106 urn:lsid:arphahub.com:pub:dddd9632-2f62-529d-aa08-fcb37c695039 urn:lsid:zoobank.org:pub:73014291-F6B9-403A-8AC3-FAC9C44FEF36 Acta Ichthyologica et Piscatoria AIeP 0137-1592 1734-1515 Pensoft Publishers 10.3897/aiep.56.207218 207218 Research Article Actinopterygii Bothidae Pleuronectiformes Taxonomy Asia Pacific Ocean No poverty First records of two reversed flatfishes, Engyprosopon maldivense and Engyprosopon grandisquama (Actinopterygii, Pleuronectiformes, Bothidae), from Taiwan, northwestern Pacific Ocean Amaoka Kunio https://orcid.org/0009-0005-9439-6994 1 Conceptualization Writing - original draft Data curation Formal analysis Hо Hsuan-Ching https://orcid.org/0000-0003-1154-601X 2 3 4 5 Writing - review and editing Funding acquisition Su Yo scott03150315@gmail.com https://orcid.org/0000-0002-3576-9229 5 Conceptualization Writing - review and editing Visualization Hokkaido University, Hakodate, Hokkaido 040, Japan Hokkaido University Hokkaido Japan Department of Aquaculture, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan Department of Aquaculture, National Kaohsiung University of Science and Technology Kaohsiung Taiwan https://ror.org/00hfj7g70 National Museum of Marine Biology and Aquarium, Pingtung, Taiwan National Museum of Marine Biology and Aquarium Pingtung Taiwan https://ror.org/02apq7b82 Australian Museum, Sydney, Australia Australian Museum Sydney Australia International Doctoral Program of Marine Science and Technology, National Sun Yat-sen University, Kaohsiung, Taiwan International Doctoral Program of Marine Science and Technology, National Sun Yat-sen University Kaohsiung Taiwan https://ror.org/02zv4ka60

Corresponding author: Yo Su (scott03150315@gmail.com)

Ronald Fricke

 2026 28 09 2026 56 431 439 32AA747C-03A5-5B7C-868B-DDD3D5D0A1E9 33F21AE7-53F9-4DE3-AFA3-C6214A09BE95 09 07 2026 31 08 2026 Kunio Amaoka, Hsuan-Ching Hо, Yo Su This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. https://zoobank.org/33F21AE7-53F9-4DE3-AFA3-C6214A09BE95

Two species belonging to the genus Engyprosopon in the flatfish family Bothidae, exhibiting the reversed condition not only externally but also internally, were recently collected from southwestern Taiwan. A detailed description of each specimen is provided, and situs inversus is discussed. Previously, reversed specimens in the family Bothidae were extremely rare, with only three reported cases, whereas the reversed condition was commonly found in the families Paralichthyidae and Pleuronectidae. This rarity is possibly due to their low commercial value, which results in a low chance of detecting an inversion. Previously, it was thought that the family Bothidae was the most specialized group within the order Pleuronectiformes, making the reversed condition less likely to occur. However, this idea may need reevaluation, and the reversed condition in Bothidae may be more common than previously thought.

 Abnormality Actinopterygii ichthyology morphology situs inversus viscerum National Kaohsiung University of Science and Technology and National Science and Technology Council, Taiwan (NSTC 113-2327-B-992-001- and 114-2327-B-992-001-) Introduction

Flatfishes are a unique group of fishes that are outwardly asymmetrical because both eyes and coloration are on one side of the body rather than the other, although the internal organs are not symmetrical in position or morphology, as is also the case in other vertebrates. Currently, 826 species across 19 families are recognized as valid (Fricke et al. 2026). In most flatfish families, eye position is fixed; for example, families such as Paralichthyidae, Bothidae, and Scophthalmidae have both eyes located on the left side of the body. On the other hand, families such as Pleuronectidae, Poecilopsettidae, and Samaridae have both eyes on the right side (Norman 1934; Hensley 2001a; Amaoka 2016). In contrast, members of Psettodidae exhibit variable eye orientation, with individuals occurring in approximately equal proportions on either the left or right side (Hensley 2001b). Furthermore, within Citharidae, eye orientation varies at the generic level, with some genera having eyes on the left side and others on the right (Hubbs 1945; Amaoka 1969; Hensley 2001c).

In very rare cases, individuals with a reversed left-right position are found in families with fixed eye positions (e.g., Follett et al. 1960; Amaoka 1964; Dawson 1964, 1966, 1971; Okiyama and Tomi 1970; Dawson and Heal 1976; Ruiz-Carus and Rider 1998; Da Silva Jr et al. 2007; Ivankov et al. 2008; Goto 2009). However, for a long time, this phenomenon had not been found in the family Bothidae, as summarized by Amaoka (1964) and Kamei (1983). Surprisingly, three new cases have recently been found and reported in Bothidae (Kuroshima et al. 2022; Su et al. 2024; Lin and Ho 2026).

The two species reported here belong to the bothid genus Engyprosopon and are the first records of the reversed condition among congeners. Moreover, both show reversed internal organs (i.e., situs inversus), representing the second and third records in Bothidae. Here, these specimens are described in detail, and the values of numerical characters and morphometric features are compared with those of normal specimens collected from the same place. Some considerations regarding the reversal, such as asymmetry, frequency, and factors, are also discussed.

 Materials and methods

The reversed specimens and comparative specimens examined in this study were collected from the waters of Taiwan and preserved in the Pisces Collection of the National Museum of Marine Biology and Aquarium, Pingtung, Taiwan (NMMB-P).

Counts and proportional measurements follow those of Hubbs and Lagler (1958) and Amaoka et al. (1993). Standard length (SL) and head length (HL) are used throughout. All measurements were made to the nearest 0.1 mm with calipers. The dorsal-, anal-, and caudal-fin rays and vertebrae were counted from digital radiographs from NMMB and the Hokkaido University Museum, Hakodate, Japan (HUMZ). The features of the caudal skeleton were observed in the radiographs.

 Results

Family Bothidae

Genus Engyprosopon Günther, 1862

Species composition and diversity in Taiwan

Among the 35 valid species of Engyprosopon, 12 species are currently recognized in Taiwanese waters (Amaoka and Ho 2018; Su et al. 2025), of which three species, namely Engyprosopon brevifrontale Amaoka et Ho, 2018, Engyprosopon keliaoense Amaoka et Ho, 2022, and Engyprosopon parvipectorale Amaoka et Ho, 2018, were originally described based on specimens collected from Taiwan (Amaoka and Ho 2018, 2022). An identification key to species was provided by Su et al. (2025).

Animalia Pleuronectiformes Bothidae 8FF2650A-34C1-5CE7-AC9E-7D646AE7A9FD Engyprosopon maldivense (Regan, 1908) English name: olive wide-eyed flounder Chinese name: 馬爾地夫短額鮃 Figs 1 , 3C, D ; Table 1 Scaeops maldivensis Regan, 1908.—Regan (1908): 234, pl. 25. (Type locality: Maldives, Indian Ocean, depth 27–44 fathoms [49.3–80.5 m]) Arnoglossus maculipinnis Fowler, 1934.—Fowler (1934): 329, fig. 84 (Type locality: Vicinity of Jolo, Sulu Province, the Philippines, 6°08'45''N, 121°03'E, depth 20–76 fathoms [36.6–139.0 m]) Engyprosopon borneensis Chabanaud, 1948.—Chabanaud (1948): 64, figs 1, 2 (Type locality: Northeast of Datoe [Datoae] Point, north coast of Borneo, Malaysia) Engyprosopon macroptera Amaoka, 1963.—Amaoka (1963): 115, fig. 5 (Type locality: off Mimase, Kochi Prefecture, Japan) Specimen examined.

NMMB-P42769, 1 male, 82.3 mm SL, off Ke-Tzu- Liao Fishing port, ca. 22°42'53''N, 120°13'12''E, Kaohsiung, southern Taiwan, 3 Feb. 2024, collected by Y. Su.

 Comparative specimens.

As listed in Amaoka and Ho (2018).

 Description of reversed individual.

Counts and proportional measurements as shown in Table 1.

Comparison of proportional measurements and counts between reversed (dextral) and normal (sinistral) specimens of Engyprosopon maldivense and E. grandisquama.

Character E. maldivense E. grandisquama
Reversed Normal Reversed Normal
1 M 6 M, 6 F 1 F 6 M, 4 F
Absolute values [mm]
Standard length (SL) 82.3 71.1–180.5 63.2 65.7–82.5
Head length (HL) 22.1 19.3–40.8 13.9 17.0–20.2
Relative values [in SL]
Head length (HL) 3.72 3.68–4.42 3.7 3.65–4.16
Body depth 1.85 1.82–2.10 1.77 1.63–1.86
Relative values [in HL]
Snout length (from lower eye) 4.17 3.78–5.20 5.72 4.61–5.89
Upper-eye diameter 3.2 3.12–3.96 2.98 2.66–3.32
Lower-eye diameter 2.95 3.04–4.16 3.04 2.69–3.43
Interorbital width in M 2.72 1.65–2.31 — 2.52–3.47
Interorbital width in F — 2.72–3.89 6.81 4.57–5.31
Upper-jaw length (O) 2.7 2.59–2.77 3.25 3.07–3.47
Upper-jaw length (B) 2.48 2.56–2.78 3.33 3.12–3.55
Lower-jaw length (O) 2.1 1.95–2.18 2.27 2.21–2.41
Lower-jaw length (B) 1.94 1.99–2.18 2.1 2.02–2.29
Caudal-peduncle depth 2.19 1.74–2.13 2.13 1.74–2.14
Pectoral-fin length (O) in M 0.56 0.43–0.79 — 1.01–1.04
Pectoral-fin length (O) in F — 0.44–0.61 1.13 1.03–1.14
Pectoral-fin length (B) 1.92 1.56–2.00 2.01 1.72–2.04
Pelvic-fin length (O) 2.05 1.73–2.33 2.2 2.05–2.76
Pelvic-fin length (B) 2.19 1.85–2.72 2.34 2.03–2.44
Length of pelvic-fin base (O) 2.19 2.05–2.42 2.55 2.17–2.73
Length of pelvic-fin base (B) 5.26 5.52–7.25 6.81 5.72–8.14
Longest dorsal-fin ray 1.96 1.55–1.98 1.81 1.77–2.02
Longest anal-fin ray 1.89 1.44–2.00 1.91 1.69–2.14
Counts
Dorsal-fin rays 89 85–94 84 81–87
Anal-fin rays 65 64–69 65 61–65
Caudal-fin rays (unbranched + branched + unbranched) 3 + 11 + 3 3 + 11 + 3 3+ 11 + 3 3 + 11 + 3
Pectoral-fin rays (O) 11 10–12 11 10–12
Pectoral-fin rays (B) 10 9–11 9 8–10
Scales in lateral line 44 44–48 40 40–45
Gill rakers (O) 0 + 9 0–3 + 8–11 0 + 7 0 + 5–7
Vertebrae (abdominal + caudal) 10 + 25 10 + 25–26 10 + 24 10 + 24

Abbreviations: B, blind side; F, female; HL, head length; M, male; O, ocular side; SL, standard length.

Body ovoid, deepest slightly before middle, its depth approximately half of SL. Head small, its length less than half of body depth; anterior margin of head about vertical with slight concave before upper margin of upper eye. Snout length shorter than eye diameter; short spine on near tip of snout. Eyes rather large, anterior margin of upper eye attached to body margin; both eyes separated by deep and wide space, interorbital width wider than eye diameter.

10.3897/aiep.56.207218.figure1 FABB38C0-3D3E-5F52-AF9B-36A1467DAC39

Fresh (A) and preserved (B, C) conditions of a reversed specimen of Engyprosopon maldivense, NMMB-P42769, 82.3 mm SL. (A, B) Ocular (right) side; (C) blind (left) side. Photo by Y.-C. Hsu (A).

https://binary.pensoft.net/fig/1792369

Mouth large, maxillary extending below middle of lower eye; synthesis of lower jaw with extremely small process at ventral tip. Teeth on upper jaw in two rows, outer row stronger and wider in interval than those of inner row; some anterior teeth canine-like; teeth on lower jaw strong and single row.

Gill rakers elongate and sharply tipped, without serration on posterior margin, no gill rakers on upper limb.

Scales on ocular side ctenoid with short spines along posterior margin, those on blind side cycloid. Pectoral fin on ocular side elongate, filamentous, longer than head; that on blind side very small, shorter than approximately 1/3 of that on ocular side. Origin of pelvic fin on ocular side below middle of lower eye, width of fin-base very wide; that on blind side originating between fourth and fifth rays on ocular side, and narrow base.

Caudal fin rounded, upper and lower three rays simple and others branched. Anus on blind side before origin of anal fin, genital papilla on ocular side opposite side of anus.

Caudal skeleton with deeply branched parhypural and three hypurals. Suborbital bones three on blind side, but absent on ocular side. Liver situated on ocular side, and a large part of the stomach on blind side (Fig. 3C, D).

 Coloration.

When fresh, body somewhat yellow, with scale pockets dark. Yellow spots scattered on upper margin of eyes, snout, and anterior dorsal-fin base. When preserved, body and fins on ocular side light greyish green, that on blind side yellowish white stained with dark.

 Remarks.

The specimen was identified as a reversed individual of E. maldivense based on having no blotches on the caudal fin, no serrations on the inner gill rakers, biserial upper-jaw teeth, a wide interorbital width (2.72 in HL), 44 lateral-line scales, 0 + 9 gill rakers, and eyes positioned away from the anterior margin of the head (Amaoka and Ho 2018; Su et al. 2025). The meristic and morphometric data of the reversed (dextral) specimen generally agree with those of normal (sinistral) specimens (Table 1), except that the reversed specimen possesses a slightly larger lower eye (2.95 vs. 3.04–4.16 in HL in normal specimens), a slightly smaller interorbital width (2.72 vs. 1.65–2.31 in HL), a slightly larger blind-side upper jaw (2.48 vs. 2.56–2.78 in HL), a slightly larger blind-side lower jaw (1.94 vs. 1.99–2.18 in HL), and a slightly larger blind-side pelvic-fin base (5.26 vs. 5.52–7.25 in HL).

 Animalia Pleuronectiformes Bothidae F198A8DB-1098-5B63-B57D-DFF66E43FEBF Engyprosopon grandisquama (Temminck et Schlegel, 1846) English name: largescale flounder Chinese name: 偉鱗短額鮃 Figs 2 , 3E, F ; Table 1 Rhombus grandisquama Temminck et Schlegel, 1846.—Temminck and Schlegel (1846): 183, pl. 92. (Type locality: Nagasaki, Japan) Rhomboidichthys spilurus Günther, 1880.—Günther (1880): 47, pl. 21. (Type locality: South, Zebu, Paoua New Guinea, depth 28 fathoms [51.2 m]) Rhomboidichthys spiniceps Macleay, 1881.—Macleay (1881): 127 (Type locality: Port Jackson, New South Wales, Australia) Rhombus poecilurus Bleeker, 1852.—Bleeker (1852): 293 (Type locality: Ambon Island, Molucca Islands, Indonesia) Specimen examined.

NMMB-P42768, 1 female, 51.4 mm SL, off Ke-Tzu- Liao Fishing port, ca. 22°42'53''N, 120°13'12''E, Kaohsiung, southern Taiwan, 18 May 2024, collected by C.-A. Lin.

 Comparative specimens.

As listed in Amaoka and Ho (2018).

 Description of reversed individual.

Counts and proportional measurements as shown in Table 1.

Body ovoid, deepest near middle, its depth slightly deeper than half of SL. Head small, its length less than half of body depth; anterior margin of head somewhat steep with a slight concave before upper margin of upper eye. Snout length much shorter than eye diameter; no spine on near tip of snout. Eyes rather large, anterior margin of upper eye very close to dorsal edge of body; both eyes separated by deep and somewhat wide space, interorbital width narrower, approximately half of eye diameter.

Mouth small, maxillary extending below anterior margin of lower eye. Teeth on upper jaw two rows, outer row stronger and wider in interval than those of inner row; teeth on lower jaw strong and close-set in single row.

Gill rakers short, rounded at tip, without serration on posterior margin, no gill rakers on upper limb.

Scales on ocular side ctenoid with short spines along posterior margin, those on blind side cycloid. Pectoral fin on ocular side short, shorter than head of ocular side; that on blind side very small, shorter than approximately half of head on blind side. Origin of pelvic fin on ocular side below middle of lower eye, width of fin-base very wide; that on blind side originating at fourth ray on ocular side, and narrow base. Caudal fin rounded, upper and lower three rays simple and others branched. Anus on blind side before origin of anal fin, genital papilla on ocular side opposite side of anus.

10.3897/aiep.56.207218.figure2 C2B85BBF-7737-5768-9001-200D627D5D36

Preserved condition of a reversed specimen of Engyprosopon grandisquama, NMMB-P42768, 51.4 mm SL. (A) Ocular (right) side; (B) blind (left) side.

https://binary.pensoft.net/fig/1792370

Caudal skeleton with deeply branched parhypural and three hypurals. Suborbital bones three on blind side, but absent on ocular side. Liver situated on ocular side, and a large part of the stomach on blind side (Fig. 3E, F).

10.3897/aiep.56.207218.figure3 FAC7C974-2FFA-5778-AA49-8EA2A726AE83

Comparison of the positions of visceral organs (liver and stomach) between normal (sinistral) specimens and reversed (dextral) specimens of Engyprosopon grandisquama (A, B, E, F) and E. maldivense (C, D). (A, C, E) Ocular side; (B, D, F) blind side. (A, B) Normal specimen; (C, D, E, F) reversed specimen. Abbreviations: L, liver; S, stomach.

https://binary.pensoft.net/fig/1792371 Coloration.

When preserved, body and fins on ocular side light greenish grey, that on blind side whiteish.

 Remarks.

The specimen was identified as a reversed individual of Engyprosopon grandisquama based on having a pair of black blotches on the caudal fin, 40 lateral-line scales, 0 + 7 gill rakers, body depth 1.77 in SL, and interorbital width 6.81 in HL (Amaoka and Ho 2018; Su et al. 2025). The meristic and morphometric data of the reversed specimen also agree with those of normal specimens (Table 1), except that the reversed specimen possesses a slightly smaller interorbital width (6.81 vs. 4.57–5.31 in HL in normal specimens).

 Discussion

In the present inverted specimens, a complete left-right reversal of all organs was found, although optic-nerve crossing was not confirmed. According to Su et al. (2024) and Lin and Ho (2026), visceral inversion (situs inversus) occurred in Parabothus taiwanensis Amaoka et Shen, 1993 and Crossorhombus valderostratus (Alcock, 1890), but not in Chascanopsetta lugubris Alcock, 1894 (see Kuroshima et al. 2022). Thus, both types have been observed in the family Bothidae. The two reversed specimens of Engyprosopon reported here represent the third and fourth records of visceral inversion in Bothidae.

According to Amaoka (1964), the manifestation of left-right inversion had previously been reported in one species in the family Citharidae (Hubbs 1945). It occurs most frequently in Paralichthyidae and Pleuronectidae but had not been observed in Bothidae or Samaridae at that time. Until now, only three records of reversed bothids have been reported (Kuroshima et al. 2022; Su et al. 2024; Lin and Ho 2026). Moreover, Amaoka and Ho (2019) pointed out that the specimen of Laeops kitaharae (Smith et Pope, 1906) depicted in Shen and Wu (2012) is, in fact, a reversed Plagiopsetta glossa Franz, 1910 (Samaridae). From this, Amaoka discussed the frequency of reversed occurrence in flatfishes from the perspective of flatfish evolution and phylogenetic relationships.

The most primitive flounders, Psettodoidei, naturally occur in roughly equal proportions, with both eyes on the left side in half of the individuals and on the right side in the other half, and there are two types of optic-nerve crossing. The upper eye is located on the back of the body, and the eyed and eyeless sides have roughly the same color. They have one spine on the pelvic fin, roughly bilaterally symmetrical bodies and paired fins, and a basic perciform feature of the caudal skeleton.

The family Citharidae includes the genus Citharoides Hubbs, 1915, with both eyes on the left, and the genus Lepidoblepharon Weber, 1913, with both eyes on the right, and it is thought that the left-right orientation of the family has not yet been fixed. It has one spine on its pelvic fin, and its caudal-fin skeleton resembles that of Psettodoidei, retaining its primitive form. However, in other organs, left-right asymmetry is quite advanced. For this reason, the family is considered to be intermediate between Psettodoidei and two families, Paralichthyidae and Pleuronectidae (Hubbs 1945; Amaoka 1969). These families have both eyes on the left and on the right, respectively, and are groups with a low degree of asymmetrical development.

On the other hand, in the family Bothidae, which has both eyes on the left, and the family Samaridae, which has both eyes on the right, the left and right sides are significantly asymmetrical (e.g., the lateral line is developed on the ocular side and weakly developed or absent on the blind side, and the pectoral fin on the ocular side is longer than that on the blind side; Hensley 2001a; Hensley and Amaoka 2001; Amaoka 2016). For these reasons, Amaoka (1964) considered that the lower the level of specialization, the more likely inversion phenomena are to occur. However, the recent discovery of inverted individuals in three genera and four species of the family Bothidae has called this idea into question.

Therefore, a different estimation of the frequency of reversal in flatfishes was attempted. Reversal is most commonly reported in the families Pleuronectidae and Paralichthyidae, which contain many species important to fisheries (Amaoka and Hensley 2001; Amaoka 2016). It is thought that abnormal individuals with both eyes on the opposite side of the body are easily discovered during the distribution process of these fish. Furthermore, when processed into dried fish, many individuals of the same species are lined up, making inverted individuals highly conspicuous.

On the other hand, Bothidae and Samaridae are generally small, contain few species important to fisheries, and are mostly discarded or used as feed for other fish, making the chances of discovering inverted individuals extremely low (Amaoka 2016, 2019). From this, it can be speculated that the observed frequency of inverted individuals is due to differences in the chances of discovery and that there is little difference among families of Pleuronectiformes.

Policansky (1982) conducted various combinations of crossbreeding experiments between individuals with both eyes on the left and those with both eyes on the right in Platichthys stellatus (Pallas, 1787) (100% of Japanese individuals have both eyes on the left side of the body, whereas 50% of American individuals have both eyes on the right side of the body and 50% on the left side). As a result, it was revealed that the appearance of inversion is caused by heredity. In crossbreeding experiments between left-eyed males and females and between right-eyed males and females, inverted individuals appeared at a rate of approximately 13–18% in both cases. The survival percentage of inversions occurring in nature is thought to be significantly lower than that observed in the experimental results.

Regarding reproduction in Bothidae, it is known that reproductive behavior involves males and females overlapping away from the seabed (Moyer et al. 1985). If the position of the genital pores is reversed, there will be a disadvantage during mating. Accordingly, it is thought that there is a geographical bias in the frequency of reversal, as in Platichthys stellatus, with the dextral type having both eyes on the right side occurring on the American side and the sinistral type occurring on the opposite side in Japan.

 Acknowledgments

The authors thank Tung-Hai Fishery Company and C.-A. Lin (NCHU) for assistance with specimen collection; Y.-C. Hsu (NSYSU) for taking photographs; P.-N. Lee (NMMBA) for curatorial assistance; and T. Kawai and F. Tashiro (HUMZ) for various forms of assistance. This study was supported by the National Kaohsiung University of Science and Technology and the National Science and Technology Council, Taiwan (NSTC 113-2327-B-992-001- and 114-2327-B-992-001-).

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The authors have declared that no competing interests exist.

 Ethical statement

No ethical statement was reported.

 Artificial Intelligence (AI) use

The authors accept full responsibility for the content of the manuscript, including the disclosure of any use of AI.

No AI tools were used in the preparation of this manuscript.

 Funding

This study was supported by the National Kaohsiung University of Science and Technology and the National Science and Technology Council, Taiwan (NSTC 113-2327-B-992-001- and 114-2327-B-992-001-).

 Author contributions

Conceptualization: KA, YS. Data curation: KA. Formal analysis: KA. Funding acquisition: HCH. Visualization: YS. Writing - original draft: KA. Writing - review and editing: HCH, YS.

 Author ORCIDs

K. Amaoka https://orcid.org/0009-0005-9439-6994

H.-C. Hо https://orcid.org/0000-0003-1154-601X

Y. Su https://orcid.org/0000-0002-3576-9229

 Data availability

All of the data that support the findings of this study are available in the main text.