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New record and morphometry of the bluntnose sixgill shark Hexanchus griseus (Chondrichthyes: Hexanchidae) in Baja California Sur, Mexico.

The bluntnose sixgill shark, Hexanchus griseus, is the only species of the genus Hexanchus recorded in the Mexican Pacific (Compagno, 1984; Fischer et al., 1995). It is one of the largest carnivorous sharks, occupying the highest trophic level, reaching 4.82 m in total length (Ebert, 2015). The species possesses six pairs of gill slits, a subterminal mouth and a short, broadly rounded blunt snout. It is grey with or without irregular brown spots. Also, the distance from the dorsal fin to the caudal fin is equal to the length of the dorsal fin base (Compagno, 1984).

Records of H. griseus in Mexico have come from catches and fish lists drawn up in Sonora (Santana-Morales, 2005; Robertson & Allen, 2015), Baja California (Miller & Lea, 1972; Castro-Aguirre & Garcia-Dominguez, 1988; Goodson, 1988; Ruiz-Campos et al., 2010) and Baja California Sur (Castro-Aguirre et al., 2003; Robertson & Allen, 2015). Of the studies reporting the occurrence of this species, that of Castro-Aguirre et al. (2003) is the only one, however, that includes morphometric measurements, those of the head of an unsexed specimen recorded in Bahia Magdalena, Baja California Sur, Mexico. Here, we present the first record and morphometry of a complete specimen of 107 cm in total length, caught 180 km south of Bahia Magdalena, off the fishing camp of Punta Lobos, Baja California Sur, Mexico.

A juvenile female of H. griseus was caught incidentally on 18 December 2013 by coastal fishermen using longlines. Punta Lobos is located at 23[degrees]25'N, 110[degrees]14'W, on the west coast of the Baja California Peninsula (Fig. 1).

In this area, oceanographic conditions at a depth of 200 to 2500 m are characterized by a temperature range of 4 to 12[degrees]C, a stable salinity of 34 to 35, and dissolved oxygen values of 2.5 to 4 mL [L.sup.-1] (Locarnini et al., 2013; Zweng et al., 2013; Garcia et al., 2014). Such conditions coincide with the known habitat of H. griseus in its area of distribution, i.e., hypoxic environments with low temperature and high hydrostatic pressure (Compagno, 1984; Barnett et al., 2012). Although this species mainly inhabits the continental slope, juveniles have been reported to make incursions into neritic environments (over the continental shelf) during the night, when they undergo vertical migration possibly related to feeding (Compagno, 1984; Andrews et al, 2007, 2009, 2010; Comfort, 2012).

The caught specimen was dark grey without spots on the back and white on the belly, with no clear boundary between the two colours (Fig. 2). Its flabby body, typical of organisms inhabiting the mesopelagic or bathypelagic zone (Randall & Farrell, 1997), had an epidermis entirely covered with mucus, with no ectoparasites detected.

Taxonomic identification was performed using Fischer et al. (1995) dichotomous key as well as morphometry of the whole organism (Table 1). The shark specimen was transported on ice to the Laboratory of Fish Ecology of the Interdisciplinary Centre of Marine Sciences (CICIMAR-IPN, by its Spanish acronym) in La Paz, BCS, Mexico.

Despite the fact that H. griseus is a top predator, knowledge of this shark species is scarce and insufficient (Ebert, 2002, 2003), in both coastal and deep-water ecosystems (Barnett et al., 2012). This study provides the first record of H. griseus that includes detailed morphometric data of a complete individual. This individual was caught in shallow waters off the west coast of Baja California Sur, providing additional information on the species' distribution in Mexico. As for its conservation status, the blunt-nose sixgill shark is categorised as "near threatened" by the IUCN (Cook & Compagno, 2005).

In Mexico, information relative to H. griseus is insufficient for realize a proper assessment of the status of its population; furthermore, the limited information on its reproductive biology suggests that this taxon may be vulnerable to fishing (Barnett et al, 2012). The punctual catches recording of H. griseus taking advantage of them to produce scientific knowledge about basic aspects of its biology, will allow better understanding of the ecological niche occupied by this species in the national territory.

DOI: 10.3856/vol45-issue4-fulltext-20

ACKNOWLEDGEMENTS

We thank the fishermen of Punta Lobos, and Nefertiti Tayde Roldan-Wong, and Janna Dominguez-Boza. Nicole Reguera-Rouzaud for her help in drawing the figures. FGM thanks the Instituto Politecnico Nacional for funding from the Commission of Operation and Promotion of Academic Activities (COFAA) and Performance Incentive for Researchers (EDI) grants.

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Received: 20 September 2016; Accepted: 24 January 2017

Edgar Eduardo Becerril-Garcia (1,3), Carlos Augusto Aguilar-Cruz (2) Alexis Alejandro Jimenez-Perez (2) & Felipe Galvan-Magana (1)

(1) Instituto Politecnico Nacional, Centro Interdisciplinario de Ciencias Marinas, La Paz, BCS, Mexico

(2) Universidad Autonoma de Baja California Sur, La Paz, BCS, Mexico

(3) Pelagios Kakunja A.C., La Paz, BCS, Mexico

Corresponding author: Felipe Galvan-Magana (galvan.felipe@gmail.com)

Corresponding editor: Diego Giberto

Caption: Figure 1. Previous records of Hexanchus griseus and new location of Punta Lobos, BCS, Mexico.

Caption: Figure 2. Juvenile female of Hexanchus griseus caught off the fishing camp of Punta Lobos, BCS, Mexico.
Table 1. Morphometry of the bluntnose sixgill shark female
captured off Punta Lobos, BCS, Mexico.

Measure                             Length   Proportion
                                     (cm)       (%)

TL (total length)                   107.2      100.00
FL (fork length)                     84.6      78.92
PCL (pre-caudal-fin length)          74.9      69.87
HDL (head length)                    22.1      20.62
POB (preorbital length)              5.2        4.85
PP1 (prepectoral-fin length)         22.1      20.62
PP2 (prepelvic-fin length)           27.8      25.93
PAL (preanal-fin length)             63.4      59.14
EYL (eye length)                     1.6        1.49
ING (intergill length)               1.4        1.31
P1A (pectoral-fin anterior margin)   5.4        5.04
P1B (pectoral-fin base)              15.3      14.27
CDM (dorsal caudal-fin margin)       10.3       9.61
CPV (oreventral caudal-fin margin)   30.1      28.08
CFW (caudal-fin fork width)          3.4        3.17
CFL (caudal-fin fork length)         5.4        5.04
P2L (pelvic-fin length)              2.9        2.71
P2B (pelvic-fin base)                8.7        8.12
ANL (anal-fin length)                1.7        1.59
ANA (anal-fin anterior margin)       8.2        7.65
ANB (anal-fin base)                  5.4        5.04
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Title Annotation:Short Communication
Author:Becerril-Garcia, Edgar Eduardo; Aguilar-Cruz, Carlos Augusto; Jimenez-Perez, Alexis Alejandro; Galva
Publication:Latin American Journal of Aquatic Research
Date:Sep 1, 2017
Words:1762
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