1. Effects of the eye fluke (Diplostomum sp
Effects of the eye fluke (Diplostomum sp.) on the energetics and feeding of Arctic charr (Salvelinus alpinus)
Ari Voutilainen,
Kaisa Figueiredo, and Hannu Huuskonen
Ecological Research Institute, University of Joensuu

2. Arctic charr (Salvelinus alpinus)
The northernmost freshwater fish; energetically and ecologically adapted to low temperature
Both landlocked and anadromous populations
In this study: hatchery-bred one-summer old Arctic charr from the Lake Saimaa population
High prevalences and abundances of Diplostomum spp. -infections have been reported occurring in lake resident Arctic charr stocks.
A Holarctic polymorphic salmonid fish, the most cold-adapted freshwater fish in the world, a habitat generalist
In Finland: Lake Saimaa Arctic charr is critically endangered

3. Diplostomum sp. - the eye fluke
The trematode fluke Diplostomum sp. uses aquatic snails (Lymnaeidae) and fish as intermediate and fish eating birds (mainly gulls) as main hosts.
Free-swimming cercaria-larvae shed from infected snails penetrate the fish surface, migrate inside the fish and reached their target, i.e. the eye lens, in hours
In the fish eye cercariae develop into long-lived metacercaria-larvae, which, by their metabolic excretion, cause cataract opacities in the lens

4. Experimental infections
Three fish at a time (total n = 42) were placed in an 6-litre glass aquarium in which Diplostomum sp. cercariae were shed from infected snails (Lymnaea stagnalis)
For the respirometer experiments duration of the exposure was 30 ± 5 min and the cercaria density in the suspension about 400 Diplostomum sp. fish-1
For the feeding experiment duration of the exposure was 1, 5 or 10 min and the cercaria density Diplostomum sp. fish-1
Temperature was 15.0 ± 0.5ºC and photoperiod 18L:6D throughout the study
The purpose of the infection procedure for the feeding experiment was to obtain fish with a high variation in infection abundance

5. Measurement of oxygen consumption
One hour (acute stage, n = 24) or 10 weeks (chronic stage, n = 17) after the Diplostomum sp. exposure oxygen consumption of Arctic charr was measured with an automated three-chamber intermittent-flow respirometer (Cyclobios®).

6. Feeding experiment
Infected and control Arctic charr were distributed into three groups so that each group included two “blind”, four “partially blind”, and four healthy-eyed fish
The fish school was placed in a 45-litre aquarium
Daphnia magna (n = 20) were added into the water
One fish at a time was taken for observation
The number of successful and unsuccessful catching attempts in 5 min were counted
Also the time lag between the adding of D. magna and the first catching attempt was controlled
After 5 min the fish and D. magna left were removed
After a break the procedure was repeated and feeding of the second fish was observed

7. Energetic response to the infection (1/2)
Acute stage: Difference in oxygen consumption between infected and control fish in the first eight hours after the exposure (P <0.005)

8. Energetic response to the infection (2/2)
Chronic stage: Difference in oxygen consumption between infected and control fish the whole measured period through (P = 0.006)
In darkness healthy-eyed Arctic charr increased their oxygen consumption rate unlike cataract-bearing individuals

9. Feeding capability
Cataract impaired both the Arctic charr ability to catch food items (upper figure) and to react to offered food (lower figure).
The number of zooplankters caught declined linearly with increasing cataract score.
Relationship between reaction time to food and cataract was exponential as cataract score rose above five.
The overall cataract score: from 0 (no cataract) to 24 (wide and dense cataract in both eyes)

10. Conclusions
In chronic phase of the Diplostomum sp. infection oxygen consumption rate of infected Arctic charr was higher than that of healthy-eyed non-infected Arctic charr.
Eye lens opacities impaired feeding capability of Arctic charr. Healthy-eyed fish reacted to zooplankton faster, made more catching attempts and caught zooplankton with better success compared to cataract-bearing conspecifics in the same school.
Impaired vision due to parasite induced cataract may alter remarkably nocturnal behaviour of the species.
To compensate the energetic cost caused by Diplostomum-infection cataract-bearing Arctic charr have to spend more time foraging, which may result in decreased growth rate and increased predation risk.
Acknowledgements: We thank the Finnish Game and Fisheries Research Institute for a supply of Arctic charr, and Prof. J. Taskinen (University of Jyväskylä) for valuable advice concerning the experimental Diplostomum-infections. The study has been financially supported by the Graduate School in Biological Interactions with a grant to A.V

11. Thank you