The family Poecilopsettidae comprises just 21 valid species in three genera: two species in Marleyella, four in Nematops, and 15 in Poecilopsetta, the genus to which the new Japanese specimens belong. These fishes are characterized by a suite of anatomical features that set them apart from other flatfishes: eyes on the right side of the body, an ocular-side lateral line that curves upward above the pectoral fin, a blind-side lateral line that is reduced or absent entirely, a dorsal fin that originates above the upper eye, pectoral fins present on both sides of the body, no lachrymal bone on the blind side, and an autogenous first preural centrum and five hypurals in the caudal skeleton. Until now, Poecilopsetta colorata had been recorded only as far north as the South China Sea off Hong Kong, while Poecilopsetta praelonga had been known only from southern Taiwan at its northern limit. Both species had previously been documented from tropical and subtropical waters of the eastern Indian Ocean and western Pacific, from the Mozambique Channel and Arabian Sea in the west to the Philippines and northwestern Australia in the east, generally in depths of 216 to 732 meters.

Identifying deep-sea flatfishes is notoriously difficult, and the researchers behind the new report did not rely on a single character. Instead, they reassessed the diagnostic features of each species using a combination of fin-ray counts, scale counts, body proportions, skeletal details, and coloration. The two specimens of Poecilopsetta colorata, one from Tosa Bay measuring 103.1 millimeters in standard length and one from the Enshu-nada Sea measuring 91.0 millimeters, were identified by a specific combination of characters: dorsal-fin rays numbering 48 to 62, anal-fin rays 46 to 53, ocular-side pectoral-fin rays 9 to 12, and lateral-line scales 90 to 124. The body is deep, with its greatest depth at mid-body, and the eyes are separated by a flat, scaled interorbital area rather than being contiguous. Critically, the middle pectoral-fin rays on the ocular side are branched, a feature that distinguishes this species from close relatives in which all pectoral-fin rays are simple. A single large dark blotch on the ocular-side pectoral fin completes the identification.

The single specimen of Poecilopsetta praelonga, measuring 104.5 millimeters in standard length and collected from the same Enshu-nada Sea trawl station, presented a different anatomical profile. Its dorsal-fin rays number 57 to 65, anal-fin rays 45 to 55, ocular-side pectoral-fin rays 6 to 11, and lateral-line scales 91 to 111. The body is slender and oblong rather than deep and round, and the eyes are contiguous, touching each other rather than being separated by an interorbital space. All pectoral-fin rays are simple on both sides, with no branched rays, and the straight section of the lateral line is about three times the length of the lateral-line curve width. The pectoral fin is uniformly black, a striking feature that contrasts with the single large dark blotch of Poecilopsetta colorata. The researchers confirmed these identifications by comparing the Japanese specimens against type and non-type material held in collections in Japan and India, including the holotype of Poecilopsetta maculosa, a junior synonym of Poecilopsetta colorata, and the syntypes of Poecilopsetta praelonga from the Bay of Bengal off Colombo, Sri Lanka.

To bolster the morphological evidence, the team extracted total genomic DNA from muscle tissue preserved in 99.5 percent ethanol and amplified a partial mitochondrial cytochrome c oxidase subunit I gene, the standard DNA barcode for fishes. The polymerase chain reaction was run for 30 cycles with denaturation at 94 degrees Celsius for 15 seconds, annealing at 56 degrees Celsius for 15 seconds, and extension at 72 degrees Celsius for 30 seconds. The resulting sequences were registered in the International Nucleotide Sequence Database Collaboration under accession numbers LC924618, LC924620, and LC924619, providing a permanent molecular reference for the Japanese specimens. X-ray images were used to count unpaired-fin rays and vertebrae and to examine the caudal skeleton, following established terminology for flatfish osteology. All measurements were made to the nearest 0.1 millimeter with digital calipers and a divider, and the specimens are now deposited in the fish collections of Kochi University and the Kanagawa Prefectural Museum of Natural History.

Beyond the identification itself, the study produced a valuable reassessment of which characters are actually reliable for distinguishing species of Poecilopsetta. The presence or absence of branched ocular-side pectoral-fin rays has long been treated as species-specific, and the new evidence supports that view. The researchers examined 16 adult specimens of Poecilopsetta plinthus, a common Japanese species, and found that all had branched pectoral-fin rays, consistent with the species diagnosis. In contrast, the presence or absence of ocular-side transverse bands, long considered a species-level character, now appears less dependable. The Japanese specimen of Poecilopsetta praelonga had faint transverse bands on the ocular-side body, even though such bands have been considered absent in other adult specimens of the species previously examined. Similarly, the presence or absence of black blotches on the caudal fin, once treated as diagnostic, shows intraspecific variation in Poecilopsetta praelonga: some specimens have two pairs of dark blotches, others lack them entirely. The ocular-side head and body coloration of Poecilopsetta colorata also appears variable, with some adults showing many small black dots and others lacking them.

The most compelling explanation for how these tropical deep-sea flatfishes arrived in Japanese waters lies in the life history of the group and the physics of the western Pacific. Poecilopsettid species are thought to have a long pelagic larval stage, as evidenced by their large-sized pelagic larvae, which can reach roughly 26.5 to 29.0 millimeters in standard length in related species such as Poecilopsetta hawaiiensis. The Japanese specimens of Poecilopsetta colorata and Poecilopsetta praelonga retained faint black spots on the blind-side body, spots that are distinct in larval and juvenile Poecilopsetta but tend to disappear with growth and are lost in larger adults. This suggests the specimens had spent a relatively short time post-settlement before collection. Western Pacific localities where these species have been previously recorded, including the Philippines and Taiwan, sit squarely in the pathway of the Kuroshio Current, the strong western boundary current that flows northeastward past Taiwan and along the Pacific coast of Japan. The researchers therefore believe the larvae were transported northward by the Kuroshio during their pelagic stage and subsequently settled near their collection sites.

The Enshu-nada Sea records are the northernmost for both species, and the finding fits a broader pattern. Considerable range extensions probably driven by ocean-current dispersal have already been documented for the Atlantic species Poecilopsetta beanii, which has been recorded far outside its previously known distribution. The Kuroshio Current is well known to marine biologists as a dispersal mechanism for shallow-water tropical fishes that reach temperate Japan as larvae, contributing to the phenomenon of tropicalization in coastal Japanese waters. The new report extends that paradigm to deep-sea flatfishes, showing that the same current can carry larvae of deep-dwelling tropical species into temperate seas at bathyal depths. Because the specimens were collected by bottom trawl at 300 meters, a depth routinely worked by Japanese research and commercial vessels, it is plausible that other tropical deep-sea fishes are settling undetected in Japanese waters, waiting to be recognized by taxonomists with the right expertise.

The researchers proposed new standard Japanese names for both species: Gomafu-kawara-garei for Poecilopsetta colorata, referring to the black dots scattered on the ocular-side head and body of the reported specimens, and Hōba-kawara-garei for Poecilopsetta praelonga, referring to the body shape reminiscent of a leaf of the Japanese bigleaf magnolia. These names, based on the Enshu-nada Sea specimens, anchor the species in Japanese ichthyological tradition and will aid communication about the fishes among researchers, fishers, and conservationists. The study also highlights the importance of museum collections and comparative material: the identification of the Japanese specimens depended on access to type specimens collected more than a century ago in Sri Lankan waters, as well as modern comparative material from the Sulu Sea, the Indian Ocean off Java, and northwestern Australia. Without such collections, deep-sea biodiversity would remain largely invisible, and range extensions of the kind reported here would go unnoticed.

For deep-sea biologists, the Japanese records raise questions about whether these populations are self-sustaining or ephemeral, dependent on continued larval input from the south. The specimens were adults, as indicated by the lack of large round black spots on the ocular-side head and body, a feature characteristic of larvae and juveniles of Poecilopsetta colorata. Whether they can reproduce in Japanese waters, and whether their offspring would survive the cooler temperatures at the northern margin of the species ranges, remains unknown. What is clear is that the Kuroshio Current continues to reshape the biogeography of the western Pacific deep sea, one larva at a time, and that careful taxonomic work at trawl stations off Japan can catch these biogeographic shifts as they happen. The three specimens now preserved in Japanese museums are more than curiosities; they are evidence that the deep-sea fauna of temperate Asia is being actively replenished from the tropics by one of the ocean’s most powerful currents.

Subject of Research: The deep-sea flatfishes Poecilopsetta colorata and P. praelonga ( Actinopterygii , Pleuronectiformes , Poecilopsettidae ) from the Pacific coast of southern Japan

Article Title: The deep-sea flatfishes Poecilopsetta colorata and P. praelonga ( Actinopterygii , Pleuronectiformes , Poecilopsettidae ) from the Pacific coast of southern Japan

Article References: Obata, K., Mizumoto, Y., Kawai, T., & Wada, H. (2026). The deep-sea flatfishes Poecilopsetta colorata and P. praelonga (Actinopterygii, Pleuronectiformes, Poecilopsettidae) from the Pacific coast of southern Japan. Acta Ichthyologica et Piscatoria, 56, 299-314. https://doi.org/10.3897/aiep.56.197471

Image Credits: AI Generated

DOI: 10.3897/aiep.56.197471

Keywords: deep-sea, flatfishes, Poecilopsetta, colorata, praelonga, Actinopterygii, Pleuronectiformes, Poecilopsettidae, Pacific, coast, southern, Japan