Rare ‘reclusive’ tropical fish found for first time off South Korea by divers
Researchers scuba diving off the coast of South Korea’s Ulleung Island encountered the rare Valenciennea wardii, or Ward’s sleeper goby, making it the northernmost record of the tropical species.
The species, known for its “reclusive behavior and low abundance,” primarily lives in tropical waters from the Indian Ocean, as well as the Red Sea and Western Pacific, according to a study published Sept. 17 in the journal Acta Ichthyologica et Piscatoria.
Though the species has a wide east-to-west distribution, this marks a notable “poleward” or northward range extension into temperate water for the Ward’s sleeper goby, according to the study.
Researchers said “climate-driven increases in seawater temperatures” may be a “major factor contributing to the poleward expansion of tropical marine species,” which are appearing more often recently in the waters surrounding the Korean Peninsula.
In 2024, the waters reached a high of 83.5 degrees Fahrenheit. When compared with 2012, the average water temperatures off the Korean Peninsula was more than 3 degrees higher.
Despite the warming waters, temperatures in the winter, which can drop to about 50 degrees, are “not conducive to the survival of the species in this region,” researchers said.
Lower temperatures can inhibit feeding, swimming and osmotic abilities in warm-water fish leading to death, researchers said.
The Ward’s sleeper goby “typically inhabit sandy or muddy substrates, digging burrows and feeding on small invertebrates by sifting sand through their mouths,” researchers said.
The single specimen found off Ulleung Island was recorded at a depth of about 82 feet.
Researchers described it as long and “slender,” measuring just over an inch.
In life, its head is light brown and body is light gray with three dark brown bands. It has a distinct yellow strip bordered by a “black-edged blue stripe” running from its upper jaw to its gill region, according to the study.
Its fins are mostly white with black, orange and yellowish coloring.
Researchers said the single specimen may represent a new cryptic species of Ward’s sleeper goby.
The researcher team included Si-Won Choi, Se Hun Myoung, Chang-Soo Bae, Min-Su Woo, Yun-Bae Kim and Joo Myun Park.
This story was originally published September 17, 2025 at 1:45 PM with the headline "Rare ‘reclusive’ tropical fish found for first time off South Korea by divers."
