Ontogenetic comparison of larvae and juveniles of Diaphus garmani and Benthosema pterotum (Myctophidae, Pisces) collected from Korea
Fisheries and Aquatic Sciences volume 23, Article number: 18 (2020)
During June 2017, we collected two postflexion larvae (6.01 and 7.56 mm in standard length [SL]) and two juveniles (7.72 and 9.62 mm SL) belonging to Myctophidae in the waters of Jejudo Island. Those four individuals were identified as Diaphus garmani, which had not been reported in Korea. They were distinguished from Benthosema pterotum by melanophores in the abdominal cavity (absent in D. garmani vs. present in B. pterotum) and the development of photophores (developed in D. garmani vs. rudimentary in B. pterotum) when shorter than 10.0 mm SL. Analysis of 16S rRNA sequences showed that the sequences of four individuals matched those of adult D. garmani (Kimura 2-parameter distance: 0.6–0.8%). This is the first record of larvae and juveniles of D. garmani in Korean waters, and we propose a new Korean name, Gar-ma-ni-sat-bi-neul-chi.
The family Myctophidae in the order Myctophiformes contains 251 species in 33 genera worldwide (Fricke et al. 2019), of which four species in three genera (Benthosema pterotum, Myctophum asperum, Myctophum affine, and Notoscopelus japonicus) occur in Korean waters (MABIK 2019; Park et al. 2019). The lanternfishes (Myctophidae) are mesopelagic fish that undertake diel vertical migration as a unique environmental adaptation (Ozawa 1986; Mini and James 1990; Moser and Ahlstrom 1996; Watanabe et al. 1999). They have a compressed body shape, large mouth, rows of small teeth, a swim bladder, bioluminescent photophores, and adipose fins (Nafpaktitis 1978, 1982, Nafpaktitis et al. 1995; Paxton et al. 1984; Martin et al. 2018).
The genus Diaphus in the family Myctophidae comprises 77 recognized species worldwide (Nelson et al. 2016; Froese and Pauly 2019) and has not been reported in Korea. The species in the genus Diaphus are classified into the Diaphus A and B groups depending on the presence and absence, respectively, of photophores on the suborbital organ (So), mouth morphology of early larvae, position of the anus, expression of melanophores and photophores, and timing of metamorphosis(Javadzadeh et al. 2012). Of which, Diaphus garmani is a small fish (~ 60 mm total length) belonging to the Diaphus B group and thus lacks photophores on the So (Nafpaktitis 1978; Nelson et al. 2016). Some studies of Diaphus have examined its early life history (Bineesh et al. 2010) and the first recorded species (Sassa et al. 2003). In Korea, studies of mesopelagic fish have examined the early life history of Maurolicus muelleri (Kim and Yoo 1999; Kim et al. 2007) and a new record for Sigmops gracilis (Lee and Kim 2013).
As the Myctophidae larvae collected in this study had similar morphological features to those of Benthosema pterotum, we confirmed the correctness of species identification by using 16S rRNA sequences. Afterward, in-depth morphological data related to ontogenetic features of these two species were compared with each other, which are barely distinguishable at the larval stage.
Four larval specimens (PKUI 618-621) of Diaphus garmani were collected from the southern Jejudo Island in June 2017 by a research vessel from the National Institute of Fisheries Science using a bongo net (mouth diameter 80 cm; mesh size 500 μm). The collected specimens were immediately fixed in 5% formalin for 1 h on the vessel and then washed before being placed in a 1L polyethylene container, where they were fixed in 99% alcohol. Four Benthosema pterotum larvae (PKUI 614-617) were also collected from the western Jejudo Island to the East Sea between July and December 2018 (Fig. 1). After the study, the specimens were deposited at Ichthyoplankton Laboratory of Pukyong National University (PKUI).
The larvae were identified according to their morphological characteristics and photophore features observed, following the methods described by Richards (2006), Okiyama (2014), Nakabo (2013), and Martin et al. (2018). Morphometric characters included total length (TL), standard length (SL), body depth (BD), head depth (HD), head length (HL), preanus length (PaL), eye diameter (ED), pre-dorsal fin length (PDFL) and pre-anal fin length (PAFL), which were observed under a stereomicroscope (SZH-16, Olympus, Japan) and then measured to the nearest 0.01 mm using an Image-Pro plus (ver. 2.0, Media Cybernetics) (Fig. 2). Meristic characters included the dorsal fin rays (D), anal fin rays (A), pectoral fin rays (P1), and pelvic fin rays (P2), which were stained with alizarin red S for counting under a stereomicroscope (SZH-16, Olympus, Japan). Each stage of larval development was sketched using a camera lucida attached to the stereomicroscope.
Total DNA was extracted from eyeballs removed from the right side of the larvae using 150 μL Chelex 100 resin (Bio-Rad Laboratories, USA). The mitochondrial 16S rRNA region was amplified using the primers 5′-CGC CTG TTT ATC AAA AAC AT-3′ and 3′-CCG GTC TGA ACT CAG ATC ACG T-5′ (Ivanova et al. 2007). The polymerase chain reaction (PCR) conditions were as follows: initial denaturation at 95 °C for 11 min; 35 cycles of denaturation at 94 °C for 1 min, annealing 52 °C for 1 min, extension at 72 °C for 1 min; and a final extension at 72 °C for 5 min. The same conditions were used for PCR analysis of B. pterotum, except that the annealing temperature was 54 °C. The sequences were aligned using ClustalW in the program BioEdit (ver. 126.96.36.199; Hall 1999), and genetic distances were calculated using the Kimura 2-parameter model (Kimura 1980) using Mega X (ver. 10.0.5). The genetic relationships were analyzed by constructing a neighbor-joining tree using Mega X (ver. 10.0.5) with 1000 bootstrap replications. The 16S rRNA sequences of D. garmani (KR231737) and B. pterotum (JX133756) were also obtained from the National Centre for Biology Information database to compare eight specimens of Myctophidae larvae between the two species. The 16S rRNA sequence of B. pterotum and D. garmani used in this study was assigned the following registration number from NCBI(MT242581-242587).
A 479 bp of 16S rRNA sequence was obtained from each of the four Myctophidae larvae for comparison with the sequence from adults of the candidate species D. garmani (KR231737). The genetic distances ranged from 0.6 to 0.8%, indicating close consistency. We also compared the 16S rRNA gene sequences of the four B. pterotum larvae with that of the adult B. pterotum (JX133756) and found complete consistency, with a genetic distance of 0%. The D. garmani larvae, juveniles, and adult were clearly separated from the four Diaphus spp. and B. pterotum in the neighbor joining tree (Fig. 3).
Materials examined: PKUI 618, one specimen, 6.01 mm in standard length (SL), 32° 99.61′ N, 127° 05.18′ E, southern Jejudo Island, 24 June 2017, Bongo net; PKUI 619, one specimen, 7.50 mm SL, 32° 99.61′ N, 127° 05.18′ E, southern Jejudo Island, 24 June 2017, Bongo net; PKUI 620, one specimen, 7.72 mm SL, 32° 99.61′ N, 127° 05.18′ E, southern Jejudo Island, 24 June 2017, Bongo net; PKUI 621, one specimen, 9.62 mm SL, 32° 99.61′ N, 127° 05.18′ E, southern Jejudo Island, 24 June 2017, Bongo net.
The 6.01 mm standard length (SL) post-flexion larva of D. garmani (Fig. 4a) has large eyes with a pair of small nostrils situated in front. The mouth is large and slopes, with the rear end of the upper jaw reaching the posterior rim of the eye. There is a row of small sharp teeth in the jaws. The anus is positioned slightly behind the middle of the body. The end of the operculum tapers to a triangular shape. The myomeres show a clear M shape. Br2 photophores are found on the middle part of the lower jaw, and only a trace of PO5 photophore remain in front of the pelvic fin. The larva has an adipose fin, 13 dorsal fin rays, 14 anal fin rays, nine pectoral fin rays, and seven pelvic fin rays (Table 1). A few melanophores are present on the opercular organ, a series of three melanophores on the ventral side, one star-shaped melanophore on the anus, and one large dot-shaped melanophore at the posterior end of the anal fin base.
The 7.56 mm SL post-flexion larva of D. garmani (Fig. 4b) has a heavier head. Br2 photophores are observed on the middle part of the lower jaw, a line of PO1235 photophores is arranged along the ventral side of the abdominal cavity, and a raised PO4 photophore is situated between PO3 and PO5. A PVO12 photophore is observed between the base of the opercular organ and the pectoral fin, a VO1 photophore on the base of the pelvic fin, and VO45 photophores on the anus. There are 14 dorsal fin rays, 15 anal fin rays, 10 pectoral fin rays, and 8 pelvic fin rays (Table 1). The occurrence of melanophores is similar to that of the 6.01 mm SL larva, but with the addition of two star-shaped melanophores in front of the pectoral fin and one and two on the upper and lower lobes, respectively, of the caudal fins.
The 7.72 mm SL juvenile of D. garmani (Fig. 4c) has a sharper snout than that at the previous stage, with the anus shifted forward toward the head. Br123 photophores are arranged in a line in the lower jaw. OP2 is recognized in the opercular organ under the eye. PLO is situated posterior to the operculum between the lateral line and pectoral fin. PO1235 photophores are aligned in the abdominal cavity. The PO4 photophore is raised between PO3 and PO5. The PVO12 photophore is situated obliquely between the base of the opercular organ and the pectoral fin. VLO is raised slightly from the middle between the base of the pelvic fin and the lateral line. VO145 photophores are arranged in a line behind the pelvic fin. The VO2 photophore is raised between VO1 and VO4. AOa23 photophores are lined on the anal fin. There are 14 dorsal fin rays, 15 anal fin rays, 13 pectoral fin rays, and 8 pelvic fin rays (Table 1). Several melanophores are present on the opercular organ. Two star-shaped melanophores are aligned on the abdominal cavity. One star-shaped melanophore is present on the anus. A dot-shaped melanophore is present at the end of the anal fin base. There are one and two melanophores dotted on the upper and lower lobes, respectively, of the caudal fin.
The 9.62 mm SL juvenile of D. garmani (Fig. 4d) has a similar morphology to that of the previous developmental stage. A Dn photophore is present in front of the eye. Br123 photophores are lined in the lower jaw. A OP2 is located under the eye. PO1235 photophores are arranged in a line in the abdominal cavity. PO4 is raised between PO3 and PO5. The PVO12 photophore is situated obliquely between the base of the opercular organ and the pectoral fin. PLO is situated between the base of the pectoral fin and the lateral line. VLO is raised slightly from the middle between the base of the pelvic fin and the lateral line. The VO145 photophores are arranged in a line behind the pelvic fin. The VO23 photophores are situated obliquely between VO1 and VO4. The SAO123 photophores are gradually raised behind VO5. AOa1 is slightly raised from the anal fin base and follows the line of AOa23456 photophores arranged along the anal fin base. AOa7 is raised more. Pol is near the lateral line at the end of the anal fin base. The AOp1234 photophores are arranged in a line along the caudal peduncle base behind the anal fin base. A line of Prc123 photophores is near the caudal peduncle, and Prc4 is observed under the lateral line of the caudal peduncle. There are 14 dorsal fin rays, 14 anal fin rays, and 8 pelvic fin rays (Table 1). Of the melanophores, one is observed above the left side of the eye, a few are recognized on the top of the head and the opercular organ with two star-shaped melanophores nearby. One melanophore is dotted on the anus, and one on the upper lobe and two dotted on the lower lobe of the caudal fin.
Scopelus pterotus Alcock, 1890: 217 (type locality: Madras coast, India)
Materials examined: PKUI 614, one specimen, 9.81 mm SL, 34° 83.28′ N, 129° 10.75′ E, eastern Korea Strait, 9 Dec 2018, Bongo net; PKUI 615, one specimen, 5.75 mm in standard length (SL), 32° 68.40′ N, 123° 88.52′ E, western Jejudo Island, 16 Sep 2018, Bongo net; PKUI 616, one specimen, 7.27 mm SL, 33° 09.68′ N, 126° 38.57′ E, East Sea, 9 Dec 2018, Bongo net; PKUI 617, one specimen, 8.07 mm SL, 37° 18.53′ N, 129° 67.90′ E, East Sea, 20 July 2018, Bongo net.
The 5.75 mm SL post-flexion larva of B. pterotum (Fig. 5a) has a small body depth and large head depth, with the head depth decreasing from the nape. At this stage of development, the larva has large eyes and a sharp snout. The anus is positioned slightly behind the middle of the body. A Br2 photophore is found in the lower jaw under the eye and Bu under the rear of the eye. The adipose fin is present. There are 10 dorsal fin rays, 14 anal fin rays, 11 pectoral fin rays, and 7 pelvic fin rays (Table 2). Observed melanophores include one on the end of the lower jaw snout, a line of melanophores in the lower jaw, one on the central upper part of the gut, many melanophore dots around the anus, and a line of very short, indistinct melanophores on the anal fin.
The 7.27 mm SL juvenile of B. pterotum (Fig. 5b) has a Dn photophore in front of the eye, Br2 on the lower jaw under the eye, and Bu under the rear of the eye. There are 12 dorsal fin rays, 18 anal fin rays, 11 pectoral fin rays, and 8 pelvic fin rays (Table 2). Observed melanophores include one on the end of the lower jaw, one around the nostrils, a line of four melanophores at the base of the opercular organ, in addition to irregularly distributed small dots of melanophores, and a few on the anus. During this stage of development, existing melanophores disappear from the central anus and central muscle of the anal fin.
The 8.07 mm SL juvenile of B. pterotum (Fig. 5c) has a Dn photophore in front of the eye, Br2 on the lower jaw under the eye, and Bu under the rear of the eye. There are 12 dorsal fin rays, 18 anal fin rays, 11 pectoral fin rays, and 8 pelvic fin rays (Table 2). Observed melanophores include one at the tip of the lower jaw snout, one under the nostrils, a line of dark dots on the base of the opercular organ, and a large melanophore dot on the anus.
The 9.81 mm SL juvenile of B. pterotum (Fig. 5d) has a Dn photophore in front of the eye, Br2 on the lower jaw under the eye, Cp on the opercular organ, a series of PO1234 photophores followed by a raised PO5 in the isthmus, PVO1 in front of the base of the pectoral fin behind the operculum organ, VO1 under the pelvic fin followed by a raised VO2, AOa12 on the anal fin, and Prc2 below the lateral line of the anal fin. There are 10 (?) dorsal fin rays, 18 anal fin rays, 11 pectoral fin rays, and 8 pelvic fin rays (Table 2). Observed melanophores include a series of dots under the operculum organ, one dot on the anus, one melanophore each on the upper and lower lobes of the base of the caudal fin.
We performed a molecular identification of Myctophidae larvae and juveniles collected in the waters around Jeju Island and the East Sea in June 2017 and found that four of the larvae had 99.2–99.4% identity to the sequence of a D. garmani adult and the other four larvae 100% identity to the sequence of a B. pterotum adult. Diaphus garmani has not been recorded in Korea, and its morphological features and measurements according to developmental stage obtained in this study were found to be generally consistent with previous reports (Gilbert 1906; Kawaguchi and Shimizu 1978; Sassa et al. 2003; Richards 2006; Bineesh et al. 2010; Okiyama 2014). Okiyama (2014) suggested that the completion of metamorphosis and photophore development occurs later in B. pterotum (12–13 mm SL) than in D. garmani (11 mm SL), which is consistent with our results. In this study, a juvenile D. garmani (PKUI 621, 9.62 mm BL) developed AOa1–7. This result is consistent with that in the original descriptive paper (Gilbert 1906), but differs from the results of Nakabo (2013) and Bineesh et al. (2010) which described only AOa1–6. In addition, D. garmani juveniles in this study had a Prc4 photophore under the lateral line inside of the caudal fin, which is consistent with Bineesh et al. (2010) but clearly different than that present under the lateral line outside of the caudal fin from Nakabo (2013). It is thought that the discrepant results for the AOa photophores are due to individual variations, but further study will be needed with regard to the Prc4 photophore location. The species D. garmani and B. pterotum share the characteristics of body shape, eye size, and photophore arrangement, but the two species can be differentiated by the photophore development process and relative growth of individuals of similar size. In terms of photophore development, B. pterotum first develops Dn in front of the eye, whereas D. garmani first develops OP2 at the base of the opercular organ. B. pterotum has Bu under the right eye and Cp on the isthmus, which are not present in D. garmani. The two species can also be differentiated by the position of the PO photophores; D. garmani has PO4 at the highest raised position, whereas B. pterotum has PO5 as the highest PO photophore. The arrangement of VO photophores can also clearly differentiate the two species: VO3 is located at the top in D. garmani versus VO2 in B. pterotum. AOa1 is positioned higher than the other AOa photophores in D. garmani, whereas the AOa photophores are arranged horizontally in B. pterotum.
In the adult stage, the two species can be clearly identified by the number of Prc photophores: four in D. garmani and two in B. pterotum (Nakabo 2013). We also found four Prc photophores in D. garmani and one in B. pterotum. Comparing the body ratios of the two species at each developmental stage, the ratios of head depth, head length, preanus length, eye diameter, and pre-anal fin length to SL differentiated the two species (Fig. 6a–e), except the ratio of pre-dorsal fin length to SL (Fig. 6a–f). Okiyama (2014) suggested that in D. garmani, the ratios of pre-anal fin length, head length, and body depth to SL increase linearly until the larvae grow to 7 mm SL, after which the ratios gradually decrease until they reach the juvenile stage. In contrast to that previous study, we showed that the pre-anal fin length increased with SL in D. garmani, but with no change in head length. This result may be related to the trivial difference in genetic distance (0.6–0.8%) in terms of the 16S rRNA sequence between Japanese D. garmani adults and the D. garmani larvae collected in Korean waters, which requires further study.
Based on mitochondrial DNA 16S rRNA sequences, two larvae and two juveniles were identified as Diaphus garmani, which is the first record in Korean waters. Among myctophid species previously recorded in Korea, Benthosema pterotum seems to be very similar to D. garmani in external morphology. The two species were distinguished by melanophores in the abdominal cavity (absent in D. garmani vs. present in B. pterotum) and the development of photophores (developed in D. garmani vs. rudimentary in B. pterotum) when shorter than 10.0 mm standard length. We propose a new Korean name Gar-ma-ni-sat-bi-neul-chi, a combination of the specific name garmani and the Korean name of the family Myctophidae Sat-bi-neul-chi.
Availability of data and materials
All datasets analyzed during the current study are available from the corresponding author on reasonable request.
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We are grateful to crews of research vessel of NIFS for their sampling and anonymous reviewers for their valuable comments.
This work was supported by a grant from the National Institute of Fisheries Science (R2020021) and also supported by the Marine Fish Resource Bank of Korea (No. 20170431) under the Ministry of Oceans and Fisheries, Korea.
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Lee, HL., Kim, JK., Yu, HJ. et al. Ontogenetic comparison of larvae and juveniles of Diaphus garmani and Benthosema pterotum (Myctophidae, Pisces) collected from Korea. Fish Aquatic Sci 23, 18 (2020). https://doi.org/10.1186/s41240-020-00163-w
- Diaphus garmani
- Benthosema pterotum
- New record
- 16S rRNA