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. 2026 Mar 25;109(1):605–609. doi: 10.1111/jfb.70432

First genetic confirmation of the smooth hammerhead shark ( Sphyrna zygaena , Linnaeus, 1758) in French Polynesian waters

Clémentine J M Séguigne 1,2,✉, Chloé A Pupier 1,2, Temehani Chand 1,2, Julie Grall 1,2, Théo Guillaume 1,2, Keanu Robert 1,2, Claire‐Sophie Azam 2, Nicolas Buray 2, Jean‐Marie Jeandel 3, Alexander Mataarere 3, Anne‐Marie Trinh 4, Jodie Rummer 5
PMCID: PMC13397259  PMID: 41879721

Abstract

The smooth hammerhead shark (Sphyrna zygaena) has never been formally confirmed in French Polynesian waters. Observations of solitary hammerhead sharks suggested the presence of an unrecorded species. Two specimens captured off Tahiti were morphologically and genetically examined. Both matched traits of the smooth hammerhead and COI barcoding confirmed >99% similarity to reference sequences in the NCBI and BOLD databases. This record extends the known range of S. zygaena to the central South Pacific and highlights the role of citizen observations in detecting rare oceanic sharks, which is important to regional conservation frameworks.

Keywords: citizen science, DNA barcoding, shark conservation, shark distribution, species range extensions

French Polynesia encompasses 5.5 million km2 of ocean across 118 islands and constitutes the world's largest shark sanctuary. Such large‐scale protection measures are associated with significantly higher shark abundances compared to regions lacking equivalent conservation initiatives (MacNeil et al., 2020; Séguigne et al., 2023; Ward‐Paige, 2017). To date, 34 shark species have been documented (Siu et al., 2017), including the Pacific prickly shark (Echinorhinus cookei), recorded at ~500 m depth during a 2021 submersible dive conducted outside of scientific programmes (Clua et al., 2021). Opportunistic observations from ocean users are increasingly complementing scientific monitoring, and citizen‐science initiatives have recently expanded elasmobranch records throughout French Polynesia through the Polynesian Shark Observatory (ORP) (Séguigne et al., 2023).

Hammerhead sharks previously recorded in French Polynesia include the scalloped hammerhead Sphyrna lewini and the great hammerhead Sphyrna mokarran (Séguigne et al., 2023; Siu et al., 2017). For several years, however, observations reported by ocean users through citizen‐science initiatives and informal exchanges among local shark enthusiasts have described hammerhead sharks whose appearance and/or behaviour did not match either of these two species. These reports consistently referred to individuals with a smooth and curved cephalofoil, differing from S. mokarran, which displays a nearly straight anterior cephalofoil margin, and from S. lewini, characterized by a broad arch with a pronounced median notch (Compagno, 1984). Most sightings also involved solitary individuals, contrasting with the schooling behaviour commonly observed in S. lewini (Klimley, 1985).

Although these observations suggested the possible presence of Sphyrna zygaena, they were based on informal field reports and visual assesments alone, and no confirmed record existed for French Polynesia. These repeated reports therefore warranted scientific verification to formally establish the species' presence in the region.

Here, we document the first confirmed record of the smooth hammerhead S. zygaena in French Polynesian waters based on morphological examination and DNA barcoding.

A non‐lethal shark monitoring programme targeting tiger (Galeocerdo cuvier) and silky (Carcharhinus falciformis) sharks is conducted off Tahiti (French Polynesia) since November 2024. Activities are carried out under local research authorization. As part of this programme, any incidentally caught shark individuals are briefly restrained alongside the vessel for examination following ethical procedures.

Total length (TL) is measured as the straight‐line distance from the snout tip to the upper caudal‐fin lobe, and high‐resolution surface and underwater photographs are taken to document head and fin morphology. A small caudal‐fin tissue sample is collected for genetic analysis, and individuals are released once regular swimming behaviour is observed.

Morphometric ratios are extracted from photographs using ImageJ (Abràmoff et al., 2004) following diagnostic criteria in Compagno (1984) to distinguish S. lewini, S. mokarran and S. zygaena.

DNA extraction and COI barcoding are performed by GenoScreen (Lille, France) following the DNA barcoding protocol of Ward et al. (2005), using FishF1 and FishR1 primers. The mitochondrial COI fragments amplified and sequenced are 711–712 bp long, corresponding to the final edited consensus sequences obtained after quality trimming. Consensus sequences are edited and compared to public reference sequences using the BLASTn algorithm (Madden, 2013; Sharma et al., 2024) and subsequently query against the BOLD version 4 barcode database (Ratnasingham et al., 2024) to support species‐level assignment.

Two female hammerhead sharks were captured offshore Tahiti in August 2025 (Figure 1a). The first individual (‘Tere’) measured 264 cm TL and the second (‘Poe’) measured 249 cm TL. Both specimens were examined for external morphology and morphometric analyses, when photographs allowed accurate measurements (Figure 1b). Both sharks were released in good condition after handling.

FIGURE 1.

FIGURE 1

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(a) Map of the study area, located in the Northwest of Tahiti, French Polynesia. (b) Capture of Poe on 13 August 2025. Photo credit: Théo Guillaume.

Morphological traits visible on photographs matched diagnostic characteristics of S. zygaena. Both sharks exhibited a smooth, broadly curved anterior cephalofoil margin with no median indentation. In Tere, the posterior margin of the eye extended slightly beyond the upper jaw symphysis, whereas this trait could not be assessed for Poe. Poe provided a clear view of anterior teeth, which were moderately long with stout cusps and smooth cutting edges. Second‐dorsal‐fin proportions and positions were consistent with S. zygaena, and both sharks displayed dusky ventral pectoral‐fin tips. These morphological features align with diagnostic descriptions for S. zygaena and differ from those typical of S. lewini and S. mokarran (Table 1, Supplementary Material: Data S1).

TABLE 1.

Comparison of external morphological characters observed in the two captured individuals with diagnostic features described for Sphyrna lewini, Sphyrna mokarran and Sphyrna zygaena according to Compagno (1994).

Scalloped hammerhead shark S. lewini Great hammerhead shark S. mokarran Smooth hammerhead shark S. zygaena
General
Large hammerhead, to over 3 m Large to gigantic hammerhead, to 3–5.5+ m Large hammerhead, to over 3 m
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Dusky to black pectoral‐fin tips No fin marking Undersides of pectoral‐fin tips dusky
X Tere, Poe X Tere, Poe ✓ Tere, Poe
Head
Anterior margin of head very broadly arched Anterior margin of head nearly straight in adults Anterior margin of head very broadly arched
Prominent median and lateral indentations Prominent median and lateral indentations Prominent lateral indentations but no medial indentation
X Tere, Poe X Tere, Poe ✓ Tere, Poe
Posterior margins of head wide, angled posterolaterally and generally broader than mouth width Posterior margins of head long, angled transverse in adults and about as broad as mouth width Posterior margins of head wide, angled posterolaterally and generally broader than mouth width
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Well‐developed prenarial grooves present anteromedial to nostrils Prenarial grooves absent or hardly developed Well‐developed prenarial grooves present anteromedial to nostrils
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Rear ends of eyes slightly anterior to upper symphysis of mouth Rear ends of eyes anterior to upper symphysis of mouth Rear ends of eyes slightly behind upper symphysis of mouth
X Tere X Tere ✓ Tere
Teeth
Anterior teeth with moderately long, stout to slender cusps Anterior teeth with moderately long stout cusps Anterior teeth with moderately long, very stout cusps
Smooth or weakly serrated Strongly serrated edges Smooth or weakly serrated edges
X Poe X Poe ✓ Poe
Dorsal fins
First dorsal moderately falcate First dorsal strongly falcate First dorsal moderately falcate
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Second dorsal fin low, less than anal height Second dorsal fin high, about equal to anal height Second dorsal fin low, less than anal height
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Second dorsal with a shallow concave posterior margin Second dorsal with a strongly concave posterior margin, about twice the fin height, and ending almost opposite upper caudal origin Second dorsal with a shallow concave posterior margin
✓ Tere X Tere ✓ Tere
Second dorsal inner margin long, about twice the fin height, and ending almost opposite upper caudal origin Second dorsal inner margin short, about equal to fin height, and ending well in front of upper caudal origin Second dorsal inner margin long, about twice the fin height, and ending well in front of upper caudal origin
X Tere X Tere ✓ Tere
Pelvic and anal fins
Pelvic fins not falcate Pelvic fins strongly falcate Pelvic fins not falcate
Straight or slightly concave posterior margins Strongly concave posterior margins Straight or slightly concave posterior margins
✓ Tere X Tere ✓ Tere

Anal fin larger than second dorsal fin and rather long

✓ Tere

Anal fin about as large as or larger than second dorsal fin and moderately long

X Tere

Anal fin slightly larger than second dorsal fin and rather long

✓ Tere

Anal‐fin origin well ahead of second dorsal origin Anal‐fin origin well ahead of second dorsal origin Anal‐fin origin slightly ahead of second dorsal origin
✓ Tere, Poe X Tere, Poe ✓ Tere, Poe
Anal‐fin posterior margin shallowly concave to nearly straight Anal‐fin posterior margin deeply notched Anal‐fin posterior margin deeply notched
X Tere ✓ Tere ✓ Tere
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Note: Characters common to all three species or not observable in the specimens are omitted. When information was only available for one of the two sharks, only that individual is reported.

COI sequences from both individuals showed the highest similarity to S. zygaena reference sequences (99.56% for Tere and 99.28% for Poe; GenBank reference: ON695879.1). BOLD database queries returned a 100% match with the S. zygaena reference record (BOLD:AAA6753). Molecular results corroborate the morphological identification.

This study provides the first genetic and morphometric confirmation of the smooth hammerhead S. zygaena in French Polynesia. Morphological traits diagnostic of the species, including a smooth and broadly curved cephalofoil lacking a median indentation, together with COI barcoding, supported the identification. Although identification relied on a single mitochondrial marker and limited regional reference data, the congruence between genetic and morphometric traits supports the robustness of the finding. Future analyses incorporating nuclear genes and broader Polynesian comparisons will help refine population‐level understanding. Until 2025, hammerhead observations in the region have been reported and catalogued by sea users as S. lewini and S. mokarran (Séguigne et al., 2023; Siu et al., 2017) based on visual identification. Although occasional misidentification of S. zygaena cannot be excluded in historical records, no morphological or genetic evidence had previously confirmed its presence in French Polynesian waters, making this the first verified occurrence of a third hammerhead species in French Polynesian waters.

The smooth hammerhead has a circumglobal distribution in temperate waters and, in some regions, extends into tropical areas (Bezerra et al., 2017; Ebert et al., 2013; Last & Stevens, 2009). Its detection in French Polynesia therefore aligns with its broad Pacific range and migratory behaviour (López‐Martínez et al., 2020; Rigby et al., 2019). Population genetic studies indicate differentiation between Northern and Southern Pacific groups, with lower genetic diversity reported in the Northern Hemisphere (Bolaño‐Martínez et al., 2019; Testerman et al., 2008). This record provides an important geographic datapoint in the central Pacific and motivates future work to evaluate potential connectivity among Pacific populations.

From a conservation perspective, this finding has direct implications for regional management, as S. zygaena is listed as vulnerable (Rigby et al., 2019) and faces exploitation through targeted fisheries and by‐catch throughout its range (Huynh & Tsai, 2023; López‐Martínez et al., 2020; Tsai et al., 2018). This species also accounts for approximately 1.7% of the global shark fin trade and ranks among the most imported hammerhead species in Hong Kong markets (Clarke et al., 2006; Dent & Clarke, 2015; Fields et al., 2018). Explicit inclusion of S. zygaena within the Polynesian Shark Sanctuary framework would therefore strengthen protections for this vulnerable species across 5.5 million km2 of ocean.

Finally, the detection of a rarely observed pelagic shark in offshore waters highlights the difficulty in documenting wide‐ranging elasmobranchs in remote marine regions, similar to deep and rarely sighted species (Clua et al., 2021; Ebert et al., 2013; Finucci, 2018). This record reinforces the value of involving ocean users in biodiversity observation programmes, which can complement scientific surveys and contribute to improved understanding of species distribution in French Polynesia (Jürgens et al., 2018; Séguigne et al., 2023). Continued observations and genetic sampling will help clarify occurrence patterns, seasonality and potential connectivity within Pacific populations.

AUTHOR CONTRIBUTIONS

Writing – original draft: Clémentine J.M. Séguigne. Writing – review and editing: all authors. Field investigation: Clémentine J.M. Séguigne, Chloé A. Pupier, Temehani Chand, Julie Grall, Théo Guillaume and Keanu Robert. Data curation: Clémentine J.M. Séguigne and Chloé A. Pupier. Visualization and data analyses: Clémentine J.M. Séguigne and Chloé A. Pupier. Citizen‐science network coordination and data sharing: Clémentine J.M. Séguigne, Claire‐Sophie Azam, Nicolas Buray, Jean‐Marie Jeandel, Alexander Mataarere. Funding: Anne‐Marie Trinh. Supervision: Clémentine J.M. Séguigne, Claire‐Sophie Azam, Anne‐Marie Trinh and Jodie Rummer.

FUNDING INFORMATION

This study was conducted by ORP and IREMP under the authorization of the DIREN (permit No. 4466). Financial support was provided by the Direction des Ressources Marines (DRM) and the Institut pour la Recherche sur les Écosystèmes Mésophotiques et Profonds (IREMP). This project was co‐funded by the European Union (BESTLIFE2030), 2024A69. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.

CONFLICT OF INTEREST STATEMENT

None of the authors have a conflict of interest to disclose.

Supporting information

Data S1. Body parts from the captured specimens used for morphometric analysis. Photo credits: 2 Alexandre Allioux (2.2.1), Théo Guillaume (1.1.1, 1.1.2, 1.3.1, 1.3.2, 1.4.1, 2.1.1, 2.3.2, 2.4.1), Chloé Pupier 3 (2.3.1).

JFB-109-605-s001.pdf (2MB, pdf)

ACKNOWLEDGEMENTS

The authors thank the Mote Marine Laboratory, the Rummer Lab and the Ma'o Mana Foundation for their valuable advice. The authors acknowledge the strong involvement of the working group of the Ma'o Cycling project, including the CAPL, the DAG, the DBS, the local IUCN France Office and the Port de Pêche de Papeete. The authors are particularly grateful to Erwin Amavassee, Jacques Auraa, Chloé Delisle, Coralie Deniot, Chloé Desmots, Elena Gorchakova, Fabien Ho‐Ying, Fanny Martre, Laurent Maunas, Laurent Pasco, Heinui Tepahuaitaipari and Thibaut Thellier for their active participation in co‐ordination efforts. Finally, they extend their sincere thanks to local marine users and naturalists, in particular Anthony Berberian, who had previously raised the possibility of the presence of S. zygaena, and Maxence Hazard, as well as to all ORP observers who contributed valuable observations and field support to this study.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Data S1. Body parts from the captured specimens used for morphometric analysis. Photo credits: 2 Alexandre Allioux (2.2.1), Théo Guillaume (1.1.1, 1.1.2, 1.3.1, 1.3.2, 1.4.1, 2.1.1, 2.3.2, 2.4.1), Chloé Pupier 3 (2.3.1).

JFB-109-605-s001.pdf (2MB, pdf)

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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