In this study, the mtDNA CO1 and 16S rRNA genes of deep-sea eel specimen (sample ID: EBS01) were sequenced and the lengths of the markers were 721 bp and 608 bp, respectively. For the deep-sea eel specimen (sample ID: BT04), the mtDNA CO1 and 16S rRNA genes were sequenced and the lengths of the markers were 655 bp and 600 bp, respectively. The amplified sequences of mitochondrial CO1 of two deep-sea eels were confirmed using BLAST searches on the NCBI website. One deep-sea eel (sample ID: EBS01) showed 87.22% identity with Bassozetus zenkevitchi (acc. no. AP004405.1) and the other deep-sea eel (sample ID: BT04) showed 94.66% identity with Synaphobranchus kaupii (acc. no. AP002977.2), suggesting that the two deep-sea eels belong to the genera Bassozetus and Synaphobranchus, respectively. Indeed, the molecular phylogenetic tree based on mtDNA CO1 gene sequence showed that the deep-sea eel specimen (sample ID: EBS01) clustered together with B. zenkevitchi, B. glutinosus, and B. compressus (Fig. 3A). In addition, the phylogenetic tree based on 16S rRNA showed that the deep-sea eel specimen (sample ID: EBS01) and Bassozetus zenkevitchi were placed together as sister groups, suggesting that EBS01 belongs to the genus Bassozetus (Fig. 3B). Regarding the sample BT04, the molecular phylogenetic tree based on the mtDNA CO1 gene sequence showed that this specimen clustered together with S. kaupii, S. brevidorsalis, and S. affinnis (Fig. 4A). The phylogenetic tree based on 16S rRNA showed that the deep-sea eel specimen (sample ID: BT04) and Synaphobranchus kaupii were placed together as sister groups, suggesting that BT04 belongs to the genus Bassozetus (Fig. 4B).
Species identification of the genus Bassozetus has been described based on significant morphological characteristics, such as large head, eyes much smaller than the snout, 9–22 long gill rakers on the anterior arch, dorsal margin of the maxilla sheathed by skin of the cheek region, elongated body tapering caudally, opercula spine absent or weak, and 21–29 pectoral-fin rays not reaching the anus [28, 29, 30]. Additionally, the genus Synaphobranchus has been described based on morphological characteristics, such as a conical head, slender and large mouth, dark brown color, oval scales, gill slits confluent along the ventral midline, and irregularly placed teeth [31]. Although taxonomical information of the genera Bassozetus and Synaphobranchus, based on morphological characteristics, are available, it is difficult to accurately identify the similar species only by morphological characteristics due to morphological diversity and ontogenetic change during development [19, 32, 33, 34, 35]. In this context, our two deep-sea eel specimens are morphologically similar to the genera Bassozetus and Synaphobranchus, respectively, which positively correlated with the phylogenetic analysis based on mitochondrial DNA markers. This suggests that mitochondrial DNA markers are useful for accurate species identification of the genera Bassozetus and Synaphobranchus collected from the western Pacific Ocean.
The deep-sea cusk-eel genus Bassozetus Gill 1883 (Ophidiiformes: Ophidiidae, Neobythitinae) currently comprises 13 species [36] and is commonly found at depths ranging between 1,000 and 5,500 m (except for Bassozetus zenkevitchi [37]) in tropical and temperate areas [24, 29]. To date, Bassozetus has been recorded in the Atlantic, Indian, and Pacific Oceans. For example, six species (B. galatheae, B. glutinosus, B. compressus, B. elongatus, B. levistomatus, and B. robustus) have been described in the western Indian Ocean [38]. Bassozetus elongatus was described from the Celebes (Sulawesi, Indonesia) (West Pacific) [39]. Bassozetus compressus and B. elongatus have also been described from the Indo-West Pacific Ocean [24, 29, 30, 40] and the Atlantic Ocean [24, 29], respectively. The cutthroat eel genus Synaphobranchus currently includes six valid species [41]: Kaup’s arrowtooth eel, S. kaupii Johnson, 1862 [42]; Gray’s cutthroat, S. affinis Günther, 1877 [43]; Shortdorsal cutthroat eel, S. brevidorsalis Günther, 1887 [44]; cutthroat eels, S. dolichorhynchus [45]; S. oregoni [46]; and S. calvus Melo, 2007 [25]. All species in Synaphobranchus have been recorded from the North Atlantic Ocean except for S. calvus [31, 45, 47]. However, to date, no species have been identified belonging to Bassozetus and Synaphobranchus from the western Pacific Ocean.
In conclusion, this study introduces two newly recorded specimens from the genera Bassozetus and Synaphobranchus, respectively, in the western Pacific Ocean. In addition, this study provides information on mitochondrial DNA markers (CO1 and 16S rRNA) for identifying deep-sea eels. However, more samples from the western Pacific Ocean are needed to accurately identify these species.