Natural history study on Riccardoella tokyoensis: life history with ontogeny and host distribution records

Riccardoella tokyoensis is a snail parasite recently recorded in Japan. To understand the basic ecology of this mite, we surveyed the life history of a population, including seasonal dynamics, host range, and geographical distribution in East Japan. Seasonal dynamics of the mite were studied in Rinshi-no-mori Park, Tokyo, Japan, from June 2016 to May 2018; thereafter, the morphology of each stage was described. In this park, the host population was infected with the mite in all seasons; however, the prevalence and infection intensities were highest during the warm seasons. Larvae and nymphs were found in the warm seasons. Proto- and tritonymphs were uncommon. To study the host range and geographical distribution, we sampled 1135 terrestrial mollusks from 16 families at 21 locations in Kanto and east Tokai areas from June 2016 to May 2020. Mites were detected at 13 locations, and only clausiliid snails were infected with the mite. Through a laboratory experiment, our observations confirmed that the mite was transmitted to the host lung via the pneumostome. None of the artificially infected snails died, although we maintained the snails for 2 months. No inflammation was found in naturally infected host tissue, suggesting that their virulence against the host snails was low. Riccardoella tokyoensis was not found in the litter layer in the field surveys, and no mite was observed on the external surface of the hosts, suggesting that the major habitat of this mite was the host lung.