A new deep-sea species of Barathronus Goode & Bean from Brazil, with notes on Barathronus bicolor Goode & Bean (Ophidiiformes: Aphyonidae)

A new species of Barathronus (Ophidiiformes: Aphyonidae) is described from a single, mature male specimen (101 mm SL) bottom trawled on the continental slope of Rio Grande do Norte, northeastern Brazil, between 1,964 and 2,045 m depth. The new species is diagnosed among congeners by the following combination of characters: peritoneum transparent, deep-set eyes not visible, eight fangs on vomer, anal fin rays 69, predorsal length 42.0% SL, preanal length 49.5% SL, penis long, slender, and lacking a pair of lobes at its base, and presence of a ventral flexure of the anterior 2-3 vertebrae. Additionally, morphological data of three specimens of Barathronus bicolor collected in Brazilian waters are presented and compared with those from 51 specimens from the western Central Atlantic.


Introduction
The Aphyonidae (Ophidiiformes) is composed of six genera and 22 valid species of mostly small to mediumsized, viviparous, bathypelagic and benthopelagic fishes. Diagnostic features of the family include a loose, transparent and gelatinous skin, poorly developed (or not visible) eyes, basibranchial tooth patches absent, and long dorsal and anal fins joined to the caudal fin (Nielsen et al., 1999). Previous records of the family in the western South Atlantic are restricted to two specimens of Barathronus bicolor Goode & Bean, 1886 and three specimens of Aphyonus gelatinosus Günther, 1878, all collected off southeastern Brazil (Séret & Andreata, 1992;Franco et al., 2007;Franco, 2010).
Four species of Barathronus are reported from the Atlantic Ocean: B. bicolor in the West Atlantic from off South Carolina to off Rio de Janeiro State; B. multidens Nielsen, 1984 from off Florida and Morocco;B. unicolor Nielsen, 1984 from the western North Atlantic (38°28'N, 70°52'W); B. parfaiti (Vaillant, 1888) from between the Azores and France. Nielsen (1969: 53) listed two specimens of B. parfaiti, the holotype (MNHN 86-554) and a 100mm SL specimen collected in the Azores in 1896 (MOM, uncatalogued). The holotype of B. parfaiti is a poorly preserved 40-mm SL specimen, and few characters stated in the original description of this species can be verified today from examination of the specimen. Additional specimens of Barathronus strongly indicate that the MOM specimen does not belong to B. parfaiti, but rather to an undescribed species of the genus not treated herein. Six species of Barathronus are reported from the Indian and Pacific oceans: B. affinis Brauer, 1906, from off the Maldive Islands, Indian Ocean; B. bruuni Nielsen, 1969, from the western South Indian Ocean; B. diaphanus Brauer, 1906 andB. maculatus Shcherbachev, 1976, both from the Indian and 54 western Pacific oceans; B. pacificus Nielsen & Eagle, 1974, from the eastern North Pacific; B. solomonensis Nielsen & Møller, 2008, known from the Solomon Sea, Pacific Ocean (Nielsen et al., 1999;Nielsen & Møller, 2008).
In 2011, a deep-sea expedition performed by the R/V Seward Johnson on the continental slope off Rio Grande do Norte State, northeastern Brazil, collected several deep-sea fishes, including one specimen of Barathronus taken at approximately 2,000 m depth. Comparison with all valid species of the genus (Tables 1-2) revealed that this specimen belongs to a new species, which is described herein. In addition, a 117-mm male specimen of B. bicolor collected off Rio de Janeiro State, between 990 and 994 m depth, represents the third specimen of the species from the western South Atlantic. The three specimens of B. bicolor from Brazilian waters are compared to specimens from the western Central Atlantic.

Material and Methods
Material examined includes 95 specimens of all valid species of Barathronus and the holotype of the new species described herein (see Comparative Material for a complete list). Measurements and counts (Tables 1-2) follow those of Nielsen et al. (1999) Nielsen (1969;42 specimens) and Rannou et al. (1975;9 specimens). ** Based on 10 specimens; tip of rays often broken.    Barathronus linsi and the six Indo-Pacific congeners differ in a series of features, which are summarized in Table 2. In addition, no species of Barathronus have so far been recorded in both the Atlantic and Indo-Pacific oceans. Among Indo-Pacific species, B. linsi is most similar to B. affinis (known from a single juvenile specimen) in meristic characters, by the presence of a ventral flexure of the anterior vertebrae, and the transparent peritoneum, but they differ by the number of fangs on vomer (8 vs. 2) and the number of long gill rakers (25 vs. 20).

Barathronus linsi, new species
Description. Meristic and morphometric data of holotype are presented in Tables 1 and 2. Scales absent, skin loose, gelatinous and translucent, sensory pores highly indistinct. Head down-bent, slightly thicker than body. Dorsal, caudal and anal fins united. Dorsal-fin origin well anterior to vertical through midpoint of body, anal-fin origin at midpoint of body. Pelvic fins each with a single, slender ray; pelvic-fin base below hind part of operculum. Pectoral peduncle as broad as long. Eyes not visible. Mouth opening oblique. Nostrils with low rim. Opercular spine covered by skin. Musculi infracarinalis mediales yellowish white; ratio between length and height of "middle fields" (Nielsen, 1969: 9) ca. 0.5. Anterior gill arch with 26 prolonged rakers (up to 2.2% SL): four on upper branch, one in the angle and 21 on lower branch. About 30 very small gill filaments (0.6% SL). Pseudobranchial filaments apparently absent. Testes large (22% SL), nearly filling abdominal cavity. Intromittent organ a 9-mm long slender penis, covered proximally by urogenital hood. Lobes at base of penis absent.
Otolith: Otolith 2.2 mm long, compact, with nearly flat inner face and strongly convex outer face. Outline round, with somewhat depressed predorsal rim. Otolith height 1.2 in length; otolith thickness 1.7 in height. Sulcus moderately large, positioned centrally on inner face and terminating at some distance from otolith rims. A single, undivided, small, narrow colliculum located in anterior part of sulcus. Small bulge on inner face in front of sulcus (Fig. 2). with long and slightly laterally compressed neural spines decreasing in length posteriorly. Short parapophyses developed on vertebrae 4-33. Pleural and epipleural ribs absent. Vertebral centrae hourglass-shaped (Fig. 3).
Color. Recently collected specimen (Fig. 1A) overall yellowish to reddish white due mostly to the underlying, somewhat darker, muscle coloration and blood vessels (mostly skin capillaries). No black pigmentation observed either in skin or on peritoneum. Preserved specimen (Fig. 1B) yellowish white (no evidence of bleaching after two years of preservation).

Distribution.
Known from the holotype, collected between 1,965 and 2,045 m depth in the Potiguar basin, of Rio Grande do Norte, northeastern Brazil (Fig. 4).  Notes on Barathronus bicolor in the western South Atlantic. A female specimen (73 mm SL) of B. bicolor, collected in 1987 at 610 m depth off Rio de Janeiro State, southeastern Brazil (Séret & Andreata, 1992), represented until recently the single record of this species in the western South Atlantic. Twenty years later, Franco et al. (2007) reported an additional specimen (98 mm SL, male) collected between 1,605 to 1,640 m off Rio de Janeiro State. Meristic and morphometric data of these two specimens and of one additional specimen reported herein (NPM 1243;Fig. 5) are compared to those from the currently known 51 specimens of B. bicolor collected in the western Central Atlantic (Table 1). Morphological characters examined are highly congruent between the two clusters, even though the geographic distance between southeastern Brazil and the western Central Atlantic is more than 5,000 km. The only significant difference between examined characters occurs in the penis length, which measures about 4% SL in the only fully mature male collected off southeastern Brazil (NPM 1243), vs. 10-14% SL in specimens from the western Central Atlantic.

Discussion
Records from both the north and south hemispheres show that B. bicolor is widely distributed in the western Atlantic, although a disjunctive distribution cannot be ruled out given that the species has not been recorded in the South Atlantic north of Rio de Janeiro State. The geographic distribution of B. linsi is obviously highly speculative, as the species is known from the holotype. However, it seems reasonable to suppose that B. bicolor occurs off northeastern Brazil, and that therefore the distributions of B. linsi and B. bicolor probably overlap partially. Barathronus linsi was collected at a depth of approximately 2,000 m, whereas B. bicolor is more commonly collected between 500 and 900 m, at least in the western Central Atlantic (Nielsen, 1969;Rannou et al., 1975). If those two benthopelagic species are partially sympatric, it is possible that they might live in different depths on the continental slope.
In the last five years, two new deep-sea species of the Ophidiiformes have been described from Brazilian waters (Nielsen, 2009;this paper), and more than a dozen species of the group were recorded for the first time in the region (Franco et al., 2007;Nielsen et al., 2009). There is also an increase in the knowledge on the diversity of several Brazilian deep-sea fish groups in addition to the Ophidiiformes (e.g., Carvalho et al., 2005;Anderson & Mincarone, 2006;Mincarone & Anderson, 2008;Franco et al., 2009;Melo et al., 2009Melo et al., , 2010Lima et al., 2011;Mincarone et al., 2014 ). That situation results largely from the fact that only in the last decade collections made in recent fishing surveys performed offshore Brazil started to be properly studied. There is still an immense potential for biological discoveries if those collections are further examined, but more collecting efforts in the region are necessary. However, the relatively few deep-sea fishing surveys conducted in Brazilian waters in the last years, especially after the end of REVIZEE (Program for Assessment of the Sustainable Yield of Living Resources of the Exclusive Economic Zone, Brazilian Government), have been totally focused in regions of deep-water oil drilling, such as the Campos Basin, off Rio de Janeiro State. That situation is not negative in itself, but the knowledge on the Brazilian deep-sea diversity will continue to be geographically fragmented and mostly restricted to the few regions were offshore oil drilling is conducted if more encompassing surveys are not planned for the next years.