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Life cycle of the introduced trematode Dollfustrema invadens in Japan. Illustration: Iwata Sho, Makito Hayashi and Tsukasa Waki
Life cycle of the introduced trematode Dollfustrema invadens in Japan. Illustration: Iwata Sho, Makito Hayashi and Tsukasa Waki
A newly identified parasite from mainland China has established itself in Japan’s Tone River system, using a complex network of both native and invasive species to complete its life cycle, according to researchers. Their findings reveal how introduced species can carry hitchhiking parasites that eventually affect native wildlife.
The collaborative research team from Toho University, Japan Fisheries Research and Education Agency, Nihon University, the Global Environmental Forum, and the Museum Park Ibaraki Nature Museum identified the parasite as a new species, naming it Dollfustrema invadens in their study published January 20, 2025, in the Journal of Helminthology.
While examining fish and mussels collected between 2021 and 2023, researchers discovered the distinctive yellow-colored parasites in the fins and fin bases of several fish species. The parasite, which belongs to a group called bucephalid trematodes, appears to have entered Japan’s ecosystems around 2020, though its exact introduction route remains unclear.
“We found that the golden mussel, an invasive species, acts as the infection source,” explained lead researcher Yoshiki Saito. “The parasite then uses both native freshwater fish, such as Tridentiger brevispinis and Tridentiger obscurus, as well as non-native species to complete its life cycle.”
What stands out about this particular parasite is its remarkable ability to adapt to new environments by exploiting available host species. Researchers found particularly high parasite loads in the channel catfish and bluegill, both designated as invasive species in Japan. These findings suggest these three invasive species—the golden mussel, channel catfish, and bluegill—play a crucial role in sustaining the parasite’s life cycle and enabling its transmission to native fish.
A Complex Life Cycle
The research team conducted extensive DNA analysis to track different stages of the parasite’s development across various host species. Their work revealed how the parasite moves through the ecosystem in a way that differs from typical patterns.
Like other members of its family, D. invadens typically requires three different hosts to complete its life cycle. The parasite begins in the golden mussel, where it develops into a stage called a sporocyst. These sporocysts release cercariae—free-swimming larvae—which then infect small freshwater fish, becoming encysted in their tissues as metacercariae.
Normally, these metacercariae would need to be consumed by a predatory fish to develop into adults. However, the researchers made a surprising discovery: in some cases, the metacercariae can develop directly into adults while still inside the second host’s tissues.
“We found adult worms with eggs in the fins, fin bases, and gills of small fishes, where both adults and metacercariae co-occurred,” noted the researchers in their publication. “This suggests that metacercariae can develop directly into adults within the tissues of second intermediate hosts.”
This unusual flexibility in its life cycle may help explain how the parasite has established itself so quickly in a new environment.
The Invasive Species Connection
What makes this case particularly interesting is that most of the hosts involved are themselves non-native species. Of the eight fish species found harboring the parasite, six were introduced from western Japan, mainland China, or North America.
The golden mussel, which serves as the primary infection source, is native to East Asia and was likely introduced to Japan around 2005 with Asian clam seeds imported for aquaculture. Researchers believe the parasite may have arrived more recently through repeated introductions of these mussels or possibly with specific fish species.
Dr. Tsukasa Waki, one of the study’s authors, expressed concern about the potential spread: “Given that a single adult likely generates eggs through self-fertilization, the population of D. invadens can expand rapidly when newly introduced into water systems.”
Potential Impact
While no human infections have been reported, the presence of the visible brown or yellow worms in fish muscle reduces the commercial value of affected freshwater fish. The research team notes that intense bucephalid infections have been reported to negatively affect host fish in other contexts, potentially causing hemorrhages, abnormal swimming, and sometimes leading to the host’s death.
This isn’t the first introduced parasite to establish itself in Japan’s freshwater systems. The study also detected another introduced bucephalid trematode, Prosorhynchoides ozakii, which was previously reported in the Tone River system. In some cases, researchers found fish co-infected with both parasites.
The golden mussel, which carries these parasites, has now spread to at least eight river systems in Japan. The potential fish hosts are also becoming more widespread in these river systems, creating conditions that could allow the parasite to expand its range further.
To prevent the spread of the species, the researchers recommend avoiding the introduction of potential hosts from the Tone River system into other water systems.
This study illustrates how invasive species can create complex ecological ripple effects, sometimes facilitating the establishment of additional non-native organisms that might otherwise be unable to gain a foothold in new environments. As climate change and global trade continue to facilitate species movement worldwide, understanding these ecological relationships becomes increasingly important for conservation and management of freshwater ecosystems.
The study was published in the Journal of Helminthology on January 20, 2025.
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