Rhachotropis flamina, Bellan-Santini, 2006

Bellan-Santini, Denise, 2006, Rhachotropis species (Crustacea: Amphipoda: Eusiridae) of hydrothermal vents and surroundings on the Mid- Atlantic Ridge, Azores Triple Junction zone, Journal of Natural History 40 (23 - 24), pp. 1407-1424 : 1408-1411

Rhachotropis flamina n. sp.

( Figures 1 , 2 )

Type locality

Mid-Atlantic Ridge, 36 ° 149N, 33 ° 549W, 2250 m depth, at 500 m to the north of Rainbow vent area.

Material examined

One female, holotype, 8 mm length. FLAME 2 cruise, 10 September 1997, sampled by the sediment trap PPS1-B 1, 160 m above the bottom, MNHN-Am7453 .

Diagnosis

Body delicate. Head without rostrum. Segments 1 and 2 of the pleon bear on the back a pair of processes as long as the segments. Segment 3 bears a long median process with two smaller on the side. Accessory flagellum of antenna 1 present, reduced to a small article. Gnathopods 1 and 2 similar, subchelate. Telson entire, triangular, long, and narrow.

Description

Female with oostegites, length 8 mm.

Body delicate, some articles are broken or distorted. Segments 1 and 2 of the pleon bear on the back a pair of processes as long as the segment. Segment 3 bears a long median process with a smaller one on each side. Epimeral plates 1 and 2 rounded, plate 3 indented on the posterior and distal edges. Urosome without process or keel.

Head without rostrum, lateral lobes rounded. Eyes absent. Antennae equal in length, equal to half of body. Antenna 1 peduncle articles 1 and 2 equal, article 3 equal to 0.44 of article 2; accessory flagellum reduced to a small article ended by a seta, flagellum with 12 articles. Antenna 2 peduncle articles 3 and 4 equal, flagellum with 14 articles. Mandible with incisor process well developed, lacinia mobilis denticulate, molar process conical, palp triarticulate, article 1 short, articles 2 and 3 long and little setose. Maxilla 1 inner plate bearing only one subterminal seta, outer plate with four large denticulate spines, palp biarticulate, article 2 setose. Maxilla 2 inner and outer plates equal in length, both largely setose. Maxilliped inner plate very short, outer plate narrow and longer than inner, palp long and setose.

Coxae 1 and 2 rounded, 4 and 5 subquadrate, all with four to six small setae on the posterior margin. Gnathopods 1 and 2 similar, subchelate. Gnathopod 1 slightly smaller than gnathopod 2. Gnathopod 1 basis fringed at the anterior side by many short spines, carpus lobate but short, propodus narrowly oval, palm long and fringed with many small setae, some of them are longer, palm limited by a row of spines, dactylus as long as propodus, slightly curved. Gnathopod 2 slightly larger than gnathopod 1.

Pereopods 3 and 4 articles narrow and elongate. Pereopod 5 basis oval with the posterodistal corner notched, ischium short, merus long and narrow fringed on both sides by small spines, carpus short fringed on both sides by small spines, propodus long fringed by small spines only anteriorly, dactylus long; ratio basis/ischium/merus/carpus/propodus54/0.3/ 12.5/5.5/15.6. Pereopod 6 basis longer than basis of pereopod 5, bearing two small spines on the postero-distal corner and eight small spines on the posterior edge, ischium short, merus long fringed on both sides by small spines, carpus short, propodus long, both articles fringed on both sides by small spines, dactylus long; ratio basis/ischium/merus/carpus/ propodus/dactylus55.5/0.5/11/5/17.5/6. Pereopod 7 basis longer than basis of pereopod 6, ischium short, merus long and fringed on both sides by small spines, carpus broken.

Uropod 1 rami slightly unequal, without spine, shorter than peduncle. Uropod 2 rami slightly unequal, longer than peduncle, bearing small spines. Uropod 3 rami lanceolate, longer than peduncle. Telson entire, triangular, long, and narrow.

Etymology

Name of the species is derived from the name of the FLAME cruise.

Relationship

Rhachotropis flamina has a certain number of specific characteristics that are just conspicuous: no rostrum, blind, gnathopods subchelate and little different in shape and in size; segments 1 and 2 of pleon with two long processes; segment 3 of pleon with a long median process and two smaller lateral processes, epimeral plate 3 denticulate, telson long and entire.

In the 22 species of Rhachotropis now found in the Atlantic Ocean, only two have an entire telson, R. caeca Ledoyer, 1977 and R. integricauda Carausu, 1948 . Rhachotropis caeca has only a small median process on segments 1 and 2 of the pleon and one small median process and two lateral on segment 3, epimeral plate 3 is smooth. Rhachotropis integricauda has a pointed rostrum, eyes well developed, the three segments of the pleon are armed with three short spines but no long process. Rhochotropis gloriosae Ledoyer, 1982 is the third species of Rhachotropis in the world with an entire telson, it is known from two fragments from Glorious Islands; this species has a long rostrum, short lateral spines on segments 1 and 2 of the pleon, the basis of pereopod 7 has an angulate infero-posterior lobe.

Distribution and habitat

This single specimen was collected at 500 m north of the vent site Rainbow (Mid-Atlantic Ridge, 2250 m depth), thus in an area without vent, and at 160 m above the bottom. This species with a delicate body, blind, with long pereopods, is probably a bathypelagic species. It was not observed in the samples collected on the vent area.

Figures

Figure 1. Rhachotropis flamina, FLAME 2, PPS1-B1, holotype female, 8 mm. (1) Habitus; (2) antenna 1; (3) mandible; (4) maxilla 1; (5) maxilla 2; (6) gnathopod 1; (7) gnathopod 2. Scale bars: 100 mm.

Figure 2. Rhachotropis flamina, FLAME 2, PPS1-B1, holotype female, 8 mm. (1) Maxilliped; (2) pleosomite 1; (3) pleosomite 2; (4) pleosomite 3; (5) pereopod 3; (6) pereopod 5; (7) pereopod 6; (8) pereopod 7; (9) uropod 1; (10) uropod 2; (11) uropod 3; (12) telson. Scale bars: 100 mm.