Rhachotropis licornia, 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 : 1412-1416

Rhachotropis licornia n. sp.

( Figures 3–5 )

Type locality

Mid-Atlantic Ridge, 36 ° 13.459N, 33 ° 52.819W, 1937 m, at 2000 m to the vent site Rainbow.

Material examined

Holotype, 10 mm length. FLAME 2 Cruise, 20 May 1997, collected by the sediment trap PPS3-H 20, 300 m above the bottom, MNHN-Am7454.

Diagnosis

Body smooth. Eyes absent. Antenna 1 longer than antenna 2, article 1 robust, twice as long as head; bearing at the infero-distal corner a long curved spine covered with hook-shaped scales, no accessory flagellum. Gnathopods subchelate. Telson long and tapered, deeply cleft.

Description

Length 10 mm.

Head shorter than the first segment of the pereon. Body smooth. Eyes absent. Rostrum short, lateral lobe rounded. Antenna 1 longer than antenna 2, article 1 robust, twice as long as head; bearing at the infero-distal corner a long curved spine covered with hook-shaped scales in the distal half; the inferior edge fringed with five tufts of setae; article 2 1.5 times as long as article 1, bearing at the inferior edge two rows of calceoli. The calceoli with attachment stalks in the central line and the distal element orientated toward the exterior. Accessory flagellum absent; flagellum with 80 articles each bearing a calceolus at the posterior edge and a seta at the anterior edge. Antenna 2 shorter than antenna 1, peduncle article 4 a little shorter than 5, bearing 14 tufts of setae on the anterior margin and more at the posterior margin; article 5 fringed anteriorly with calceoli and bearing distally three long plumose setae; flagellum 34-articulate, the 21 proximal articles each bear a calceolus. Maxilla 1 palp biarticulate, inner plate rounded with two setae on the inner edge, outer plate with seven dentate spines. Maxilla 2 inner plate broader than outer, both fringed with numerous setae. Maxilliped inner plate short, outer plate large, both hardly setose, palp four-articulate, articles 2 and 3 ovate, densely setose, article 4 falcate.

Coxa 1 quadrate, distal edge rounded, distal posterior corner notched. Coxa 2 subrectangular, distal posterior corner notched, posterior edge with a spine in the middle. Coxa 3 subrectangular, distal posterior corner notched, posterior edge with a spine in the middle. Coxa 4 anteriorly rounded, the posterior edge weakly concave.

Gnathopods subchelate. Gnathopod 1 anterior edge of basis fringed by numerous setae, ischium and merus short, carpus lobate, propodus enlarged, length/breadth ratio 3.5:2.5, palm fringed with numerous short setae, delimited by a tuft of spines, dactylus as long as palm. Gnathopod 2 basis weakly setose on the anterodistal part, ischium and merus short, carpus lobate posteriorly, lobe extended, propodus less wide than gnathopod 1, length/ breadth ratio 2:1, palm fringed with numerous short setae, palm delimited by a tuft of spines, dactylus as long as palm.

Pereopods 3–4 slender and elongate, each article except ischium long and slender. Pereopod 5 basis lobate, anterior edge rounded and fringed with small spines, ischium short, carpus, propodus, and dactylus elongate. Pereopod 6 basis distally lobate, lobe reaching merus, ischium short, merus, carpus, propodus, and dactylus slender and elongate. Pereopod 7 basis distally lobate, lobe triangular, reaching anterior third of the merus, ischium short, merus, carpus elongate, propodus broken.

Epimeral plates rounded. Uropod 1 peduncle long and spinose, rami short, outer ramus/ inner ramus ratio 1:2, inner ramus fringed with small spines, outer ramus with one small spine. Uropod 2 peduncle slightly shorter than inner ramus, outer ramus shorter than inner ramus, peduncle and rami bear short spines. Uropod 3 peduncle short, rami subequal and foliaceous. Telson long and tapered, deeply cleft.

Remarks

This species is characterized by the long spine at the inferodistal corner of article 1 of antenna 1. This long curved spine is fluted on the proximal surface and is scaly and hooked on the distal part. Article 2 of antenna 1 bears two rows of calceoli on the inferior edge in front of the spine of the first article.

The hypothesis of sonar or sound receptor function for the calceoli put forward by Lincoln and Hurley (1981), based on the radar reflector form of the calceoli, may be reinforced by the presence in Rachotropis licornia of the spine-bearing scales, similar to a vibrator positioned in front of the double rows of calceoli on article 2.

Etymology

The name is suggested by the spine of antenna 1, with a strange morphology resembling the horn of the mythological Licorne.

Relationship

A similar antenna 1 has not been described before. This species, with a smooth body, lacking a rostrum, with a long deeply cleft telson, is assigned to the complex Rhachotropis genus but it perhaps represents a new genus.

Distribution and habitat

Mid-Atlantic Ridge, Central Atlantic Ocean. This species was sampled in a sediment trap deployed 2000 m of the vent site Rainbow, at 300 m above the bottom. Thus, it is considered to be a bathypelagic species.

Figures

Figure 3. Rhachotropis licornia, FLAME 2, PPS3-H20, holotype, 10 mm. (1) Habitus; (2) antenna 1; (3) antenna 2; (4) mandible; (5) maxilla 2; (6) maxilliped; (7) gnathopod 1; (8) gnathopod 2. Scale bars: 100 mm.

Figure 4. Rhachotropis licornia, FLAME 2, PPS3-H20, holotype, 10 mm. (1) Labium; (2) maxilla 1; (3) pereopod 3; (4) pereopod 4; (5) pereopod 5; (6) pereopod 6; (7) pereopod 7; (8) uropod 1; (9) uropod 2; (10) uropod 3; (11) telson. Scale bars: 100 mm.

Figure 5. Rhachotropis licornia, FLAME 2, PPS3-H20, holotype, 10 mm. (1) Antenna 1, spine of the infero distal corner, article 1 of the peduncle; (2) antenna 1, article 1, shaft of the spine; (3) antenna 1, article 1, scales hookshaped in the half distal part of the spine; (4) antenna 1, calceoli of article 2; (5) antenna 2, setae tufts of the anterior margin of article 4; (6) antenna 2, calceolus of article 5. Scale bars: 100 mm (a); 20 mm (b); 10 mm (c).