Authors

Discussion

Trophically transmitted parasites may increase their transmission efficiency by altering the behaviour of infected hosts, thus increasing their susceptibility to predation by next hosts. The strigeid trematode Cardiocephaloides longicollis (Rudolphi, 1819) Dubois, 1982 parasitises 31 fish species, including the gilt-head seabream (Sparus aurata L.), one of the most important fish in Mediterranean aquaculture. Actually, this parasite has recently been found in gilt-head seabreams from aquaculture facilities, with a prevalence of infection up to 54%. The cercariae penetrate the skin and migrate into the brain, where they encyst as metacercariae, and the definitive hosts, seabirds, become infected by consumption of infected fish. It is commonly believed that the parasite larvae can cause significant alterations in fish behaviour, thus increasing their transmission to the definitive host, as evidenced in other brain-infecting trematodes (e.g. Euhaplorchis californiensis). However, the behavioral pathology suggested to be provoked by C. longicollis has never been experimentally studied. In this study, an experiment to detect differences in the behaviour of infected and uninfected fish was performed. First, 14 fish were experimentally infected with 180 cercariae of C. longicollis, emerged from their first intermediate host, the snail Nassarius reticulatus. Preliminary assays showed an infection success around 50%, so that a high number of metacercariae should be accumulated in the fish brain. Furthermore, behaviour experiments were run 6 months post-infection to ensure that metacercariae were infective. Fish were placed in a plexiglass tube (200 cm height, 30 cm diameter) where an effective light-dark gradient was generated. The water column was vertically divided into 20 cm sections and the position of each fish at 1-min intervals for 30 minutes every 2 hours during 3 days was recorded in three assays, i.e. control (i.e. uninfected), infected and mixed fish group. Preliminary results show significant differences in the distribution of control and infected fish along the sections in the tube, in both separated and mixed groups, suggesting a different use of the space, and thus behaviour. This may indicate that encysted metacercariae might provoke this behavioral alteration in infected fish within the tube associated to a neuronal disorder. Despite of the fact that most metacercariae infect the optical lobes, this behaviour might not be a consequence of a decrease of light perception, as infected fish occupied generally deeper and so darker positions. However, other aspects of fish vision to be important in the nervous control of behaviour or the host´s antipredator responses, such as visual acuity, could be affected, which has to be further studied.

This study was supported by projects MSM200961706 (Czech Academy of Sciences), AGL2015-68405-R (MINECO/FEDER, UE), Prometeo/2015/018 and Revidpaqua ISIC/2012/003 (Valencian Regional Government).