The Atlantic Hallibut (Hippoglossus hippoglossus) is an important commercial species, and is farmed in globally. Although it has been commercially farmed for almost 40 years there are still issues arising in early developmet caused by opportunistic bacteria including Flexibacter ovolyticus, Vibrio salmonicida and Vibrio anguilarum. One particular disease, known as “gaping jaw” is thought to be caused by bacteria infecting lesions caused by the rearing container in larvae. s the larvae have very little immune system they are unable to fight it off and the disease itself eventually prevents effective feeding causing the animal to die from starvation.
This paper looks at the analysis of the culturable bacteria found in larvae yolk sacs of those individuals exhibiting symptoms of gaping jaw. Samples were taken from 40 larvae, grown on agar plates and amplified using PCR. Although the results showed a heterogeneity for many bacteria, (32 out of 44 found), there were a number that were only found in larvae with gaping jaw. These included species from Vibrio, Photobacterium, Flavobacterium and Bacillus genera suggesting that they may be linked to the jaw deformity.
The team also assessed the level of expression of three immune genes and found that expression was significantly elevated for two out of the three genes in larvae with gaping jaw. One in particular, Hepcidin, was elevated 1196 times when compared to levels from healthy larvae.
One of the Vibrio species isolated had a high similarity to Vibrio alginolyticus which is a known pathogen. In addition there are also previously described pathogens from the Photobacterium genera. Even though Flavobacterium are generally harmless there are again a number that are associated with fish disease. The presence of Bacillus however could be linked to the disease but there is also a possibility that they are there as an antimicrobial agent as they are able to produce bacteriocins.
The increase in levels of Hepcidin in the presence of the bacteria found on diseased larvae is interesting as it is know to up-regulate in other fish such as the Atlantic cod in response to bacterial infections. Similar increases in levels of Hepcidin have also been seen in sea bass in response to infection from both Photobacterium and Vibrio species.
Given that previous studies into this disease have found little correlation between other environmental factors such as temperature, salinity and larval density it is possible that it is indeed caused by a bacterial infection. Further study will now be required to determine which if any of these culturable bacteria may be responsible for the disease. It would also be of interest to conduct metagenomic studies to determine which if any VBNC bacteria may be present and also complete individual tests to see what effect Hepcidin might have on the bacteria specific to the diseased larvae.
Urtubia, R., Gallardo, P., Lavin, P., Brown, N., & González, M. (2014). Characterization of culturable bacterial flora in yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus L.) with" gaping jaws" syndrome. Latin American Journal of Aquatic Research, 42(1).