Growth patterns of Ceratothoa oxyrrhynchaena parasitizing Doederleinia berycoides in the northeastern Sea of Japan

Abstract Ceratothoa oxyrrhynchaena Koelbel, 1879 is a cymothoid isopod that parasitizes marine fishes including the blackthroat seaperch Doederleinia berycoides (Hilgendorf, 1879). In 2025, 50 individuals of D. berycoides parasitized by C. oxyrrhynchaena were collected from the northeastern Sea of Japan. Total length (TL) and body weight (BW) of the host fish and body length (BL) of the isopods were measured. Mostly fewer than three isopods parasitized a single fish, with only one individual hosting five isopods. This suggests the possible existence of a mechanism that limits the number of parasites per host. No difference in size was detected between the smallest fish parasitized by a single parasite and the smallest fish by two parasites, suggesting that dual parasitism begins when the host is still young. Significant positive correlations were detected between the host body size (TL and BW) and BL of parasites, indicating that the isopods grow concurrently with their host fish. The smallest host fish was indirectly estimated to be younger than 1 year of age, whereas the age of the largest fish was estimated to be close to 5 years of age. This finding suggests that the isopod parasite may inhabit the host’s buccal cavity for at least 4 years.