Stenothoidae (Crustacea: Amphipoda) of hydrothermal vents and surroundings on the Mid‐Atlantic Ridge, Azores Triple Junction zone

Four new species of Stenothoidae were collected from the Azores Triple Junction zone during different French cruises on the Mid‐Atlantic Ridge. One of the species belongs to the genus Torometopa (T. saldanhae) and is the first record of this genus in the Atlantic Ocean. The three other species belong to the genus Stenothoe (S. divae, S. marvela, and S. menezgweni). It is not possible to determine from morphological and ecological characters whether these amphipod species are endemic to hydrothermal systems or are bathyal species that may be found away from vent sites.


Introduction
While we possess a lot of data on Atlantic bathyal amphipods, this is not true for amphipods that live on the Mid-Atlantic Ridge. Amphipods are not especially abundant at most locales on the Mid-Atlantic Ridge and sampling is difficult; we thus have had little material to study from each cruise. It was necessary to wait for the development of multiple programmes to obtain sufficient specimens for proper study. For example, in the samples from the FARA programme (June 1993) on Lucky Strike and Snake Pit sites, the genus Stenothoe was not present (Bellan-Santini and Thurston 1996).
During five Ifremer (Brest, France) diving cruises on the Mid-Atlantic Ridge, some specimens of Stenothoidae were found. Four new species were identified in this material: Torometopa saldanhae, Stenothoe divae, S. marvela, and S. menezgweni.
Coxa 1 small, covered by coxa 2, rounded anteriorly, coxa 3 subrectangular, coxa 4 enlarged, ovate, not posterodistally excavate. Gnathopods subchelate. Gnathopod 1 feeble, merus distally lobate, setose on the posterior side, bearing four long setae distally, carpus as long as propodus, not lobate, propodus with a short and oblique palm, defined by a spine, posterior margin with a double row of spines and a long seta. Gnathopod 2 large, merus and carpus short and lobate, propodus large, distally expanded with palm transverse, strongly indented, limited at the distal corner by a tooth, posterior margin fringed with six small spines, dactylus as long as the palm, margin smooth. Pereiopods 3-4 rectilinear. Pereiopod 5, basis not lobate in the middle of the article but a postero-distal lobe expanded to the end of ischium. Pereiopods 6-7 with expanded and lobate basis, more expanded in pereiopod 7. Merus with a large postero-distal triangular lobe. Pleonite 3 without dorsal process.
Female. Gnathopods with palm oblique, less indented than male.

Etymology
This first species of the genus Torometopa in the Atlantic Ocean is dedicated to our friend Professeur Luiz Saldanha who was one of the promoters of the Franco-Lusitanian cooperation in oceanography, especially in the hydrothermal domain.

Relationship
According to Krapp-Schickel (1996a , Table 1), Scaphodactylus and Torometopa are the only two genera of Stenothoidae with the basis of pereiopod 5 with an expanded distal lobe, and Torometopa is alone with zero to two articles in the accessory flagellum. With these characters our species is attributed to the genus Torometopa.
Torometopa was erected by Barnard and Karaman (1987). Presently, 22 species are described, only one of which is found in the northern hemisphere, from the Arctic Ocean, T. medipa . The others are known from antarctic or subantarctic zones. Krapp-Schickel (1996b) described T. medipa from Greenland at 310 m depth.
Torometopa saldanhae differs from T. medipa in the presence of eyes, the shape of the gnathopods and telson.
In consideration of the characters analysed by Krapp-Schickel (1996b) for Torometopa species, T. saldanhae is characterized by the peduncle of antenna 1 with article 1 length/ breadth ,3/1; the coxa 1 is oval with length/breadth ,3/1; the postero-distal lobe of the basis of pereiopod 5 is elongate and broadened; propodus of gnathopod 1 is regularly convex, palm not well defined.

Distribution and habitat
Mid-Atlantic Ridge, central Atlantic Ocean between 839 to 2364 m depth. The specimens coming from the sites Menez Gwen (839 m) and Lucky Strike (1648 m) were collected at 1 or 2 m from the hydrothermal fluid emissions, among sponges (Menez Gwen) or dead mussels (Lucky Strike). The specimens of Rainbow (2273 m) and of Famous zone (2364 m) were collected among gorgonians, out of the active hydrothermal areas. Therefore, this species, occurring as well on the hydrothermal sites as in the bathyal environment, can be regarded as an opportunist among the hydrothermal communities.

Diagnosis
Antenna 1 lacking nasiform process on article 1, no accessory flagellum. Mandible without palp. Gnathopod 1 with merus produced to the middle of the carpus. Gnathopod 2 with the propodus enlarged, palm oblique, excavate in the proximal part and largely indented in the distal part. Pereiopods 6-7 with expanded lobate basis. Telson entire.

Description
Male. Length 6 mm. Head slightly shorter than two first pereon segments. Eyes present of moderate size. Antenna 1 lacking nasiform process on article 1. Antenna 1 exceeding length of body, articles 1 and 2 subequal, accessory flagellum absent, flagellum with 23 articles. Antenna 2 shorter than antenna 1 but longer than half of body, flagellum with 14 articles. Palp of mandible absent (represented by a single seta), cutting edge strongly toothed, without molar. Palp of maxilla 1 biarticulate. Inner plates of maxillipeds well separated, palp four-articulate.
Epimeral plates 1 and 2 rounded, 3 rounded with the distal margin oblique. Pleonite 3 lacking dorsal process. Uropod 1 peduncle with four spines, longer than rami; uropod 2 shorter than 1, outer ramus three-quarters of the inner with, respectively, three spines. Uropod 3 with three lateral and four distal spines on the peduncle, ramus biarticulate, the proximal article with two distal spines. Telson entire, longer than broad, each lateral margin with one spine and one seta, one pair of subterminal setae.
Female. Female with the propodus of gnathopod 2 without excavation, but strongly indented.

Etymology
Name of the species is derived from the name of the DIVA cruise.

Relationship
This species is referable to the genus Stenothoe, due to no mandibular palp, and maxilla 1 with two-articulate palp. Seven species occur in the deep Atlantic: S. aequicornis Stephensen 1931, S. bosphorana Sowinski, 1898, S. coutieri Chevreux, 1908, S. dollfusi Chevreux, 1887, S. macrophthalma Stephensen, 1931, S. marina Bate, 1856, and S. richardi Chevreux, 1895. Three species have been found in the North Sea or Norway Sea at more than 180 m deep: S. megacheir (Boeck, 1871), S. microps Sars, 1895, and S. tenella Sars, 1895. None of these species has a propodus of gnathopod 2 enlarged with a palm obliquely excavated in the proximal part and strongly indented in the distal part, a gnathopod 1 with merus distally produced into a setose lobe reaching to the middle of the carpus, and a telson with only one pair of spines. Stenothoe divae is considered a new species.

Distribution and habitat
Mid-Atlantic Ridge, south-west of Azores, Segment 38uN, 788 m depth. The specimens were collected on indurated sediment, among spongia, hydrozoans, gorgonians, and colonial tunicates.

Diagnosis
Species blind. Antenna 1 lacking nasiform process on article 1, as long as body, no accessory flagellum. Mandible without palp. Gnathopod 1 with merus produced to the middle of the carpus. Gnathopod 2 with the propodus enlarged, palm excavate in the distal part. Pereiopods 6-7 with basis slightly lobate. Telson entire.

Description
Female. Female with oostegites, length 4 mm. Head slightly shorter than first segment of pereon. Eyes absent. Antenna 1 as long as body. Antenna 1 without nasiform process on article 1, articles 1 and 2 subequal, article 3 one-third of article 2, accessory flagellum absent, flagellum 19-articulate. Antenna 2 shorter than antenna 1, flagellum 11-articulate. Palp of mandible absent, incisor strongly dentate, no molar. Palp of maxilla 1 biarticulate, inner plate with a distal seta, outer plate with five teeth distally. Maxilla 2 inner plate bearing two lateral setae and one distal. Inner plate of maxilliped well separated, palp elongate four-articulate.
Epimeral plates 1-2 rounded, 3 with the posterior margin oblique and the postero-distal corner rounded. Uropod 1, rami slightly unequal, shorter than peduncle. Uropod 2 shorter than uropod 1, rami subequal. Uropod 3 with peduncle shorter than biarticulate ramus, three spines on the peduncle and three on the first article of the ramus. Telson entire, triangular, longer than wide, with only one pair of proximal spines.

Etymology
The name of the species is derived from the name of the MARVEL cruise.

Relationship
The single blind species of Atlantic Stenothoe is S. dactylipotens Chevreux, 1908, from the Azores, considered by Krapp-Schickel (1976) as a synonym of S. bosphorana Sowinski, 1898, but probably different (eyes, gnathopod 2, uropod 3, telson). According to Chevreux's description of the Azores specimen, S. marvela differs in the size of the antennae, the shape of gnathopod 2, and the telson. The shape of gnathopod 2 of S. marvela is near that of S. dollfusi Chevreux, 1887, but the two species differ in the size of the antennae, the absence of eyes in S. marvela, and the number of spines on the telson. Stenothoe marvela is a new species.

Distribution and habitat
Mid-Atlantic Ridge, south-west of Azores, Segment 36uN, Famous zone, 2364 m depth. The specimen was collected among an abundant bathyal community composed of spongia, hydrozoa, gorgonians, echinoderms, and colonial tunicates.

Description
Female. Female, length 5 mm. Head as long as first two pereon segments. Eyes of moderate size, brown-red in alcohol. Antenna 1 lacking nasiform process on article 1. Antenna 1 as long as body length, articles 1 and 2 subequal, article 3 short, one-quarter of the second, accessory flagellum vestigial, flagellum with 21 articles. Antenna 2 shorter than antenna 1, flagellum with 21 articles. Palp of mandible absent, cutting edge strongly toothed, without molar. Palp of maxilla 1 biarticulate, inner lobe with one small seta, outer lobe with six large teeth. Maxilla 2 with inner lobe shorter than outer, and bearing at the inner margin six setae. Inner plates of maxillipeds well separated, palp long, four-articulate.

Relationship
This species differs from other Atlantic Stenothoe species in the shape of the propodus of gnathopod 2, the presence of a vestigial accessory flagellum (this is not a good character because it is very difficult to distinguish). Gnathopod 2 differs little from that of S. marvela, but the presence of eyes, the number of spines on the telson, and the setation of the inner lobe of maxilla 2 are good characters, according to Krapp-Schickel (1996a), that distinguish this new species from S. marvela.

Discussion
At the Azores Triple Junction, the depth of the ridge decreases from 2400 to 850 m. The Triple Junction area is a complex zone, and at the hydrothermal vents the fauna is a mix of species endemic to vent environments and bathyal species (Desbruyères et al. 2001). Stenothoidae collected in the past in deep stations near the Azores are Stenothoe coutieri Chevreux, 1908, from 38u319N, 26u50.259W, 845 m, andS. dactylipotens Chevreux, 1908 (now synonymized with S. bosphorana), from 37u409N, 26u269W 1919 m, from outside the influence of hydrothermal activity; they were not found again. It is not possible to say if the species described here have any relationship with hydrothermal phenomena. Torometopa saldanhae is present at hydrothermal sites as well as away from venting activity. Stenothoe menesgweni was also collected at a hydrothermal site; S. divae and S. marvela occur in nonhydrothermal sites.