Monotaxis heterodon (Bleeker, 1854) is widespread in the Indo-Pacific Ocean, however, it was reported that Monotaxis grandoculis (Forsskål, 1775) was the single species of Monotaxis in this area. We collected four M. heterodon specimens from the lagoon waters of Mischief Reef in South China Sea. A morphometric study was taken to confirm the occurrence of M. heterodon in the seawaters of the South China Sea and thoroughly separate them from M. grandoculis. In addition, DNA barcoding was taken for the classification of specimens. The mean genetic distance within M. heterodon group was 0.24 percentage points, group mean distance between M. heterodon and M. grandoculis was 8.71 percentage points. The phylogenetic analysis confirmed the existence of M. heterodon in the lagoon waters of the South China Sea. This study will contribute to species identification within this genus distributed in the South China Sea.

COI DNA, first record, fish taxonomy, Monotaxis heterodon, South China Sea

The family Lethrinidae consists of 5 genera and 43 species, including 28 species of Lethrinus, 11 species of Gymnocranius, 2 species of Monotaxis, and one each of Gnathodentex and Wattsia (see Chen and Borsa 2020; Fricke et al. 2021). However, only 4 genera and 33 species have been recorded from China (Chen and Zhang 2015).

There are some controversies on the taxonomy of Monotaxis heterodon (Bleeker, 1854), some authorities (Carpenter and Allen 1989; Nakabo 2002) indicated that it was a junior synonym of Monotaxis grandoculis (Forsskål, 1775), while others confirmed that the two species can be completely distinguished (Randall 2005; Senou et al. 2007; Chen and Borsa 2020).

Our purpose here was to confirm the existence of M. heterodon in the seawaters of the South China Sea and separating them from M. grandoculis. In addition, a mitochondrial DNA barcoding approach was also employed in order to better solve the identification problems of M. heterodon at the genetic level. This study reports for the first time M. heterodon from the South China Sea.

A total of 4 individuals of Monotaxis heterodon were collected from the lagoon waters of Mischief Reef (09°54′29′′N, 115°32′05′′E). All collected specimens were identified based on morphological characteristics used by Randall (2005).

For the genetic study, a piece of muscle tissue was obtained from each individual and preserved in 95% ethanol to carry out DNA extraction. The classical phenol-chloroform technique was used for DNA extraction. PCR was subsequently conducted. The primer sequences used for COI amplification were 5′-TCGACTAATCATAAAGATATCGGCAC-3′ and 5′-ACTTCAGGGTGACCGAAGAATCAGAA-3′ (Ivanova et al. 2007). PCR was carried out in a 25 μL reaction mix containing DNA template (1 μL, 50 ng · uL^–1), forward primer (F, 1 μL, 10 uM · L^–1), reverse primer (R, 1 μL, 10 uM · L^–1), dNTPs (2 μL, 2.5 mM · L^–1 each), EasyTaq DNA Polymerase (0.15 μL, 5 U · μL^–1) and 10 × PCR buffer (2.5 μL, 25 uM · L^–1). A Biometra thermal cycler (Göttingen, Germany) with the following given procedure: one initial denaturation (95 °C, 5 min), thirty-five cycles consisting of denaturation (94 °C, 35 s), annealing (54 °C, 35 s), and extension (72 °C, 35 s), and one final extension (72 °C, 10 min), was employed to put PCR amplification into effect. PCR products were preserved at 4 °C. After agarose gel electrophoresis, the PCR products were sequenced by a biotechnology company. All 4 individuals’ original sequences were successfully obtained and revised by DNASTAR software (DNASTAR Inc., Madison, WI, USA). Six COI sequences of M. grandoculis and two COI sequences of M. heterodon were downloaded from NCBI for phylogenetic study (Table 1). Lethrinus miniatus (Forster, 1801) (see Muallil et al. 2020) (EU148534.1) was chosen as the out-group to root the tree. MEGA X (Kumar et al. 2018) was used to construct a neighbor-joining (NJ) tree under the Kimura 2-parameter (K2P) model.

The general morphological features of Monotaxis heterodon are shown in Fig. 1. Body oblong and compressed laterally; eye very large; The operculum, cheek, and inner surface of pectoral fin base covered with scales; 5 rows of scales between lateral line and base of middle dorsal spines; greenish-gray dorsally, shading to silvery white ventrally. All color descriptions are in accordance with Randall (2005).

Figure 1. 

Left lateral view of Monotaxis heterodon (No. M238; length 186 mm SL).

All the sequences have been deposited in GenBank. The neighbor-joining phylogenetic tree is shown in Fig. 2. Specimens of M. heterodon in the presently reported study clustered in a group, and those of M. grandoculis and Lethrinus miniatus clustered in another two groups, respectively. The genetic distance of the COI sequence within M. heterodon group was only 0.24 percentage points, and the mean distance between M. heterodon and M. grandoculis was up to 8.71 percentage points (Table 2). Together, both the morphological and genetic analysis strongly supported the validity of M. heterodon as a new record in Mischief Reef.

Figure 2. 

Phylogenetic tree based on neighbor-joining analysis of COI sequence. Lethrinus miniatus (EU148534.1) was chosen as the out-group to root the tree. Numbers above branches indicate neighbor-joining bootstrap percentages. Only Bootstrap values >50 percentage points are shown in the NJ tree.

Located in the center of the Indo-western Pacific region, the South China Sea is the largest semi-closed sea in the western tropical Pacific Ocean with extremely rich biodiversity. With more than 1120 species of reef fish species, the South China Sea is one of the world’s major centers of coral reef diversity (Zhang et al. 2021). Mischief Reef is oval and located in the eastern and central waters of the Nansha Islands with a large lagoon. Benefiting from the tropical monsoon climate and warm waters, Mischief Reef is well suited for marine fishing (Yu et al. 2006). In addition, Mischief Reef area is a good spawning and nursery ground for many fish species like skipjack tuna, Katsuwonus pelamis (Linnaeus, 1758); yellowfin tuna, Thunnus albacares (Bonnaterre, 1788); blue round scad, Decapterus maruadsi (Temminck et Schlegel, 1843); and kawakawa, Euthynnus affinis (Cantor, 1849) (see Ma et al. 2016). Sun et al. (2019) found 37 species of fishes and 7 species of invertebrates collected with gill nets and trawls in Mischief Reef. Shan et al. (2021) identified 209 individuals using morphological and molecular techniques in Mischief Reef,including 101 species in 62 genera, 27 families, 8 orders, and 1 class. Biodiversity is based on species diversity and is closely related to ecosystems and human well being (Barman et al. 2018). The presently reported survey of Mischief Reef will hopefully contribute to a better understanding of this complex ecosystem.

Monotaxis heterodon is widespread in the Indo-Pacific Ocean, with records from the Seychelles, the Maldives, Sri Lanka, Indonesia, Malaysia, New Guinea, Great Barrier Reef, Marshall Islands, and New Caledonia (Randall 2005); the Tawi-Tawi Islands (Muallil et al. 2020), New Ireland (Fricke et al. 2019), the Andaman and Nicobar Islands (Devi et al. 2015), Madang (Fricke et al. 2014), and Japan (Senou et al. 2007; Akita et al. 2016). Previous studies on the fishes of the South China Sea have not reported M. heterodon, presumably because the controversies on the taxonomy of this species. Monotaxis heterodon and M. grandoculis are very similar, but morphologically, the two species can be distinguished by some characteristics. The first one is the number of transversal scale rows on the flank: the count of scales below the lateral line to origin of anal fin is 12.5 for M. heterodon while it is 13.5 for M. grandoculis. Monotaxis heterodon lacks the black spots at the base of the dorsal and anal fins. Furthermore, M. heterodon usually have brown or reddish lips but M. grandoculis’ lips are often yellowish (Randall 2005; Chen and Borsa 2020). Although the lip color of our samples cannot be recognized, the count of scales below the lateral line to origin of anal fin is 12.5 and there are no spots at the base of the dorsal and anal fins of our samples.

Monotaxis heterodon was once thought to be a different color morph of M. grandoculis (see Bilecenoglu 2007) but Chen and Borsa (2020) recently confirmed the occurrence of the two species in the genus Monotaxis. Some different methods were adopted as the indicator of speciation. Such as the “2 percentage points rule” proposed by Ward et al. (2009), that is, individuals were much more likely to be congeneric than conspecific if that at a distance level of 2 percentage points or higher. The mean evolutionary distance within the species M. heterodon was 0.24 percentage points, the use of the 2 percentage points rule suggests that M. heterodon was here believed as a valid species.

The presently reported study constitutes the first record of M. heterodon and increases the number of Monotaxis species known to occur in the South China Sea.

1. Hainan Provincial Natural Science Foundation of China (NO. 320QN361 and NO. 320QN358).
2. Central Public-interest Scientific Institution Basal Research Fund, South China Sea Fisheries Research Institute, CAFS (NO.2021SD14).
3. Guangdong key research and development program (2020B1111030001).

The authors have declared that no competing interests exist.

The authors are indebted to Mr. Wang Zhao for collecting the fish specimens reported in this study.