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Latin american journal of aquatic research

On-line version ISSN 0718-560X

Lat. Am. J. Aquat. Res. vol.48 no.3 Valparaíso July 2020

http://dx.doi.org/10.3856/vol48-issue3-fulltext-2436 

Short Communication

New records and morphometry of the Atlantic sixgill shark Hexanchus vitulus in the Caribbean coast of Guatemala

Cristopher G. Avalos-Castillo1 

Omar Santana-Morales1  2 

Edgar E. Becerril-García3 

Elisa Areano1 

1Fundación Mundo Azul, Ciudad de Guatemala, Guatemala

2Facultad de Ciencias Marinas, Universidad Autónoma de Baja California, Ensenada, México

3Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas, La Paz, México

ABSTRACT

The present study constitutes the first records and morphometry of Hexanchus vitulus on the Caribbean coast of Guatemala. A total of 10 Atlantic six-gill sharks were captured by artisanal fishers during 2015-2019, showing a sex ratio of 2.3:1 (males:females) and a total length that ranges 61-165 cm. Morphometric measurements are provided for future comparisons in other coastal regions. Recorded sharks corresponded mostly to sexually mature individuals, which differs from that reported for different areas of the Caribbean.

Keywords: Hexanchiformes; measurements; distribution; segregation; deep-sea species; Central America

The Atlantic sixgill shark Hexanchus vitulus Springer & Waller, 1969 is one of the three species of the genus Hexanchus, along with the bluntnose sixgill shark H. griseus (Bonnaterre, 1788) and the bigeye sixgill shark H. nakamurai Teng, 1962 (Compagno et al., 2005; Daly-Engel et al., 2019). In previous decades, both H. nakamurai and H. vitulus were recognized as the same species; however, recent genetic studies have suggested the divergence between both taxa (Ebert et al., 2013; Daly-Engel et al., 2019). In the Caribbean Sea, the knowledge of H. vitulus and other deep-sea sharks is scarce, as these species were infrequently captured by local artisanal fishers, and therefore scientific research regarding its biology has not been developed (Hacohen-Domené et al., 2016, 2017). Although there are occasional records of H. vitulus in Belize (Daly-Engel et al., 2019), Colombia (Mejía-Falla & Navia, 2019), Venezuela (Ehemann et al., 2019), and the Dutch Caribbean (Van Beek et al., 2012), there is no information about its presence in Guatemalan waters or its morphometry in any locality of the Caribbean Sea. The present study aims to report the first records with morphometry data of H. vitulus in Guatemala to provide useful information for future comparisons with specimens caught in other regions.

A total of 10 sixgill sharks were captured 80 km offshore the port of El Quetzalito (15°44'25”N; 88°16'27”W) by artisanal Guatemalan fishers (Fig.1). The date of capture, fishing gear, used bait, sex, total length (TL), precaudal length (PL), and fork length (FL) were registered from each shark (Table 1). Taxonomic identification was carried out by the descriptions published by Compagno et al. (2005) and Daly-Engel et al. (2019).

Figure 1 Location site (▴) where the specimens of Hexanchus vitulus were captured in the Guatemalan Caribbean during 2015-2019. 

Table 1 Records of Hexanchus vitulus on the Caribbean coast of Guatemala during 2015-2019. TL: total length; FL: fork length; PL: precaudal length; CL: clasper length. All measurements are reported in centimeters. 

Date Fishing gear Bait Sex Maturity TL FL PL CL
21/12/2015 Gillnet F Immature 61 45 41
20/03/2016 Gillnet M Mature 152 119 110 12
15/02/2017 Longline Albula vulpes M Mature 144 112 104 14
23/05/2017 Longline Megalops atlanticus F Mature 165 127 117
26/05/2017 Longline Mixed species M Mature 153 126 116 15
03/03/2018 Gillnet F Mature 147 107 100
07/03/2018 Longline Sarda sarda M Mature 148 119 106 15
02/02/2019 Longline Sarda sarda M Mature 152 116 106 15
06/06/2019 Longline Megalops atlanticus M Mature 150 115 105 16
06/06/2019 Longline Megalops atlanticus M Mature 148 114 104 17

All the specimens were identified as H. vitulus due to the presence of six gills, a slender body with brown dorsal and white ventral coloration, narrow head and mouth, anterior serrations on lower teeth, and large eyes (Compagno et al., 2005; Fig. 2). These characte-ristics are the same described for H. nakamurai, as H. vitulus is considered a cryptic species with no evident external differences (Daly-Engel et al., 2019). The length of the specimens ranged between 61-165 cm TL, with a mean of 142 ± 29 cm TL. All the captured sharks were mature individuals, except for one juvenile female caught in 2015 (Table 1). The sex ratio was 2.3:1 (males:females), where 90% of the sharks were mature individuals. Finally, a total of 82 measurements were obtained from the last captured individual caught in June 2019 (Table 2).

Figure 2 One of the 10 Atlantic sixgill sharks Hexanchus vitulus captured in the Guatemalan Caribbean during 2015-2019; a) full body showing one dorsal fin and six-gill slits, b) upper and lower comb-shaped teeth, c) large eyes. 

Table 2 Morphometry (length (cm) and proportion (%)) of a mature male of Hexanchus vitulus caught on June 6, 2019 on the Caribbean coast of Guatemala. 

Measure Length (cm) Proportion (%)
Total length 148 100.0
Fork length 114 77.0
Precaudal length 104 70.3
Pre-first dorsal-fin length 82 55.4
Head length 37 25.0
Prebranchial length 25 16.9
Prespiracular length 20 13.5
Preorbital length 9 6.1
Prepectoral-fin length 32 21.6
Prepelvic-fin length 68.5 46.3
Snout-vent length 74 50.0
Preanal-fin length 90 60.8
Dorsal caudal-fin space 15.7 10.6
Pectoral-fin pelvic-fin space 30.1 20.3
Pelvic-fin anal-fin space 11.7 7.9
Anal-fin caudal-fin space 11.5 7.8
Pelvic-fin caudal-fin space 28.5 19.3
Vent caudal-fin length 60.1 40.6
Prenarial length 2.8 1.9
Preoral length 8 5.4
Eye length 5.3 3.6
Eye height 3 2.0
Intergill length 6.5 4.4
First-gill slit height 13 8.8
Second-gill slit height 10.1 6.8
Third-gill slit height 8.5 5.7
Fourth-gill slit height 8 5.4
Fifth-gill slit height 7.2 4.9
Sixth-gill slit height 6 4.1
Pectoral-fin anterior margin 17 11.5
Pectoral-fin base 8.7 5.9
Pectoral-fin inner margin 7.3 4.9
Pectoral-fin posterior margin 12.8 8.6
Pectoral-fin height 22.7 15.3
Pectoral-fin length 15 10.1
Dorsal caudal-fin margin 43.8 29.6
Preventral caudal-fin margin 14.1 9.5
Upper postventral caudal-fin margin 24.5 16.6
Lower postventralcaudal-fin margin 6.6 4.5
Caudal-fin fork width 11 7.4
Caudal-fin fork length 10.6 7.2
Subterminal caudal-fin margin 5 3.4
Subterminal caudal-fin width 4.5 3.0
Terminal caudal-fin margin 8.3 5.6
Terminal caudal-fin lobe 10.5 7.1
First dorsal-fin length 10.5 7.1
First dorsal-fin anterior margin 9.5 6.4
First dorsal-fin base 8.5 5.7
First dorsal-fin height 5.8 3.9
First dorsal-fin inner margin 3.4 2.3
First dorsal-fin posterior margin 7 4.7
Pelvic-fin length 22 14.9
Pelvic-fin anterior margin 7 4.7
Pelvic-fin base 10 6.8
Pelvic-fin height 5 3.4
Pelvic-fin inner margin 13 8.8
Pelvic-fin posterior margin 19 12.8
Anal-fin length 8.3 5.6
Anal-fin anterior margin 3.4 2.3
Anal-fin base 5.3 3.6
Anal-fin height 5 3.4
Anal-fin inner margin 3.6 2.4
Anal-fin posterior margin 5.3 3.6
Head height 16 10.8
Trunk height 18 12.2
Caudal-fin peduncle height 13 8.8
Second dorsal-fin origin anal-fin origin 5 3.4
pelvic-fin midpoint second dorsal-fin origin 8.5 5.7
Mouth length 10.3 7.0
Mouth width 17 11.5
Internarial space 1.5 1.0
Anterior nasal-flap length 0.5 0.3
Clasper outer length 8 5.4
Clasper inner length 17 11.5
Clasper base width 3.7 2.5
Interorbital space 11.5 7.8
Spiracle length 0.5 0.3
Eye spiracle space 5.5 3.7
Head width 18 12.2
Trunk width 29 19.6
Tail width 8.9 6.0
Caudal-fin peduncle width 4 2.7

The presence of H. vitulus in the Caribbean has been recently discussed by Daly-Engel et al. (2019) through the analysis of captured individuals in Belize. In such a study, immature sharks were obtained mostly during August, September and October, at depths of 242-333 m, and temperatures of 13-17°C. In the present work, the sharks were caught through similar fishing techniques and at a similar depth (270-350 m), but with a high presence of mature individuals during March, May and July. These preliminary observations could suggest those mature individuals may be segregated from immature sharks of northern areas like Belize, which can be related to nursery areas or differences in the feeding sites for each group.

Regarding its trophic ecology, the presence of mature and immature individuals in several areas of the Caribbean could be related to differences in feeding habits according to maturity, as it has been suggested for similar species such as H. griseus (Compagno et al., 2005; Becerril-García et al., 2017). The preference of H. vitulus for benthic fishes and crustaceans could include hunting or scavenging of other prey such as cephalopods, marine mammals, or other elasmobranchs (Compagno et al., 2005). In this manner, a potential ontogenetic dietary shift could be determined through the analysis of stomach contents and stable isotopes analysis. However, these statements are beyond the scope of this paper and should be evaluated for future studies regarding deep-sea elasmobranchs of the Caribbean Sea.

Future research related to the biodiversity of elasmobranchs in the Caribbean should include the use of non-lethal techniques, such as baited remote under-water video or environmental DNA analysis at different localities, depths and months (Hacohen-Domené et al., 2017; Daly-Engel et al., 2019), which could be useful to provide some insights about environmental conditions effects on elasmobranch's ecology. Artisanal fishers and deep-sea fisheries could increase in the area for the economic exploitation of sea bottom resources (Daly-Engel et al., 2019).

ACKNOWLEDGMENTS

We thank the fishers from El Quetzalito, Josue Ayala, and Francisco Polanco-Vásquez for all the provided support. EEBG thanks the Consejo Nacional de Ciencia y Tecnología for the scholarships provided, and D. Bernot-Simon for the English review. This work was conducted under the following permit from Consejo Nacional de Áreas Protegidas de Guatemala (I-DRNO-002-2016).

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Received: September 25, 2019; Accepted: December 12, 2019

Corresponding author: Edgar E. Becerril-García (bg.ragde@gmail.com)

Corresponding editor: Leonardo Abitia

Creative Commons License This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.