plate, with mesial and distal setules, and mesial short setae. Outer plate distally and subdistally fringed with stiff long setae.

Maxilliped (Fig. 3A) inner plate short, reaching about half length of palp article 1 (medially measured), distally truncate, apically with four cuspidate setae and one simple seta, mesial margin naked. Outer plate oblong, extending about one-third length of palp article 2, bearing two distinct rows of setae along mesial margin, (thick marginal setae and longer, slender sub-marginal setae), distal margin with two long pappose setae, lateral margin naked. Palp robust, 4-articulate; articles 1 and 2 distally broadened, article 1 acutely projecting distolaterally, with one distal seta; article 2 longest, sparsely setose with two long setae on distolateral process; article 3 laterally and distally setose; article 4 shorter than article 3, with short setae and short distal nail.

Coxal plates (Figs. 2, 4 A–E, 5A–B) coxa 1 wider than deep, projecting anterodistally, distally rounded, posterior margin concave. Coxae 2–3 deeper than wide, distally rounded, posterior margin with midward stout seta. Coxa 4 deeper than wide, slightly convex distally, posteriorly excavate. Coxae 5 and 6 bilobed, posterior lobe longer than anterior one in coxa 5, lobes subequal in coxa 6. Coxa 7 small, unilobed.

Gnathopod 1 (Figs. 2, 4A, 6A) subchelate. Basis straight and channelled anteriorly, proximally narrowed, with 9 simple setae along anterior margin. Ischium with 1 small posterodistal seta. Merus slightly longer than ischium, posterior margin with 3 short setae, posterodistal corner quadrate with 4 simple setae. Carpus 0.8x basis length, posterior lobe not exceeding merus, with 6 distal simple long setae, posterior margin channelled between lobe and apex. Propodus longest axis 0.9x carpus length, length / width ratio = 1.6; palm convex, defined by a small hump and two robust sensory setae, mesially with one row of small stout setae, laterally with one row of longer curved setae, with two pappose setae near dactylus hinge. Dactylus falcate, fitting up with palmar hump.

Gnathopod 2 (Figs. 2, 4B, 6B) similar to gnathopod 1, articles slightly longer. Basis with 5 simple seta along anterior margin. Posterior lobe of carpus with a tuft of 10 distal simple long setae. Propodus length / width ratio = 2.0; palm defined by small hump carrying 3 robust sensory setae with decreasing size.

Pereopods 3–4 (Figs. 2, 5A, B) slender, both with similar morphology each to other. Bases subequal in length, longer than merus and carpus combined, fringed with small setae along posterior margin, few of them very long in proximal part of pereopod 3. Carpus and propodus of pereopod 4 subequal in length, fringed with few short setae on posterior margin; pereopod 4 propodus with tuft of long setae on anterodistal corner. Dactylus straight, 0.6x propodus length, without nail.

Pereopods 5–7 (Figs. 2, 4 C–E) broken at merus/carpus articulation in all examined individuals, all with broad basis. Bases with anterior and posterior margin generally convex (except pereopod 7, straight anterior margin); anterior margin of pereopods 5–7 slightly serrated and setose; posterior margin smooth in pereopods 5–6, strongly serrated in pereopod 7, with one short seta within each incision; pereopods 6 with clear posterodistal lobe. Merus slightly curved (mainly in pereopod 6) with posterodistal process greatly produced (process length/merus total length: P5 = 27.0%; P6 = 30.1%; P7 = 38.4%); anterior margin setose; posterior margin serrated (incision with one seta) including posterodistal process.

Gills on coxa of pereopods 2–7. In fact, only 2 males of our collection showed the presence of coxal gill on pereopod 7. All the other ones (with preserved pereopods 7) had lost their gills, probably accidentally during sample processing.

Pleopods without conspicuous characters, long and powerfully developed (not drawn).

Uropods 1–3 (Figs. 6C, D, E) reaching equally long behind, biramous. Peduncle with decreasing length from uropod 1 to uropod 3. Rami uniarticulate, outer ramus shorter than inner one in all uropods (0.9x, 0.8x, 0.8x inner ramus length for uropods 1–3, respectively).

Telson (Fig. 6F) naked, elongate (length 2.7x maximum width), not reaching end of uropod 3, tapering distally, cleft 0.2x telson length, lobes apically acute.

Complementary morphological observations on female. General morphology similar to male.

Mandibule (Figs. 8 A–F) right incisor characterized by broad and smooth distal margin ending in a blunt bifid apex, fitting with concave depression of left incisor. Left lacinia mobilis unilamellar with six acute teeth. Right lacinia mobilis bilamellar and flabellate; main lamella with series of irregular tiny teeth; accessory lamella smaller, with many tiny teeth on distal margin and small asperities on inner face. Molar triturative as in male, with marginal ring of long blades, with unipointed apex.

Oostegites (Fig. 5C) on coxa of pereopods 2–5, weakly linguiform and ciliate in mature females, smaller and non-ciliate in immature females. In spite of the presence of long marginal setae, the brood pouch formed by these relatively narrow and non-contiguous lamellae does not constitute a very protective marsupium for the offspring.

Gills (Fig. 5C) on coxa of pereopods 2–7. 60 % of females with preserved pereopods 7 showed a coxal gill on theses appendages.

Remarks. Eusirus bonnieri sp. nov. belongs to the group of species lacking eyes (on preserved specimens): E. abyssi Stephensen 1944, E. bathybius Schellenberg 1955, E. fragilis Birstein & Vinogradov 1960, E. latirostris Ledoyer 1982 and Eusirus sp. (reported by Pirlot, 1934). It can be distinguished from E. abyssi, E. bathybius and Eusirus sp. by the length/width ratio of its gnathopod 2 propodus (≤ 2.0), from E. fragilis by the relative length of its rostrum (nearly 0.5x length of peduncle article 1 of antenna 1) and from E. latirostris by its dorsally carinate and toothed pereonite 7 and by the serrate posterior margin of its epimeron 3 (see identification key below).

In all Eusirus species with described pereopods 5–7 (published drawings), the length of the merus posterodistal process represents at the most 26.8% of the maximum length of this article (maximum value of 26.8% for P7 merus of E. microps Walker 1906). In E. bonnieri sp. nov., the relative length of the merus spiniform process is noticeably greater: 27.0% for P5, 30.1% for P6, 38.4% for P7 (see Fig. 4).

The new species is easy to distinguish from the other known European Eusirus species (ERMS area) by its singular deflexed rostrum, by the absence of eyes (present in E. biscayensis, E. cuspidatus Krøyer 1845, E. holmi Hansen 1887, E. leptocarpus, E. longipes, E. minutus G.O. Sars 1895 and E. propinquus G.O. Sars 1895), by the shape of its gnathopod 2 propodus (length/width ratio <2.0 versus > 2.0 in E. abyssi) and by the length of its antenna1 accessory flagellum (0.41 antenna 1 peduncle length versus 0.07 in E. abyssi).