Discovery of a new species of snail parasite (Trematoda: flatworm parasite) in the Institute for Nature Study in Minato Ward, Japan.

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The Institution for Nature Study has been designated as a ‘natural monument and historic site’, and despite its location in the heart of the city, it has preserved a rich natural environment since ancient times. At the same time, it is a place for learning and education, where Tokyo residents and other visitors can closely observe living creatures and nature.
Parasites, several millimeters in size, have been detected in the clausiliid snails inhabiting this site (Kuramochi et al., 2019). The parasite was considered to be a larva (metacercaria) of trematoda of the genus Brachylaima based on its morphology, but its species was not unclear.
Brachylyma spp. are trematoda that live on vertebrates and snails. Initially, the adults parasitize vertebrates that serve as their definitive hosts (i.e., hosts of adult trematoda). The eggs from the adults are released to environment via the feces of the host. When the feces containing the eggs are eaten by the snail, the first intermediate host (i.e., the larval host), the eggs hatch inside the snail and eventually develop into larvae, called sporocysts. In the sporocysts, large numbers of larvae called cercariae are produced, which in turn infect the second intermediate host snail. After infection, cercariae develop into larvae, called metacercariae. Snails infected with metacercariae are eaten by vertebrates and grow into adults.
The research group conducted a study to find out the identity of the metacercariae parasite on the door snails in the Institute for Nature Study. To establish the species of the Trematoda, it is necessary to observe the adult form, not the metacercariae. Therefore, metacercariae from the clausiliid snails were orally administered to immunosuppressed mice and allowed to grow to adulthood. The morphology of the adult worms differed from that of any previously known trematoda. Furthermore, the sequencing of part of the DNA of the metacercaria showed that they differed from previously known trematoda species at the level of a different species. Based on the above, this trematoda was described as a new species Brachylaima phaedusae.
Further investigation revealed that the larvae of Brachylaima phaedusae (metacercariae and sporocysts) are distributed over a wide area from Kanto to Kyushu and parasitized snails, mainly clausiliid, in each location. Many of the locations where they were found were familiar sites, such as parks, harbors, university campuses, and shrine precincts. These results indicate that there are species new to science that exist in our immediate surroundings.
Brachylaima phaedusae larvae have been found on various snails, but no adults have been found in the field, and the definitive hosts remain unknown. Further research is needed to reveal the definitive host vertebrates. As there was no geographical bias in the genetic variation of the new trematoda, the research group suspects that the definitive host is animals with high horizontal migratory ability, such as a bird, and that Brachylaima phaedusae migrates horizontally over long distances in a short period of time.