A Red-and-White Deep-Sea Wrasse Comes Up from the Waters off Kumejima

The fish in Okinawa Churaumi Aquarium’s small deep-sea gallery is easy to describe before it is easy to understand: a wrasse with a striking red-and-white tail, sheltering around rockwork, collected in July from 130 meters below the surface off Kumejima. Its name is Decodon erythroleukos. Until late 2025, science did not formally recognize it as a species. Until this summer, the species was known from just three preserved specimens and a small number of underwater photographs. Now one is alive in an aquarium. ^[1][3]

Churaumi announced on September 15 that the Kumejima individual is the first of its species ever kept alive and displayed publicly. That makes the story more than an aquarium rarity. Preserved specimens establish identity. Photographs establish appearance and place. A living fish can begin to reveal behavior: how it uses shelter, how it swims, what it eats, how its colors change, and how it responds to an environment modeled on the rocky deep-water habitat where remotely operated vehicles have seen the species. ^[1]

The aquarium has not published the July collection method, the fish’s sex or total length, the transport and decompression procedure, or its feeding history. Those gaps matter. A story about a rare deep-water fish is especially vulnerable to filling technical blanks with plausible-sounding aquarium lore. The defensible claim is narrower and still remarkable: a fish collected alive off Kumejima at 130 meters survived the transition into captivity and is now on display in a habitat designed to offer rock shelter. ^[1][4]

The species began with a suspiciously familiar fish

The path to the name began with a specimen collected on Okinawa Island in 2022. It looked much like the already known Decodon pacificus, the only member of the genus then recognized from Japan. But one visual detail stood out. The yellow band in front of the eye was vertical rather than horizontal. ^[2]

Taxonomists did not rely on color alone. The team compared morphology and DNA with related species. In the resulting Journal of Fish Biology paper, mitochondrial COI sequences differed by 6.2%–6.9% from D. pacificus, supporting the conclusion that the Okinawan fish belonged to a previously undescribed species. ^[3][12]

International collaboration then expanded the picture. The formal description rested on three specimens: the holotype from southern Japan and one paratype each from Jewell Reef on Australia’s Great Barrier Reef and New Caledonia. Kagoshima University says specimens had come from 110 to 350 meters, while underwater photographs also existed from Kochi Prefecture and Indonesia. ^[2][3]

Red and white became the name

Both the Japanese standard name, コウハクカザリアカボウ, and the species epithet erythroleukos point to the tail. The Greek roots mean red and white. Churaumi notes that the red area along the rear of the caudal fin is broader than in the closely related D. pacificus; Kagoshima University also highlights the orientation of the yellow band in front of the eye. ^[1][2]

Decodon itself is a small genus. Before the new description, only four species were recognized, and Japan was thought to have only D. pacificus. The new species was the first added to the genus in roughly half a century. ^[2]

That historical detail is useful because taxonomy can otherwise sound like bookkeeping. Naming a species changes the map researchers use. If two similar fishes are mistakenly treated as one, distribution, habitat and conservation data become blurred. The 2025 paper did more than name D. erythroleukos; it also re-diagnosed D. pacificus and documented the previously poorly known coloration of its juveniles. ^[3]

Three specimens became one living research subject

The species description appeared online in December 2025. About seven months later, the Kumejima fish was collected alive. Churaumi’s species guide identifies the displayed individual as the July 2026 specimen from 130 meters. ^[1][4]

The scientific value of that transition is subtle. Museum specimens are irreplaceable because future researchers can return to the physical type material and test the original diagnosis. A living fish answers different questions. Does it hug rock faces continuously or only when disturbed? Does it forage in the open? What foods does it accept? Is the red-and-white tail stable across mood, illumination and health?

Churaumi says earlier ROV and underwater-drone observations showed the species around rocky bottom habitat and that the aquarium has reproduced such structure in the display, where the fish can be seen using crevices. ^[1] That is an observation opportunity, not yet a complete behavioral description. One aquarium individual cannot represent an entire species without corroboration in the wild.

Ie Island and Ishigaki had already supplied glimpses

The living Kumejima fish did not appear from a blank map. Churaumi reports that an ROV recorded the species off Ie Island at 200 meters in May 2025. In March 2026, an underwater drone recorded it off Ishigaki Island at 150 meters. ^[1]

Those observations begin to connect multiple points across the Ryukyu region: the Okinawa type specimen, Ie Island, Ishigaki and now Kumejima. But presence is not abundance. Researchers still do not know whether the species is common but difficult to observe, naturally sparse, seasonally mobile between depths, or concentrated in habitat features that have rarely been surveyed.

A fish known from three preserved specimens has become a fish that can be watched. That changes the next questions from “What is it?” toward “How does it live?”

Churaumi has been building a deep-water field-to-tank method for years

The “Tiny Marine Life of the Deep Sea” section is designed around more than displaying unusual animals. Churaumi says its tanks reproduce environments actually observed with ROVs and pair small rare species—including newly described and first-recorded organisms—with information about their habitats. ^[5]

That approach has history. In 2017 the aquarium described using an ROV to collect the delicate benthic ctenophore Coeloplana and later display captive-born individuals. A 2018 husbandry article explained how ROV images of depressions, mud and rock were used to reproduce the microhabitats used by deep-sea animals. ^[6][7]

Wrasses are part of that history too. In 2020 Churaumi announced what it said was the world’s first display of the deep-water wrasse Bodianus tanyokidus, collected at about 150 meters north of Okinawa. In 2024, ROV observations of the golden grouper Saloptia powelli around a steep slope at roughly 200 meters helped guide repeated work with local fishers; a specimen was eventually collected at about 300 meters and placed on display. ^[8][9]

The new Decodon display therefore belongs to a research cycle: observe habitat, identify the animal, collect when possible, learn how to maintain it, and feed those observations back into field research.

Keeping a fish alive can preserve information that formalin cannot

Fresh coloration matters in fish taxonomy because preserved specimens can fade or change. In this species, the broad red area of the tail and the yellow band near the eye are diagnostic clues. Long-term observation could show whether those colors vary with age, sex, stress or social condition.

Churaumi has already illustrated how captive observation can change interpretation. In June 2026 it publicized research on a rare deep-water anthias that had appeared in two slightly different forms. Continued observations helped establish that the two forms represented males and females rather than two species. ^[11]

That does not mean the new wrasse will yield the same kind of discovery. It shows why a living individual can complement a type specimen. Taxonomy captures a species at the moment of description; husbandry can add the dimension of time.

At 130 meters, “deep sea” needs context

The Kumejima collection depth is far shallower than an abyssal plain several kilometers down. The scientific paper calls D. erythroleukos a deepwater species and uses terms including mesophotic and rariphotic in its ecological framing. ^[3] Churaumi calls it a deep-sea wrasse and exhibits it in its deep-sea area. ^[1]

What matters is not winning a vocabulary argument but recognizing that the ocean does not divide neatly into a bright reef and a black abyss. At 100–350 meters, light, temperature, current and habitat differ sharply from shallow coral reefs, while rocky slopes and shelves can still support colorful reef-associated lineages.

Churaumi’s broader “Journey to the Deep Sea” area is built around that gradient, displaying many organisms collected from deeper waters around Okinawa and treating collection, trial husbandry and research as part of the exhibit. ^[10]

Kumejima adds a new point to the species’ map

The July capture did not create a new taxonomic record in the same sense as the 2025 species description. It adds a valuable locality and a live observation to an exceptionally thin evidence base.

This is where island geography matters. Around Okinawa’s islands, seafloor slopes can descend rapidly, putting deeper rocky habitat relatively close to shore. ROVs and underwater drones can work beyond ordinary scuba depths without requiring a collector to physically occupy the same environment. Museum specialists can then connect imagery with preserved specimens, while fishers and aquarium staff contribute local operational knowledge.

Piece by piece, that builds a distribution picture extending from Okinawa across the western Pacific to Australia and New Caledonia. The great unanswered questions are still ecological: how continuous is that range, how isolated are populations, and what habitat features determine where the fish occurs?

What the aquarium still cannot tell us

The public announcement does not give the fish’s sex, age, size, collection gear, decompression protocol or diet. It does not say whether another living individual has ever been held privately or briefly without public display; Churaumi’s documented claim is that this is the world’s first successful captive keeping and exhibition of the species. ^[1]

Nor does one fish reveal population status. “Known from three specimens” describes the state of scientific collections, not necessarily the number of animals in the sea. A species can be genuinely rare, difficult to catch, restricted to little-surveyed habitat, or some combination of all three.

That is why the most interesting part of the display begins after opening day. Feeding, activity cycles, use of shelter, coloration, growth and health can now be recorded day after day. Future ROV work can test whether behavior seen in the tank also appears in the wild.

The aquarium has turned a taxonomic name into a living animal that visitors can see. For science, the deeper achievement is that a species once represented by three preserved bodies can now begin to leave a behavioral record.