First report of the black gemfish Nesiarchus nasutus ( Perciformes : Gempylidae ) in Argentinean

The gempylid Nesiarchus nasutus is reported for the first time from Argentinean waters, the southernmost occurrence of the species in the Southwestern Atlantic. This suggests that the fluctuating environmental characteristics of the area would be proper for the presence of tropical and subtropical species.


Introduction
The family Gempylidae comprises sixteen genera (Diplospinus, Epinnula, Gempylus, Lepidocybium, Nealotus, Neoepinnula, Nesiarchus, Paradiplospinus, Promethichthys, Rexea, Rexichthys, Ruvrttus, Thyrsites, Thyrsitoides, Thyrsitops and Tongaichthys) and 24 species (Nakamura and Parin, 1993;Roberts and Stewart, 1997;Nelson, 2006).This family is characterized by body oblong or elongate and compressed with two nostrils on the both sides of the snout.A large nonproctractile mouth with protruding lower jaw, strong teeth in jaws and exposed maxilla, two continuous dorsal fin, with the base of spinous dorsal fin longer than that of soft-rayed fin (excluding finlets).Anal fin similar to the second dorsal fin, with one to three spine and 8-35 soft rays; separated finlets present behind dorsal and anal fins in some species.Pectoral fin shorter than head and pelvic fin reduced or absent with one spine and five soft rays, caudal fin forked, one or two lateral lines.The number of vertebrae is 32-58 (Nakamura, 1986(Nakamura, , 1990;;Nakamura and Parin, 1993;Figuereido and Menezes, 2000;Nelson, 2006).
The black gemfish Nesiarchus nasutus Johnson 1862 is distributed worldwide in tropical and subtropical seas except in eastern Pacific and northern Indian oceans.Adults are bentho-to meso-pelagic, dwelling on the continental slope or underwater rises at about 200 to 1200 m, and migrate to midwater at night (Nakamura and Parin, 1993).Larvae and juveniles are epi-to meso-pelagic, being quite common in oceanic ichthyoplankton, but only found in the tropic (Nakamura and Parin, 1993).Feed on squids, fish and crustaceans (Nakamura and Parin, 1993).
The aim of this work is to report the range extension of the black gemfish Nesiarchus nasutus in the Southwest Atlantic.

Material and Methods
The specimen was caught using a bottom trawling [model: Nichimo MT-660-SR (MT) net trawling having 1500 m 2 of mouth opening and 60 mm mesh in the cod end] by the R/V Kaiyo Maru cruise at 100 m deep.
The specimen was identified according to Nakamura and Parin (1993), fixed in formalin 4%, preserved in 75% ethanol and deposited in the Ichthyological Collection of the Instituto Nacional de Investigación y Desarrollo Pesquero, Mar del Plata, Argentina (INIDEP 838).

Results
The specimen of N. nasutus (Figure 1) reported here was caught off the Argentinean coast (38° 32' S -52° 03' W) on 14 October 2005 (Figure 2) with a temperature of 15.7 °C and a salinity of 35.8.Diagnostic characteristics of N. nasutus agree with those described in the pertinent literature (Nakamura et al., 1983;Nakamura and Parin, 1993): body fairly elongate and strongly compressed; lower jaw beyond tip of upper jaw; vomer edentate; gill rakers degenerated; a single fairly straight lateral line, from above upper angle of opercula, gradually sloping posteriorly and running straight from mid first dorsal fin to caudal fin base.Body dark brown with violet tint; fin membranes black; margin of anus black.(Nakamura et al., 1983;Nakamura and Parin, 1993).Counts and measurements presented in Table 1.

Discussion
The black gemfish N. nasutus commonly inhabit tropical and subtropical waters; the larger specimens can reach cold-temperate waters off Iceland, Norway, northern Japan and southern New Zealand (Nakamura and Parin, 1993).The occasional presences of tropical and subtropical fishes in Argentinean waters are well documented in the literature (Cousseau and Figueroa, 1989;Díaz de Astarloa and Figueroa, 1995;Díaz de Astarloa et al., 2000;Figueroa et al., 2000;Rico and Acha, 2003;Scenna et al., 2006;Solari et al., 2010;Spath et al., 2012;Delpiani et al., 2013).The Argentinean marine biogeographic province (Briggs and Bowen, 2012) is characterized by the encounter between the warm, southward-flowing Brazilian Current, and the cold, northward flow, of the Malvinas Current (Figueroa et al., 1998) and water discharge of continental output (Lucas et al., 2005).There is not clear explanation of how tropical and subtropical fish species arrive to the Argentinean province, but Scenna et al. (2006) and Seeliger and Odebrecht (1997) proposed that species of tropical and Antarctic waters might appear sporadically because they use the Brazilian and Malvinas currents as transportation.Another explanation for the occurrence of these species south of its range distribution may be phenomena such as eddies, which can be considered a quasi-stationary structured of Brazilian-Malvinas Confluence (Gordon, 1981).The southern boundary of the Brazilian warm water is accompanied by the intermittent formation of warm core anticyclonic eddies (Gordon, 1981;Olson et al., 1988) and represent a highly complicated system reflected in composition of mesopelagic fish fauna in the surveyed area.The generation of anticyclonic frontal vortices and their further displacement southward leads to the transport of thermophilic forms out of the sub-tropical zone to the sub-Antarctic (Figueroa et al., 1998).Beyond the explanations given above, it cannot be dismissed the possibility that this fishes are not occasional, but are part of a permanent population utilizing more southerly habitats than previously documented.

Figure 2 .
Figure 2. Map from the survey area where the Nesiarchus nasutus specimen was found.The 50 and 200 m isobaths are shown as solid and dashed lines, respectively.The rectangle inset shows the location of the study area in South America.

Figure 1 .
Figure 1.Specimen of Nesiarchus nasutus collected with a bottom trawling by the R/V Kaiyo Maru cruise.

Table 1 .
Morphometric measurements and proportional relationships (% standard length = %SL; % head length = %HL) for a single specimen of Nesiarchus nasutus caught off the Argentinean waters on 2005.