Introduction
Fish in the family Trichiuridae are among the most important fisheries' target species worldwide, with a total landing of ~3.5 million tonnes in 2021 (FAO, 2021; He et al., Reference He, Luo, Zhao, Huang, Yan and Kang2022). Trichiuridae species are predominantly found in tropical and temperate areas, including ~46 species and 10 genera (WoRMS Editorial Board, 2024), of which nine species have been recorded around Taiwan (Shen and Wu, Reference Shen and Wu2011).
The deep-sea genus Aphanopus Lowe 1839, commonly known as scabbardfish, has been a significant target in commercial fisheries in the northeast Atlantic Ocean since the 17th century, with total landings of 5205 tonnes reported in 2021 (FAO, 2021). Additionally, the fisheries of scabbardfish are considered to be the first recorded commercial deep-sea longline fishing industry (Eduardo et al., Reference Eduardo, Villarins, Lucena-Frédou, Frédou, Lira, Bertrand and Mincarone2018). There are seven species in the genus Aphanopus (Parin, Reference Parin1983, Reference Parin1994; Biscoito et al., Reference Biscoito, Delgado, González, Stefani, Tuset, Isidro and Carvalho2011). The only Aphanopus species found in North Pacific Ocean is the Pacific black scabbardfish, Aphanopus arigato Parin, Reference Parin1994, which was originally described from a specimen collected in the Western North Pacific Ocean (Parin, Reference Parin1994). The distribution was then found to expand to the northwestern Pacific off southeastern Kamchatka, Russia to North America, south of Hawaiian Islands (Orlov, Reference Orlov1999). All Aphanopus species are benthopelagic, and relatively challenging to identify morphologically (Biscoito et al., Reference Biscoito, Delgado, González, Stefani, Tuset, Isidro and Carvalho2011; Delgado et al., Reference Delgado, Reis, González, Isidro, Biscoito, Freitas and Tuset2013). Alternatively, cytochrome oxidase subunit I gene (cox1) sequencing and sagittal otolith morphology are useful tools for species identification within this genus (Stefanni and Knutsen, Reference Stefanni and Knutsen2007; Tuset et al., Reference Tuset, Piretti, Lombarte and González2010).
The aim of this study was to report the first and the southernmost presence of A. arigato in the South China Sea, northwest Pacific Ocean.
Materials and methods
A single specimen was caught via bottom trawl net at ~500 m depth around Dongsha Island, South China Sea, on 14 March 2023 (18°49′ to 20°45′N and 112°46′ to 116°15′E) (Ng et al., Reference Ng, Liu, Mediodia, Lin, Ho and Lin2024; Mediodia et al., Reference Mediodia, Chang, Ho, Přikryl and Lin2024). The measurement and counting methods followed the protocol described in Biscoito et al. (Reference Biscoito, Delgado, González, Stefani, Tuset, Isidro and Carvalho2011). Description of otolith followed Lin and Chang (Reference Lin and Chang2012) and Tuset et al. (Reference Tuset, Piretti, Lombarte and González2010). The specimen was subsequently preserved at the Biodiversity Research Museum of the Academia Sinica of Taiwan under the registration code ASIZP-0082289. Comparison with the original description and additional references followed Parin (Reference Parin1994) and Orlov (Reference Orlov1999).
Genomic DNA was extracted from the muscle tissue of ASIZP-0082289 by a FavorPrep Tissue Genomic DNA Extraction Mini Kit (Favorgen, Pingtung, Taiwan). After DNA extraction, polymerase chain reaction (PCR) amplification of the partial cox1 gene (~650 bp) using the forward primer, FishF1 + F2 (5′-TCR ACY AAY CAY AAA GAY ATY GGC AC-3′) and reverse primers, FishR1 (5′-TAG ACT TCT GGG TGG CCA AAG AAT CA-3′) and FishR2 (5′-ACT TCA GGG TGA CCG AAG AAT CAG AA-3′) (Chang et al., Reference Chang, Lin, Ren, Lin and Shao2016). The final PCR products were then used for sequencing with the primer FishF1 + F2 (by Genomic Biotech Inc., New Taipei City, Taiwan), and followed the method of Chang et al. (Reference Chang, Lin, Ren, Lin and Shao2016). The cox1 sequencing results were submitted to the National Center for Biotechnology Information (NCBI) for nucleoid BLAST analysis (https://blast.ncbi.nlm.nih.gov/Blast.cgi) to confirm species identification.
Results
Family TRICHIURIDAE
Aphanopus arigato Parin, Reference Parin1994
(Figures 1–4)
Specimen: one specimen. ASIZP-0082289, 438 mm SL, from Dongsha Atolls, South China Sea, ca. 500 m depth, 14 March 2023, collected by Chien-Hsiang Lin.
Figure 1. Whole specimen of Aphanopus arigato (ASIZP-0082289) caught around Dongsha Atolls, South China Sea. Scale bar = 20 mm.
Figure 2. Details of the head profile, snout, and teeth of A. arigato (ASIZP-0082289). The red arrow indicates the posterior end of the upper jaw.
Figure 3. Details of the anal spine of A. arigato (ASIZP-0082289). The yellow arrow indicates the position of the anus; the red arrow indicates the rudimental first anal spine.
Figure 4. Right sagittal otolith of A. arigato (ASIZP-0082289). Top: ventral view; bottom: proximal view. Scale bar = 1 mm.
Description of ASIZP-0082289: The morphometric and meristic data are presented in Table 1.
Table 1. Morphometrics and meristics data for Aphanopus arigato caught around Dongsha Island, South China Sea, compared with original description and previous research by Parin (Reference Parin1994) and Orlov (Reference Orlov1999)
Measurement data from the holotype and five paratypes from Parin (Reference Parin1994).
Body extremely elongated, the highest body depth situated at the first anal-fin spine, corresponding to 6.8% standard length (SL); body depth narrows posteriorly, reaching 0.4% SL at the end of the caudal peduncle. Anus positioned about mid-point of body, with pre-anus length 52.7% SL. First anal fin spine rudimental; second anal fin spine sharp, sword-shaped, positioned posteriorly to the anus; pre-anal length 60.2% SL (Figure 3). Caudal fin very small, with slender caudal peduncle. Entire body without scales; with very thin skin; lateral line very distinct and straight.
Head large, head length (HL) 18.4% SL; eye large and slightly oval, eye diameter is 17.6% of HL, situated laterally; interorbital width 13.3% HL. Snout long and large; snout length 7.8% SL; posterior end of upper jaw reaches the middle of the eye; lower jaw projected anteriorly to upper jaw (Figure 2). Teeth sharp and strong, triangular shaped, in a uniserial arrangement on upper and lower jaws (Figure 2). Mouth large, gently curved; maxillary length 50% of HL.
Dorsal-fin origin right behind the operculum; pectoral fin short; pelvic fin absent. First dorsal-fin base slightly shorter than second dorsal-fin base; with first dorsal-fin base 37.7% SL and second dorsal fin base 45.8% SL; prepectoral length 20.8% SL.
Coloration: Body black to whitish (maybe caused by scrubbing in the bottom trawl net), separated from the blackish peritoneum. Dense black inside the mouth and gill cavities. All fins translucent.
Otolith morphology: Sagittae otoliths are fusiform (right sagittae in Figure 4). Otolith thin, spindle-shaped, with pointed anterior rim and blunt, truncated posterior rim. Dorsal rim flat to gently curved, slightly elevated anteriorly. Ventral rim gently curved. Sulcus narrow, spindle-shaped, not well-divided into ostium and cauda. Ostium triangular, opens anteriorly, deeper and wider just anteriorly to the collum. Cauda very shallow and narrow. Cristae thick, ridge-like, but become indeterminate at posterior end.
Distribution: Adults (SL > 385 mm) mainly recorded in temperate North Pacific Ocean from northern Japan to southeastern Kamchatka, Russia (Parin, Reference Parin1994; Orlov, Reference Orlov1999); and northern California (Lauth, Reference Lauth1997). Juveniles were found in subtropic and tropic central northern Pacific Ocean, most often off Hawaii (Evseyenko et al., Reference Evseyenko, Parin and Suntsov1994; Orlov, Reference Orlov1999). Specimen caught in Dongsha Island, South China Sea, northwest Pacific Ocean (present study) represents a significant range expansion to the southwest Pacific Ocean, and also the southernmost record of the Aphanopus arigato. Mainly inhabiting depths between 400 and 800 m (Parin, Reference Parin1994; Orlov, Reference Orlov1999).
Remarks: The morphological characteristics of ASIZP-0082289 mostly fall within the range described by Parin (Reference Parin1994) and previous research by Orlov (Reference Orlov1999). As ASIZP-0082289 is relatively small (438 mm SL), it shows a slightly lower body depth compared to larger individuals in previous studies: 6.8% SL vs 8.7% in the holotype HUMZ-78109 (651 mm SL) and 8.9% in IMBV-uc (710 mm SL) (Table 1) (Parin, Reference Parin1994; Orlov, Reference Orlov1999). However, the body depth of ASIZP-0082289 is consistent with paratypes of similar size, which have body depths of 6.6% SL in ZIL-50587 (504 mm SL) and 5.6% SL in ZIL-50588 (385 mm SL) (Parin, Reference Parin1994).
The specimen is clearly identified as A. arigato with the morphometric characters mentioned above. The total dorsal-fin elements (Table 1) and the sword-shaped, long second dorsal spine (Figure 3) also showed distinct differences with the congeners.
The body coloration of ASIZP-0082289 is whitish, which is slightly different compared to previous descriptions (Parin, Reference Parin1994; Parin and Nakamura, Reference Parin, Nakamura, Carpenter and De Angelis2016). However, the dense black coloration in the peritoneum and head is still observable. The white coloration could be due to the thin skin of A. arigato being rubbed off when caught by the bottom trawl net. Additionally, colour fading during the defrosting process with running water is also one major possibility (Jiang and Lee, Reference Jiang, Lee, Hui, Legarretta, Lim, Murrell and Nip2004).
Molecular: The partial cox1 sequence of ASIZP-0082289 has been submitted to NCBI under accession number PQ517214.1. The best BLAST match identified ASIZP-0082289 as A. arigato with a per cent identity exceeding 99% (NCBI accession: JQ353980.1). Furthermore, the neighbour-joining tree results also showed a high bootstrap value (93%) in the same monophyletic group of A. arigato (Figure 5). Both morphological and molecular findings consistently point to the identification of the specimen ASIZP-0082289 as A. arigato.
Figure 5. Neighbour-joining tree based on cox1 sequences of the specimens in the present study (ASIZP-0082289; NCBI accession: PQ517214.1) and congeners from NCBI.
Discussion and conclusion
The initial taxonomic review of the genus Aphanopus in 1994 by Parin restricted the distribution of Aphanopus intermedius exclusively to the tropical and temperate Atlantic Ocean, and reclassified A. intermedius recorded in the North Pacific Ocean as Aphanopus arigato (Parin, Reference Parin1994). As a single specimen was recorded from southeastern Kamchatka, Russia, there have been no updates on the distribution of A. arigato (Orlov, Reference Orlov1999). Apart from adults, larvae and juveniles of A. arigato have been discovered in the mesopelagic zone (200–500 m depth) in the northeastern region of the Hawaiian Islands (Parin, Reference Parin1994; Orlov, Reference Orlov1999).
The spawning area of the Pacific black scabbardfish has been reported in the temperate waters of the North Pacific Ocean (northern Japan, southern Russia coastal areas, and northern California), with eggs and larvae being transported by deep-sea currents to subtropical and tropical (present study) waters for further growth (Orlov, Reference Orlov1999; Kawabe et al., Reference Kawabe, Fujio and Yanagimoto2003). In this study, a single specimen of A. arigato was collected using a bottom trawl net at ~500 m depth, which is consistent with the depth range previously described for juveniles and small individuals of this species (Parin, Reference Parin1994). Furthermore, the specimen recorded in this study has a total length within the adult size range (TL > 385 mm) of A. arigato (Parin, Reference Parin1994; Orlov, Reference Orlov1999). This is the first recorded occurrence of A. arigato in the South China Sea, suggesting the possibility of a wider distribution area for the Pacific black scabbardfish. The specimen may also alternatively be a wanderer that drifted during its planktonic life and settled and survived in this region, of which there are some similar cases of fish species in the Mediterranean's Lessepsian migration (Corsini et al., Reference Corsini, Margies, Kondilatos and Economidis2005). Therefore, we recommend further research to comprehensively assess the distribution of A. arigato in the Pacific Ocean.
In conclusion, this study reports the southernmost distribution of the Pacific black scabbardfish, A. arigato, and provides a partial sequence of the cox1 gene and the otolith morphology description.
Data availability
All data supporting the findings of this study are included in the main text. The Aphanopus arigato specimen described in this study is housed at the Biodiversity Research Museum, Academia Sinica, Taiwan, under the registration code ASIZP-0082289 and the partial cox1 sequence is available on NCBI under accession number PQ517214.1.
Acknowledgement
We specially thank Hsin-Wei Liu (BRCAS) for taking the photo and registering the specimens.
Author contributions
Yen-Ting Lin mainly conducted the experiments, analysed the results, and wrote the manuscript. Chien-Hsiang Lin obtained and described the otolith. Chien-Hsiang Lin and Yu-San Han designed and supervised the experiments. All authors participated in manuscript writing and interpretation of results. All authors read and approved the final manuscript.
Financial support
The authors acknowledge the National Science and Technology Council, Executive Yuan, Taiwan (MOST 109-2313-B-002-001-MY2 and MOST 111-2313-B-002-016-MY3) for funding this project.
Competing interests
None.
Ethical standards
The material utilized in this study was acquired by us from local fisherman (Chengbin Fishing Port, Keelung). It is important to note that the specimen was deceased at the time of acquisition, hence no living animals were involved in current research.
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