Rhachotropis sido Frutos, Corbari & Sorbe, 2024

Rhachotropis sido Frutos, Corbari & Sorbe View in CoL sp. nov.

( Figs. 2 View Fig , 3, 4, 5, 6, 7, 8, 9, 10, 23, 24)

LSID: urn:lsid:zoobank.org:act:54B45BEB-920B-4C10-9D3B-1C6C6EBC8F2D

Type material. Holotype: brooding female, 19.9 mm BL (MNHN-IU-2015-801), Papua New Guinea, Marshall Bennett Archipelago, NE of Iwa Island , MADEEP cruise, Stn DW 4323, 08° 38′ S, 151° 46′ E, 720 m, 04/05/2014. GoogleMaps

Paratypes: brooding female, empty marsupium, 14.7 mm BL (MNHN-IU-2011-2707); brooding female, empty marsupium, 18.6 mm body without head (MNHN-IU-2017-333); brooding female, 18.4 mm BL (MNHN-IU-2017-334); brooding female, empty marsupium, 19.2 mm BL (MNHN- IU-2017-335); Papua New Guinea, off W Bougainville Island, seamounts, BIOPAPUA cruise, Stn CP 3747, 05° 33′ S, 153° 59′ E, 458 m, 12/10/2010 GoogleMaps .

Other material examined: Intra-marsupial specimen (MNHN-IU-2019-2148), individual carried by the holotype; same sampling data as for holotype.

Type locality. Papua New Guinea, Solomon Sea, Coral Sea province.

Etymology. The name refers to Sido, the Melanesian goddess of Fertility. Noun in apposition.

Description. Holotype brooding female (MNHN- IU-2015-801), paratype brooding female (MNHN-IU-2017-333).

Body: Head with two dorsal prominent protuberances near posterior margin ( Fig. 2 View Fig ), rostrum short (30.6% length of antenna 1 article 1), curved, eyes present, bulging laterally, without distinct ommatidia, non-pigmented (specimens preserved in ethanol). Pereonites 1 − 7 ( Fig. 2b View Fig ), each with one small mediodorsal prominent protuberance near posterior margin and larger lateral one in submarginal position. Pleonite 1 bearing short mediodorsal pointed process and elongated dorsolateral pointed processes; pleonite 2 bearing three elongated pointed processes on posterior margin, one in mediodorsal position and two ones in dorsolateral position; pleonite 3 bearing three shorter processes, a bigger one in mediodorsal position and two small ones in dorsolateral position, all elongated processes ending a keel extending along the dorsal surface of the corresponding somite ( Fig. 2a View Fig ). Epimeral plates 1–3 with a median tooth on posterior margin. Urosomites 1 − 3 without processes.

Antenna 1: Shorter than half body length and slightly shorter than antenna 2, if not broken. Peduncle article 1 ◂ Fig. 3 Rhachotropis sido sp. nov. Holotype, brooding female (MNHN- IU-2015-801). A1, antenna 1 left; A2, antenna 2 left; Mxp, maxilliped anterior face. Scale bars: 0.5 mm

with an acute dorso-distal process (Fig. 3); article 2 slightly shorter than article 1, more than 3 times as long as article 3, ornamented with subdistal plumose setae and two pointed distal processes; article 3 bearing a distal tiny 1-articulate accessory flagellum with one sensitive seta and two short simple setae. Main flagellum 14-articulate, all articles longer than wide, the 12th and 13th ones with one distal aesthetasc.

Antenna 2: Shorter than half body length. Peduncle articles 4 and 5 subequal in length (Fig. 3); ventral margin of article 4 with one longitudinal row of plumose setae; apex of article 5 with 3 subdistal plumose setae. Flagellum at least 16-articulate, each article longer than wide, with two subdistal short setae.

Upper lip: Entire, circular, triangular apex with blunt tip and subapical crown of short setules (Fig. 4).

Mandibles: Mandibles asymmetrical. Incisor process asymmetrical and well-developed on both mandibles; apex bifid on right one, partly dentate on left one. Laciniae mobilis asymmetrical, left one more massive than right one, and smoothly dentate at apex (Fig. 4). Left and right setal row with 5 cuspidate setae. Molar process cylindrical, with large triturative apex delineated by a ring of stout blades. Mandibular palp triarticulate; article 1 short; article 2 longer than article 3 with long slender setae on half distal part; article 3 tapering distally, ending with few long setae.

Lower lip: Outer lobes separated by broad gap, distally rounded and setose. Inner lobes broken, apparently small. Mandibular projections poorly developed, blunt distally (Fig. 4).

Maxilla 1: Inner plate with two subapical plumose setae and numerous setules at apex. Outer plate with 5 bifid/ multifid cuspidate setae and 3 simple cuspidate setae. Palp 2-articulate, proximal article shorter than distal one, bearing 2 distal cuspidate setae (one of them with an accessory seta) and a subdistal one; distal article with long and very long setae on distal two-third (Fig. 4).

Maxilla 2: Inner plate and outer plate subequal in length, inner one slightly wider than outer one. Both plates fringed with numerous slender simple setae and setules in their distal part. Inner margin of inner plate with two subapical plumose setae and numerous setules (Fig. 4).

Maxilliped: Inner plate shorter than outer plate, with 6 cuspidate stout setae irregularly shaped and implanted at apex, 1 long submarginal plumose seta near inner margin and numerous setules on distal half of anterior face. Outer plate not reaching half of palp article 2, with a submarginal row of stout setae near inner margin and marginal setae at apex (Fig. 3). Palp 4-articulate, long and setose; article 1 with 2 subdistal cuspidate setae near outer margin; article 2 longest, with submarginal row of simple setae near inner margin and 1 subdistal simple seta on outer margin; article 3 widened subdistally, with a transverse row of simple setae on anterior face and marginal/submarginal simple setae at apex; article 4 shorter than article 3, slightly curved (Fig. 3).

Gnathopods ( Fig. 5 View Fig ). Gnathopod 1: slightly smaller than gnathopod 2, similar in shape, both strongly subchelate. Coxa small, acute, and strongly produced anteriorly, over-reaching the anterior margin of pereonite 1. Basis weakly arched, anterior margins with short setae, distal part with 5 long setae and one long setae at postero-distal corner. Ischium and merus with 5 five long setae at postero-distal corner each. Carpus strongly produced, carpal process with 4 distal stout setae. Propodus large and oval, 2.1× as long as broad with 4 anterodistal long setae. Palmar margin with 2 rows of setae, one plumose, other long ones ending by one stout seta. Proximal part of the propodus with 4 stout setae where the tip of dactylus is folded. Subdistal palmar margin with 4 bunches of long simple setae. The dactylus is narrow and curved. Gnathopod 2: Coxa subquadrate. Basis with 6 (4 and 2 missing) anterodistal long setae and one long setae (broken) at postero-distal corner. Merus with 5 five long setae and 4 shorter setae at postero-distal corner. Carpus strongly produced, carpal process long over-reaching the proximal part of palmar margin, bearing 4 distal stout setae and 2 subdistal rows of stout setae. The propodus exhibits the same shape and proportion as in Gn1, but slightly broader with 4 anterodistal long setae (broken). Palmar margin with 2 rows of setae, one plumose, other long ones ending by one stout seta. Subdistal palmar margin with 5 long simple setae. Proximal part of the propodus with 2 stout setae where the tip of dactylus is folded. Dactylus is narrow and curved.

Pereopods. Pereopod 3 (Figs. 6 and 7): narrow and elongate, shorter than pereopods 5–7. Coxa subquadrate, postero-distal corner bluntly pointed. Basis recto-linear. Dactylus long and slightly curved, 0.73× propodus length. Pereopod 4 (Fig. 6): Similar to P3. Coxa subquadrate, distal margin concave, hind margin excavated with medial lobe. Basis rectilinear with anterior margin poorly setose. Dactylus long and slightly curved, 0.86× propodus length. Pereopod 5 (Figs. 6 and 7): Coxa distally bilobed, with small notch at postero-distal corner. Basis slightly expanded posteriorly, with bluntly median process on posterior margin bearing one plumose seta. Cuspidate setae along posterior margin of merus. Dactylus long and slightly curved. Pereopod 6 (Fig. 6): Coxa bilobed, posterior lobe bigger than anterior one and bearing small distal notch. Basis slightly expanded posteriorly, with bluntly median process on posterior margin. Cuspidate setae along posterior margin of merus as well as on anterior margin of carpus and propodus. Dactylus long and slightly curved. Pereopod 7 (Figs. 6, 7 and 24): Coxa unilobate, distally convex, with acute process at postero-distal corner. Basis slightly expanded posteriorly, with acute ◂ Fig. 4 Rhachotropis sido sp. nov. Holotype, brooding female (MNHN- IU-2015-801). Mx1, maxilla 1 left posterior face; Mx2, maxilla 2 left posterior face; Md, mandible; UL, upper lip; LL, lower lip. Scale bars: 0.5 mm

median process (holotype) or large rounded median process (paratype MNHN-IU-2017-333; Fig. 24 View Fig , see “Discussion”) on posterior margin, and a small tooth at postero-distal corner. Merus with cuspidate setae on anterior and posterior margins, ending in a spiniform process at postero-distal tip. Dactylus long and slightly curved.

Uropods ( Fig. 8 View Fig ). Uropod 1: Peduncle longer than unequal rami, fringed with a few cuspidate setae along inner margin. Outer ramus 0.91× inner ramus length, smooth. Inner ramus fringed with a few cuspidate setae along inner margin. Uropod 2: peduncle shorter than unequal rami, with only one cuspidate seta at distomedial corner. Outer ramus 0.86× inner ramus length, smooth. Inner ramus fringed with a few cuspidate setae along inner margin. Uropod 3: peduncle shorter than lanceolate rami, with a few simple and cuspidate setae along inner margin. Outer ramus 0.94× inner ramus length, smooth. Inner ramus fringed with a few cuspidate setae along inner margin.

Telson ( Fig. 8 View Fig ): Elongated (length/width ratio: 2.94), slightly tapering towards apex, cleft 6.9% of its length, with dehiscent acute lobes; dorsal proximal area with a pair of ◂ Fig. 6 Rhachotropis sido sp. nov. Holotype, brooding female (MNHN-IU-2015-801). Outer face of left pereopods 3 (P3) and 4 (P4). Outer face of pereopods 5–7 (left P 5, right P6, left P7). Scale bars: 1 mm

long plumose setae, inserted on a submarginal ridge (left seta lost).

Molecular identification. COI sequences were obtained for the following specimens (inventory numbers (GenBank accession numbers), see Supplementary Table 1): MNHN- IU-2017-333 ( PP495379 ), MNHN-IU-2017-334 ( PP495378 ), MNHN-IU-2017-335 ( PP495377 ), MNHN-IU-2011-2707 ( PP495376 ), MNHN-IU-2015-801 ( PP495375 ). Following the definition given by Pleijel et al., 2008, the holotype (MNHN- IU-2015-801) is designed as the hologenophore .

Biological considerations. Male unknown, all collected specimens were brooding females in which 2 specimens are full marsupium (i.e., holotype MNHN- IU-2015-801 bearing 30 individuals; paratype MNHN- IU-2017–334 with 2 individuals). Intramarsupial prejuvenile ( BL = 3.70 mm, MNHN-IU-2019-2148, Fig. 9 View Fig ): Eyes bulging as in mature specimens. Dorsal spiniform processes of pleonites 1–3 and urosome 1 not yet developed. Gnathopods and pereopods fully segmented. Spiniform process on posterior margin of basis of P5–7 already visible. Telson cleft and dehiscent .

Distribution. Southwestern Pacific, Solomon Sea, depth range 458– 720 m.

Remarks. R. sido sp. nov. belongs to the Rhachotropis group with long dorsal spiniform processes on pleonites 1–3, gathering Rhachotropis gubilata J.L. Barnard, 1964 ; R. lobata Shoemaker, 1934 ; R. palporum Stebbing, 1908 , and R. oweni Lörz, 2015 . R. sido sp. nov. shares strong morphological similarities with R. oweni : bulging eye species, on each pereonite presence of one small mediodorsal prominent protuberance near posterior margin, and a median process on posterior margin of P5–7 basis. R. sido sp. nov. differs from R. oweni by the presence of two subapical plumose setae on inner margin of Mx2 inner plate, by the presence of two dorsal prominent protuberances near posterior margin of the head ( Fig. 10 View Fig ), the presence of a short mediodorsal pointed process on pleonite 1; the absence of one short process in mediodorsal position on pleonite 2, and three short and equal processes on dorsolateral position on pleonite 3; and the absence of a large process at postero-distal corner of P7 basis (see “Discussion”). The genetic divergences (uncorrected p-distance) between R. sido sp. nov. and R. oweni (NIWA-82905, see Fig. 23) ranged from 10.4 to 12% and from 11.9 to 13.5% with R. cf. palporum (MG521132, see Fig. 23).

Ecological data. R. sido sp. nov. was collected at two stations with both dredge and beam trawl on hard-substrate bottoms as pictured after recovery of the sample on board, 1 and hooking the beam trawl up during towing as reported in the logbook. With a lower volume of fauna sampled with the dredge than with the beam trawl, different octocorals ( Chrysogorgidae , Isidae ), solitary scleractinian, sponges, gastropods, crinoids and decapods (galatheoids, chirostylids, nematocarcinids, pandalids) characterized the benthic megafauna at these two rocky bottoms where R. sido sp. nov. occurs. Only four amphipod specimens were reported together with the new species: an uristid Euonyx sp. , an unidentified calliopioid Pontegeneidae (both at station DW4323), a pardaliscid Pardalisca sp. , and the phronimid Phronima atlantica Guérin-Méneville, 1836 (both at station CP3747). Other than the well-known pelagic phronimid, probably a by-catch, sampled during the sampler recovery through the water column, the three bathyal species more related to a near-bottom lifestyle are new to science and will be described further.