Recent Emergence and Distributional Expansion of the Parasitic Copepod Lernaeenicus cf. ramosus in Japanese Waters Revealed by Web Image Surveys

Emerging infectious diseases are novel threats to marine ecosystems. Although elucidating the temporal and spatial dynamics of such diseases is critical for managing ecosystems, it is generally challenging because many are invisible and cryptic, making it difficult to conduct long‐term surveys at large spatial scales. Here, by analyzing web images on the social network platform X (formerly Twitter), I examined the infection dynamics of the parasitic copepod Lernaeenicus cf. ramosus , which causes pathological impacts on economically important marine fishes. Although previous studies indicated that infections increased after the 2000s in Japanese waters, there is no long‐term data. Based on 3390 posts containing images of two host fish species (yellow grouper Epinephelus awoara and grub fish Parapercis sexfasciata ) posted from 2012 to 2024, I found the copepods emerged and increased substantially around 2016–2019 in grub fish but declined in yellow grouper. Infection levels tended to be higher in autumn and winter, and methods of fish collection influenced the detectability of parasites. Moreover, I also identified new localities of the copepods, which fall outside of their previously known range, suggesting that their distribution is rapidly expanding north (east) ward. My study showed that analyzing web images sheds important insights into how aquatic diseases emerge and spread.