Flatfishes are among the most strikingly asymmetrical vertebrates on Earth. During development, one eye migrates across the top of the skull so that both eyes and coloration end up on a single side, while the other side remains eyeless and pale. Currently, 826 species across 19 families are recognized as valid. In most flatfish families, eye position is fixed: families such as Paralichthyidae, Bothidae, and Scophthalmidae have both eyes on the left side, while families such as Pleuronectidae, Poecilopsettidae, and Samaridae have both eyes on the right. In contrast, members of Psettodidae, considered the most primitive flounders, naturally occur in roughly equal proportions with eyes on either side, and within Citharidae eye orientation varies at the generic level.

For decades, reversed individuals—those with eyes on the side opposite to their family’s norm—had never been documented in the family Bothidae, as summarized by researchers in 1964 and 1983. The family was long regarded as the most specialized group within the order Pleuronectiformes, with pronounced left-right differences such as a lateral line developed only on the eyed side and a much longer pectoral fin on the eyed side than on the blind side. This high degree of asymmetry led scientists to consider that the lower the level of specialization, the more likely inversion phenomena are to occur. Reversal was reported most frequently in Paralichthyidae and Pleuronectidae, families with fixed eye positions but lower degrees of asymmetrical development.

That assumption has now been shaken. In recent years, three reversed bothid specimens have been found and reported in the genera Chascanopsetta, Parabothus, and Crossorhombus. The two new specimens from Taiwan, belonging to Engyprosopon maldivense and Engyprosopon grandisquama, represent the first records of the reversed condition among members of their genus. Moreover, both show reversed internal organs—a condition known as situs inversus—making them the third and fourth records of visceral inversion in the entire family Bothidae.

The first specimen, a male Engyprosopon maldivense measuring 82.3 millimeters in standard length, was collected off Ke-Tzu-Liao Fishing port in Kaohsiung, southern Taiwan, in February 2024. The second, a female Engyprosopon grandisquama measuring 51.4 millimeters, was collected from the same waters in May 2024. Both were preserved in the Pisces Collection of the National Museum of Marine Biology and Aquarium in Pingtung, Taiwan. Detailed examination, including digital radiographs taken in collaboration with the Hokkaido University Museum in Japan, allowed researchers to compare the reversed individuals with normal specimens collected from the same location.

The reversed Engyprosopon maldivense was identified based on several diagnostic features: the absence of blotches on the caudal fin, no serrations on the inner gill rakers, biserial upper-jaw teeth, a wide interorbital width, 44 lateral-line scales, and 0 + 9 gill rakers. Its meristic and morphometric data generally agree with those of normal sinistral specimens, with only minor differences such as a slightly larger lower eye and slightly different jaw and pelvic-fin proportions. The reversed Engyprosopon grandisquama was identified by a pair of black blotches on the caudal fin, 40 lateral-line scales, 0 + 7 gill rakers, and a distinctive interorbital width. In both specimens, the liver is situated on the eyed side and a large part of the stomach on the blind side—a complete reversal of visceral organ positions compared with normal individuals.

Previously, reversed specimens in the family Bothidae were extremely rare, with only three reported cases. The rarity is possibly due to their low commercial value, which results in a low chance of detecting an inversion. Bothidae and Samaridae are generally small, contain few species important to fisheries, and are mostly discarded or used as feed for other fish. In contrast, reversal is most commonly reported in Pleuronectidae and Paralichthyidae, which contain many species important to fisheries. Abnormal individuals with both eyes on the opposite side are easily discovered during the distribution process of these fish, and when processed into dried fish, many individuals of the same species are lined up, making inverted individuals highly conspicuous. From this, it can be speculated that the observed frequency of inverted individuals is due to differences in the chances of discovery rather than true biological differences among families.

The genetic basis of reversal has been demonstrated through crossbreeding experiments in the starry flounder Platichthys stellatus, conducted by Policansky in 1982. In Japanese populations, 100 percent of individuals have both eyes on the left side, whereas in American populations 50 percent have both eyes on the right and 50 percent on the left. Crossbreeding experiments between left-eyed males and females and between right-eyed males and females revealed that inverted individuals appeared at a rate of approximately 13 to 18 percent in both cases, confirming that the appearance of inversion is caused by heredity. The survival percentage of inversions occurring in nature is thought to be significantly lower than that observed in experimental results.

Reproduction in Bothidae may impose additional constraints on reversed individuals. It is known that reproductive behavior involves males and females overlapping away from the seabed. If the position of the genital pores is reversed, there would be a disadvantage during mating, which could explain a geographical bias in the frequency of reversal similar to that seen in Platichthys stellatus, where the dextral type occurs predominantly on the American side and the sinistral type on the Japanese side.

The discovery of inverted individuals in three genera and four species of the family Bothidae has called into question the long-held idea that the family was too specialized for reversal to occur. The two new specimens from Taiwan suggest that the reversed condition in Bothidae may be more common than previously thought, and that the observed rarity reflects sampling bias rather than evolutionary constraint. As more specimens are examined and more collections are made in understudied waters, scientists may find that reversed flatfishes are quietly swimming throughout the world’s oceans, waiting to be noticed.

Subject of Research: Marine Biology

Article Title: First records of two reversed flatfishes, Engyprosopon maldivense and Engyprosopon grandisquama ( Actinopterygii , Pleuronectiformes , Bothidae ), from Taiwan, northwestern Pacific Ocean

Article References: Amaoka, K., Hо, H.-C., & Su, Y. (2026). First records of two reversed flatfishes, Engyprosopon maldivense and Engyprosopon grandisquama (Actinopterygii, Pleuronectiformes, Bothidae), from Taiwan, northwestern Pacific Ocean. Acta Ichthyologica et Piscatoria, 56, 431-439. https://doi.org/10.3897/aiep.56.207218

Image Credits: AI Generated

DOI: 10.3897/aiep.56.207218

Keywords: flatfish, Engyprosopon, Taiwan, situs inversus, Bothidae, reversal, marine biology, First, records, reversed, flatfishes, maldivense