Churaumi Aquarium's artificial shark uterine fluid treatment eradicates parasites

Okinawa Churaumi Aquarium (Motobu Town, Okinawa Prefecture) has established a technology to eradicate parasites on purple hagfish (Eptatretus okinoseanus) by applying shark-specific "artificial uterine fluid (AUF)." Because purple hagfish in captivity can die when parasites propagate on their body surfaces, staff previously spent considerable time and effort removing the parasites one by one with tweezers. In these developments, by modifying AUF meant for baby sharks and immersing the purple hagfish in it, they found that the parasites could be exterminated in just five minutes. Applying this technique to other species could help improve captive environments in aquariums.

The purple hagfish is a type of primitive fish and a member of the "Agnatha" (jawless fish). According to Hiroko Takaoka of the Deep Sea Exhibition Section, Fish Division at the aquarium's Management Center (who is in charge of keeping deep-sea organisms), they inhabit waters deeper than 600 meters near Okinawa at water temperatures of around 10℃ and eat food by tearing it away with tooth-like projections in their mouths. In nature, they are believed to feed on the carcasses of marine animals, but much of their ecology remains a mystery.

When purple hagfish are kept in tanks, monogenean parasites attach to them. These parasites are members of the flatworm family and propagate by feeding on mucus on the body surface. As a result, the purple hagfish's body surface becomes damaged, which can lead to death in the worst-case scenario. Therefore, regular parasite eradication is essential.

Previously, "freshwater baths," in which animals are immersed in fresh water to eradicate parasites, were common at aquariums. This could not be applied to animals extremely sensitive to fresh water, such as purple hagfish. Staff instead adopted the most reliable method of picking them off with tweezers. However, because it was manual work, the animals were restrained for a long time, posing a heavy burden on both the animals and the keepers.

Dr. Taketeru Tomita of the Kuroshio Exhibition Section, Fish Division at the aquarium's Management Center, who conducts shark research, has been continuing research on AUF for sharks. When sharks are kept in aquariums for a long time, reproductive behavior sometimes occurs, leading to pregnancy and birth. In humans, artificial incubators are used during perinatal-related problems, but response methods for premature shark births had not been established. Tomita began research in 2017 and devised an artificial uterus system and AUF in 2021. AUF is created by mixing seawater with fresh water and urea. Through this, it became possible to create conditions close to the environment of a mother shark's body.

While searching for ways to reduce the burden of manual parasite removal, Takaoka wondered, "Might the components of AUF be effective?" and consulted Tomita. When they immersed the purple hagfish in the modified AUF, they found that parasites could be eradicated as expected. Subsequently, by altering the proportion of urea and seawater, they succeeded in developing a more effective solution. This solution differs from seawater in osmotic pressure and salinity, making it harmless to purple hagfish while lethal to parasites.

Takaoka stated: "Because aquariums house a wide variety of species, there are many cases that cannot be addressed by existing methods. Not limited to this method, I would like to continue developing techniques for better husbandry in the future." Tomita stated: "I want to continue research into whether this can be applied to other organisms for which freshwater baths are impossible."

This method obtained a patent on May 20, and the achievements were announced by Okinawa Churaumi Aquarium on June 9. The paper was published on July 17, 2025, in the Dutch academic journal MethodsX.