Harmful gelatinous zooplankton, especially those of the phylum Cnidaria such as jellyfish, can threaten farmed fish through various mechanisms, including both physical damage and venomous effects, and are significant contributors to gill disease development. However, there is a lack of scientific and technical understanding of the interactions between gelatinous zooplankton and fish gills and the use of laboratory based challenge models, using lab-reared organisms, can elucidate the gaps in our knowledge . Models using laboratory reared Cnidarians have not been previously reported , and the aim of this study was to develop these.
Jellyfish polyps of Aurelia aurita were successfully reared under laboratory conditions, and by artificial strobilation, juvenile medusae ( 1-2 weeks old, ~1 cm in diameter) were produced, transported and used to challenge Atlantic salmon ( Salmo salar). A preliminary exposure of the salmon gills with juveniles of A. aurita was carried out through direct contact with the gills . The challenge experiment involved the use of 3 replicate control tanks and 3 replicate challenge tanks. In each tank, 20 Atlantic salmon weighing between 100 to 400g were anesthetised , and y oung medusa were then directly applied to the gill tissue using a syringe. For c ontrol groups, a syringe containing tank water instead of jellyfish was used. To assess the effects of the challenge, 5 fish per tank per sample point (-1, 1-, 3-, and 20-days post-challenge) were randomly selected, euthanised , and sampled for histopathology and PCR analyses.
In the gross examination of gills, small necrotic patches were noted in 30% of the fish challenged on the 1st and 3rd day post-challenge. In the histopathological exam, focally extensive necrotic, inflammatory, and ha emorrhagic pathology was observed in 60% of the challenged fish 1-day post-challenge. Additionally, these lesions were associated with the presence of moderate levels of filamentous and short rod bacteria in two fish gills (see Figure). Localis ed moderate lesions persisted in subsequent sampling points, although the number of lesions decreased, and they showed signs of repair.
These preliminary results successfully demonstrated the presence of gross gill and histopathology lesions following contact with juveniles of lab-reared A. aurita. Additionally, they highlighted, for the first time, the rapidity with which bacterial colonization occurs following a primary cnidarian-induced lesion in laboratory conditions.