My passion is uncovering「how parasites interact with their hosts (and vice versa) in the wild」and examining「the determinants of parasite distribution and infections」. To answer these questions, I mainly focus on the copepods of the genus Salmincola, which mainly infects salmonids in freshwater systems. The specific research themes are as follows.
Salmicnola cf. markewitschi infecting buccal cavity of white-spotted charr Salvelinus leucomaenis
Female specimen of Salmicnola cf. markewitschi removed from white-spotted charr
1. Factors affecting distribution and infections of aquatic parasites
Most aquatic parasites must find and infect suitable hosts within a short period. During infective stages, it is assumed that parasites have difficulty to infect due to their poor swimming ability while in this period.
The members of the genus Salmincola also have the nature described above; small infective larvae (only 0.6-0.7 mm) with poor swimming ability seek suitable hosts within a few days. Interestingly, their host specificity is very strong. For instance, Salmincola carpionis generally infects on the genus Salvelinus. While they seem to have the inferior trait, their populations successfully persist in the upstream area.
We found that huge migratory fish are heavily infected by Salmincola sp.; but that there is an extremely low prevalence above the dam where migratory fish have difficulty access to. Some areas above dams have no copepod infections throughout seasons. These suggest that the move of migratory fish upstream to spawn is an important factor for the population persistence of Salmincola sp. (Hasegawa & Koizumi 2021 Ecol. Res.).
Many other factors possibly affect infection dynamics, such as numerous physical environmental and biological factors (Hasegawa et al. 2022 Parasitol. Int.). We are exploring the infection dynamics through the seasons in the field and doing mark-recapture study to explore the infection dynamics for each host individual.
White-spotted charr (anadromous type)
Salmincola cf. markewitschi on buccal cavity of anadromous charr
Erosion control dam often prevents upstream movements of ' infected hosts ' and causing local extinction of the parasites
2. Causes and consequences of parasite infections and host fitness
Assessing the parasite impacts on host fitness are very important in many fields, but their impacts are sometimes treated as negligible because infection levels of parasites in natural conditions are generally low. The copepods of the genus Salmincola is one of such parasite: while negative impacts on host feeding activities and physical damages have been often reported from reared conditions, their impacts had been ignored in field studies. However, some studies recently found outbreaks of some species and effects on host population crushes. So, the proper assessment of the parasite impacts on host fitness are required in resource managements.
We found that S. edwardsii and S. cf. markewitschi reduce host body condition in Japan (Hasegawa et al. 2022 Parasitol. Inter.). Such loss in body condition loss may extend to other ecological and evolutionary consequences like host life-history change and may cause other parasite infections. We are exploring the mechanisms of loss in body condition and other expected consequences through field surveys and laboratory experiments.
While our study suggest negative impacts of infections on host condition, this is mere correlation, so causes and consequences are reversible in some cases: fish having poor body condition are susceptible to further infections. To test these hypothesis, we are employing mark-recapture method to disentangle the causes and consequences of parasite infections and host condition.
Uninfected fish（upper）and infected fish (lower)
Copepods sometimes block the mouth of host fish, suggesting that their infections strongly regulate host feedings.