Rhachotropis chathamensis, Lörz, Anne-Nina, 2010

Lörz, Anne-Nina, 2010, Deep-sea Rhachotropis (Crustacea: Amphipoda: Eusiridae) from New Zealand and the Ross Sea with key to the Pacific, Indian Ocean and Antarctic species, Zootaxa 2482, pp. 22-48 : 28-29

Rhachotropis chathamensis sp. nov.

( Figs 1–4 )

Material examined. Holotype: NIWA 60479, female, 11.7 mm with 12 eggs, TAN0705/285, 43°47.48´S, 175°18.56´E, Brenke sled, 27 April 2007, 420 m, Chatham Rise, New Zealand.

Paratypes: NIWA 60485, 6 specimens 10.4 mm, 11.4 mm, 10.6 mm, 8.7 mm, 8.6 mm, 12.8 mm, type locality; NIWA 60480, 3 specimens sequenced; NIWA 60485, 33 paratypes, type locality.

Etymology. Rhachotropis chathamensis sp. nov. is named after the Chatham Rise, a marine area East of New Zealand where the species was collected.

Diagnosis. Body delicate. Rostrum longer than head. Eyes absent. Head twice as long as pereonite 1, lateral lobes strongly produced. Pereonites smooth. Pereopod 7 longer than body. All pleonites bearing dorsal processes, pleonite 1 and 2 also bearing dorsolateral processes. Pereonite 3 posterior margin indented. Urosomite 1 bearing dorsolateral process, urosomites 2 and 3 smooth. Telson entire, triangular, long and narrow.

Description. Antenna 1 and 2 equal in length. Antenna 1 article 2 of peduncle with several plumose setae, article 2 slightly shorter than article 1, twice as long as article 3; flagellum 10-articulate. Antenna 2 peduncle article 3 and 4 subequal in length, several plumose setae on third article; flagellum 11-articulate.

Mandible with incisor process well developed, lacinia mobilis denticulate, molar process conical; palp article 1 short, about one-third length of article 2; article 3 as long as article 2; articles 2 and 3 with long slender setae. Maxilla 1 inner plate bearing 1 subterminal seta; outer plate with 9 denticulate spines; article 2 of palp with several slender setae at tip. Maxilla 2 inner and outer plate subequal in length, margins bearing stout and slender setae, inner plate wider than outer plate, with 1 plumose seta. Maxilliped with several epibionts. Maxilliped outer plate 2.5 times as long as inner plate, reaching half of article 2 of maxillipedal palp; inner margins of palp, outer plate and terminal end of inner plate setose. Labrum entire, smooth and broadly rounded. Hypopharynx setose, outer lobes separated by broad gap.

Gnathopod 1 coxa 1 produced, reaching beyong the head; coxa 2 rounded; coxa 3 and 4 subquadrate. Gnathopods similar in shape, subchelate. Gnathopod 1 slighly smaller than gnathopod 2, basis bearing several small spines at anterior side; merus with long setae at posteroventral corner; carpus lobe extending width of propodus, spines at terminal end of lobe; propodus widened, oval; dactylus slender, not reaching end of palm.

Pereopods 3 and 4 articles long and narrow, all articles except dactylus bearing slender setae. Gill at pereopod 4 nearly as long as basis, fully-developed oostegite extending length of basis. Pereopod 5 basis small, slightly pointed midventral margin; merus and carpus equal in length, bearing short setae; prododus longer than carpus, short seta. Pereopod 6 basis larger than that of pereopod 5, small notch midventrally; gill slightly longer than basis; proportion of articles as in pereopod 5. Pereopod 7 basis widened, posterior margin serrate; merus posteroventral angle produced; propodus longer than carpus.

Pleopod 1 (right side) rami with 17 and 21 articles, respectively; rami more than twice as long as peduncle.

Uropod 1 outer rami slightly shorter than inner rami; rami about as long as peduncle. Uropod 2 peducle shorter than rami; outer rami shorter than inner rami. Uropod 3 rami lanceolote, longer than peduncle, subequal in length; peduncle produced at inner margin. Telson entire, more than three times as long as wide.

Remarks. Rhachotropis chathamensis sp. nov. from New Zealand differs from all other 55 Rhachotropis species by the combination of following characters: a long rostrum, eyes are absent, coxa 1 is anteroventrally produced, urosomite 1 has a pointed middorsal process, the telson is entire.

Only five of the 55 known Rhachotropis species have an uncleft telson. Rhachotropis caeca Ledoyer, 1977 ; R. integricauda Carausu, 1948 and R. flamina Bellan-Santini 2006 from the Atlantic, R. northriana d’Ukem, Vader & Legezinska, 2007 from the North Sea and R. glorisae Ledoyer, 1982 from the Indian Ocean, Glorious Islands North of Madagascar. Therefore Rhachotropis chathamensis sp. nov. is the first Rhachotropis species reported from the Pacific bearing an entire telson, and the sixth species worldwide.

Of the Rhachotropis species having an entire telson, only R. gloriosae bears a long rostrum and no eyes. Rhachotropis chathamensis sp. nov. differs from R. gloriosae amongst several less obvious differences in: bearing dorsal lateral spines (next to the middorsal process) on pereonites 1 and 2; coxa 1 is pointed anteroventrally; the basis of pereopod 7 is widened; urosomite 1 has a longer dorsal process and at the posterior end, whilst R. gloriosae bears a small middorsal process on urosomite 1.

The holotype has an empty “sack” attached to the inside of the basis of the right pereopod 7. The author does not know the origin or function of this unit. This structure was not found on any of the paratypes. It might be an artefact, but mentioned here in case it has a functional unit related to the epibionts (it is not a rhizocephalan structure).

Epibionts. The holotype and some paratypes had epibionts on their maxillipeds. One female paratype had a total of 43 epibionts on the maxilliped, distributed as: 9 on the left palp, 5 on the left outer lobe, 1 on the left inner lobe, 18 on the right palp, 8 on the outer lobe, 3 on the inner lobe.

Molecular data. COI sequences were obtained from three paratypes of R. chathamensis sp. nov. (BOLD Accession nos: AMPNZ09809, AMPNZ10009, AMPNZ10109).

Morphological variation. Males of Rhachotropis chathamensis sp. nov. have a slight length increase of the first flagellum of antenna 1. No other morphological variation was observed amongst the 43 paratypes of Rhachotropis chathamensis sp. nov., except that the “sack” on P7 was only present on the holotype, and only few specimens had the epibionts on their maxilliped.

Distribution. New Zealand, Chatham Rise, 420 m.

Figures

FIGURE 1. Rhachotropis chathamensis sp. nov., holotype, 11.7 mm, female, NIWA 60479: a, habitus dorsal; b, habitus lateral; c, maxilliped. Scale bars a, b 1 mm; c 0.1 mm.

FIGURE 2. Rhachotropis chathamensis sp. nov., holotype, 11.7 mm, female, NIWA 60479: a, antenna 1, enlarged plumose setae; b, antenna 2; c, mandible; d, hypopharynx; e, labrum; f, maxilla 1; g, maxilla 2. Scale bars a, b 1 mm; c – g 0.1 mm.

FIGURE 3. Rhachotropis chathamensis sp. nov., holotype, 11.7 mm, female, NIWA 60479: a, gnathopod 1; b, gnathopod 2; c, pereopod 3; d, pereopod 7; e, pereopod 5; f, pereopod 6. Scale bars 1 mm.

FIGURE 4. Rhachotropis chathamensis sp. nov., holotype, 11.7 mm, female, NIWA 60479: a, pleopod 1; b, uropod 1; c, uropod 2; d, uropods 3; e, pereopod 4; f, telson. Scale bars 1 mm.