Rhachotropis afekan Sorbe, Corbari & Frutos, 2024

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

( Figs. 10 View Fig , 17 View Fig , 18 View Fig , 19 View Fig , 20 View Fig , 21 View Fig , 22 View Fig and 23)

LSID: urn:lsid:zoobank.org:act:52013DDE-2EF2-452D-8303-2CB3DEEEB182

Type material. Holotype: brooding female, empty marsupium, 15.8 mm BL (MNHN-IU-2015-708). Papua New Guinea, off New Britain, NW of Vitu Islands, MADEEP cruise, Stn CP 4246, 04° 07′ S, 148° 09′ E, 780–850 m, 22/04/2014. GoogleMaps

Comparative material examined. Rhachotropis kergueleni Stebbing, 1888 . NHM. 1889.5.15:97. Type material. One individual, sex unknown, completely dissected on five slides, 11.4 mm BL in the original description and figured by Stebbing; one brooding female, empty marsupium, 10.7 mm BL, currently measured. Challenger expedition, southern Indian Ocean at Kerguelen Island, no station data provided.

Type locality. Papua New Guinea, Bismarck Sea , West Pacific province .

Etymology. The name refers to Afekan, the Melanesian goddess of creation and knowledge. Noun in apposition.

2 https://expeditions.mnhn.fr/campaign/salomon2/event/CP2197? area=1

https://expeditions.mnhn.fr/campaign/salomon2/event/CP2230? area=1

https://expeditions.mnhn.fr/campaign/salomon2/event/CP2231? area=1

◂ Fig.12 Rhachotropis riina sp. nov. Holotype, brooding female (MNHN- IU-2017-524). A1, antenna 1 left; A2, antenna 2 left; Mxp, maxilliped posterior face. Scale bars: 0.5 mm

Description. Holotype brooding female (MNHN-IU-2015- 708).

Body ( Fig. 17 View Fig ): Rostrum long (93% length of antenna 1 article 1), slightly curved ( Fig. 10 View Fig ); eyes absent, (specimens preserved in ethanol). Pereonites 1 − 7 smooth ( Fig. 17 View Fig ). Pleonites 1 − 3 bearing mediodorsal keel produced into a well-developed tooth (damaged on pleonite 3, apparently the largest) and dorsolateral slightly keel produced in a short tooth ( Fig. 17a View Fig ). Epimeral plates 1 ‒ 2 smooth, epimeral plate 3, postero-ventral corner rounded with serrations. Urosomite 1 with short medial keel produced into a tooth, Urosomites 2 − 3 smooth.

Antenna 1 ( Fig. 18 View Fig ): Shorter than half body length and shorter than antenna 2 (broken). Peduncle article 1 with a ventral subdistal short tooth, and bearing one long simple seta, one plumose seta, and 6 shorter simple setae along the mesial margin; article 2 shorter than article 1, more than 2 times as long as article 3; article 3 with one medial simple seta, 2 distal setae (one broken, other sensitive), and distal tiny 1-articulate accessory flagellum with one seta. Main flagellum 17 − articulate, all articles longer than wide, last one with three distal setae (aesthetasc).

Antenna 2 ( Fig. 18 View Fig ): Shorter than half body length (broken). Peduncle article 5 1.2× longer than article 4 in length; ventral margin of article 4 with one longitudinal row of simple setae, 2 subdistal plumose setae and 3 long setae; apex of article 5 with 1 subdistal plumose seta and 3 long setae. Flagellum at least 10-articulate (broken), each article longer than wide, with two subdistal long setae.

Upper lip ( Fig. 19 View Fig ): Entire, circular, triangular apex with blunt tip (folded) and subapical crown of setules.

Mandibles ( Fig. 19 View Fig ): 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 dentate at apex. On both left and right mandibles setal row with 4 cuspidate setae. Molar process cylindrical, with large triturative apex delineated by a ring of stout blades and one plumose seta. Mandibular palp 3-articulate; article 1 short; article 2 longer than article 3 with long slender setae distal parts; article 3 tapering distally, with row of long setae on inner margin and six plumose setae at the apex.

Lower lip ( Fig. 19 View Fig ): Outer lobes separated by broad gap, distally rounded and setose. Inner lobes small. Mandibular projections well developed.

Maxilla 1: Inner plate with one subapical plumose seta and one long simple seta. Outer plate with 8 bifid/multifid cuspidate setae and one simple cuspidate seta. Palp 2-articulate, proximal article shorter than distal one, bearing 3 distal long setae; distal article with long and very long setae on distal two-third ( Fig. 19 View Fig ).

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 one subapical plumose seta and subapical and apical setae on distal part ( Fig. 19 View Fig ).

Maxilliped: Inner plate shorter than outer one, with 4 ‒ 5 cuspidate stout setae irregularly shaped and implanted at apex, 3 submarginal stout setae near inner margin on anterior face. Outer plate not reaching half of palp article 2, with a submarginal row of stout setae near inner margin and marginal plumose setae at apex ( Fig. 18 View Fig ). Palp 4-articulate, long and setose; article 1 with 1 long and 2 short subdistal simple setae near outer margin; article 2 longest, with submarginal row of simple setae near inner margin and 1 subdistal short 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 subequal in length to article 3, curved ( Fig. 18 View Fig ).

Gnathopods ( Fig. 20 View Fig ). Both broken, uncompleted (carpus, propodus, and dactylus missing). Gnathopod 1: Coxa right long, produced anteriorly and rounded, reaching head anteroventral corner ( Fig. 17 View Fig ); coxa left short, produced anteriorly and rounded, reaching the post-lateral cephalic sinus ( Fig. 10 View Fig ). Basis arched, distally broader, anterior margin with a row of stout setae and a distal bunch of simple setae, postero-distal corner with 2 long and one short setae. Ischium postero-distal corner with 7 long setae. Merus, anterior margin with 2 medial setae and a bunch of long setae at postero-distal corner. Gnathopod 2: Coxa subquadrate, posterior margin slightly concave. Basis arched, distally broader, anterior several stout setae all along the anterior margin, antero-distal corer with a bunch of long setae, posterior margin with a row of stout setae, postero-distal corner with one large stout seta and 2 long simple setae. Ischium with a bunch of simple setae at postero-distal corner.

Pereopods. Pereopod 3 ( Fig. 20 View Fig ): narrow and elongate, shorter than pereopods 6 ‒ 7 (P5 partly broken). Coxa subquadrate, posterior margin slightly concave with short simple setae on inner face. Basis recto-linear, anterior margin with a row of stout setae and posterior one with several cuspidate setae, postero-distal corner with 5 large stout setae. Merus slightly enlarged distally, posterior margin with a long simple seta, antero- and posteriordistal corner with 2 ‒ 3 stout setae and 1 ‒ 2 long simple setae. Carpus with long simple setae on posterior margin. Propodus with long setae on postero- and antero-margin. ◂ Fig. 13 Rhachotropis riina sp. nov. Holotype, brooding female (MNHN-IU-2017-524). 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

Dactylus long and slightly curved, 0.8× propodus length. Pereopod 4 ( Fig. 20 View Fig ): Similar to P3. Coxa subquadrate, posteriorly excavate, posterior lobe blunt, ventral margin linear with one small setae, few setae on inner face. Basis recto-linear, anterior margin with a row of stout setae and posterior margin with several cuspidate setae, postero-distal corner with 7 stout setae. Merus and carpus with long simple setae on posterior margin. Propodus with long setae on both posterior and anterior margins. Dactylus long and slightly curved, 0.90× propodus length. Pereopod 5 ( Fig. 21 View Fig ): broken (carpus, propodus, and dactylus missing). Coxa bilobed with posterior lobe larger with postero-distal corner slightly serrate with some simple setae. Basis large, posteriorly produced, postero-distal corner acute slightly serrate with 2 distal stout setae and 2 medial short setae on posterior margin. Merus 2.88× basis length with cuspidate and stout setae all along posterior and anterior margins, postero-distal corner with a long stout seta. Pereopod 6 ( Fig. 21 View Fig ): very long. Coxa bilobed, posterior lobe bigger than anterior one with crenulation (lost setae) on hind margin. Basis large, posteriorly produced, postero-distal corner acute and slightly serrate (lost setae). Ischium with 3 medial long setae. Merus with stout setae all along postero- and antero-margin, ending in a spiniform process at postero-distal tip with a long stout seta. Carpus with stout and plumose setae all along anterior margin, one long stout seta on antero-distal corner, stout setae on posterior margin. Propodus, longest article, 3.75× basis length, with stout and plumose setae all along anterior margin, postero-distal corner with 2 long plumose setae. Dactylus (tip broken) linear. Pereopod 7 ( Fig. 21 View Fig ): broken, uncompleted (distal part of propodus, and dactylus missing). Coxa unilobate with few simple setae on anterior margin. Basis large, posteriorly produced, postero-distal corner acute, slightly serrate (lost setae) over-reaching distal margin of ischium, posterior margin with short simple setae. Merus with stout setae on anterior margin with one anterodistal long stout seta, posterior margin with stout setae, ending in a spiniform process at postero-distal tip with a long stout seta. Carpus, very long, with stout setae all along anterior and posterior margins. Propodus incomplete.

Uropods ( Fig. 22 View Fig ). Uropod 1: Peduncle longer than rami, fringed with a few cuspidate setae on outer margin and a row of stout setae along inner margin. Rami inqueal. Outer ramus 0.90× inner ramus length, smooth. Inner ramus fringed with a few stout setae along inner margin. Uropod 2: peduncle slightly longer than rami with rows of stout setae along inner and outer margins, and 2 stout setae on inner distal corner. Rami unequal. Outer ramus 0.70× inner ramus length, inner ramus fringed with a few stout setae along inner margin. Uropod 3: peduncle shorter than rami, with 3 cuspidate setae and 2 simple setae along inner margin. Rami lanceolate, subequal in length, inner ramus with a row of large stout setae on inner margin, few small stout setae on outer margin of outer ramus.

Telson ( Fig. 22 View Fig ): Elongated (length/width ratio: 2.26), slightly tapering towards apex, cleft 7.8% of its length, with slightly dehiscent slit. Dorsal proximal area with a pair of long plumose setae and few setules along inner and outer margins.

Molecular identification. COI sequence was obtained for the single specimen (inventory number (GenBank accession number), see Supplementary Table 1): MNHN- IU-2015-708 (PP495385), which is designed as the hologenophore (according to the definition given by Pleijel et al., 2008).

Biological considerations. Male unknown, the single sampled specimen was a brooding female, empty marsupium.

Distribution. Southwestern Pacific, Bismarck Sea, depth range 780– 850 m.

Remarks. R. afekan sp. nov. belongs to the blind Rhachotropis group with telson shortly cleft (less than 10%), and also characterized by a strongly anteriorly produced rounded coxa 1, pereonite 7 without mediodorsal tooth, pleonites 1 ‒ 3 strongly carinate and dorsally produced into a well-developed tooth, and urosomite 1 ending with a dorsal tooth. According to that, it is closely related to R. kergueleni and R. sibogae — the former with 1-articulate accessory flagellum bearing a distal long simple seta after detailed study of type material. R. afekan sp. nov. differs from R. kergueleni by pleonite 3 tricarinae, with dorsolateral teeth on its hind margin (vs single dorsal keel with a well-developed tooth), largest mediodorsal tooth on pleonite 3 (vs the largest one on pleonite 2), urosomite 1 carinate dorsally produced into a tooth (vs produced into sharp developed tooth), epimeron 3 slightly dentate on the inferoposterior corner (vs inferior margin straight and posterior margin with 15 upturned teeth), P7 basis postero-distal corner exceeding ischium high (vs slightly developed), and coxa 1 reaching the post-lateral cephalic sinus (vs exceeding the sinus). It differs from R. sibogae by rostrum shorter than A1 peduncle article 1 (vs as long as A1 peduncle article 1), epimeron 3 slightly dentate on the inferoposterior corner (vs inferior margin dentate and posterior margin smooth), urosomite 1 carinate dorsally produced into a well-defined tooth (vs not dorsally carinate ending by a minute tooth), telson apex with slightly dehiscent slit (vs dehiscent telson apex with two pointed lobes). The observations of R. sibogae are based on the original description (type requested but probably lost, Naturalis Biodiversity Center, Leiden). Finally, R. afekan sp. nov. do not share with either of them a ventral subdistal tooth on A1 peduncle article 1. The genetic divergences (uncorrected p-distance) between R. afekan sp. nov. and R. abyssalis (GU804297 and MN346577, see Fig. 23) ranged from 18.7 to 18.8%.

Ecological data. R. afekan sp. nov. was collected in a single station by trawling on muddy bottoms with abundant long spicules of hexactinellid sponges, beside some hard blocks and sunken wood debris (e.g., coconut and nipa palm seeds). According to the on-board pictures of the sample, 3 and preliminary notes on log book, the main benthic megafauna was represented by gorgonian and alcyonacean octocorals, sponges, as well as echinoderms (ophiurids, asteroids and echinoids), gastropods, and decapods (galatheoids, chirostylids, nematocarcinids). The epimeriid Epimeria rafaeli Coleman & Lowry, 2014 (18 specimens) represented the only amphipod species cooccurring in this station.