World Aquaculture December 2020

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2020 21 R esearchers at the Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University (Gaëtan De Gryse and Thuong Van Khuong under the guidance of Prof. Hans Nauwynck) have discovered a complex of kidneys, bladders and a large network of bags (caudal extensions) in the head of a shrimp. The entity, previously known as “the antennal gland,” is now designated as “the nephrocomplex.” The nephrocomplex plays an important role during molting. By filling the bags with water, pressure rises inside the shrimp until the old cuticule (shell) shears, allowing the animal to get out of its cuticule. By further filling the bags, the volume of the shrimp increases before the soft new cuticule, which was already present underneath the old cuticule, hardens again. This is how shrimp grow. With this work, the enigma of the molting process and the growth of shrimp has been solved. In addition, the nephrocomplex represents a major portal of entry for pathogens. Because shrimp are protected against most pathogens by an impermeable cuticle on the outside and inside, they are very resistant. However, pathogens are still able to invade and kill the animal. Many researchers have desperately searched for the portal of entry, without success. With the present publication, this enigma has been solved. After emptying the bladder (urination), a slight vacuum is Significance of the Shrimp Antennal Gland Revealed present, causing the influx of a small volume of water, possibly contaminated with pathogens (viruses and bacteria). Because the nephrocomplex is not covered with a cuticule, pathogens can invade the body. Thus, urinating is dangerous to shrimp health. Empirical observations by shrimp farmers than outbreaks of white spot syndrome virus occur following heavy rainfall are now explained by increased urination in response to reduced salinity that allows entry of the virus into the shrimp. To summarize this research in simple terms, shrimp grow by temporarily filling their head with water and shrimp may become infected with viruses and bacteria during urination. These insights into the molting process and pathogen entry open doors to the potential development of disease control measures in shrimp. The full citation of the article is: Gaëtan M.A. De Gryse, Thuong Van Khuong, Benedicte Descamps, WimVan Den Broeck, Christian Vanhove, Pieter Cornillie, Patrick Sorgeloos, Peter Bossier and Hans J Nauwynck. 2020. The shrimp nephrocomplex serves as a major portal of pathogen entry and is involved in the molting process. Proceedings of the National Academy of Sciences of the United States of America 117(45):28374-28383. www.pnas.org/cgi/doi/10.1073/ pnas.2013518117 T he deliberate introduction of captive-bred salmon into the wild is a common management response to natural or human- driven declines in salmon numbers. It is also sometimes done to increase the numbers of fish available for angling. On the face of it, introductions appear to be a good idea: wild salmon have very low survival rates as juveniles in the river (as low as 0.3%) due to natural limits of the ecosystem such as competition, predators, and food scarcity, whereas salmon bred in a protected hatchery environment can be expected to survive there at much higher rates, allowing large numbers to be moved to the wild at a suitable stage. Earlier studies of Atlantic salmon had indicated that captive- bred fish or their offspring might perform less well in the wild, but evidence across the full life cycle has thus far been lacking. New research led by scientists at University College Cork has now demonstrated, using genetic fingerprinting techniques, that captive- born salmon had as little as one third of the lifetime reproductive success as wild salmon spawning in the same river. Moreover, the overall productivity of the mixed population was much lower in years where captive-bred fish comprised a greater fraction of potential spawners. “We looked at the lifetime reproductive success of salmon Captive-bred Salmon Perform Poorly in the Wild and Do Not Enhance Wild Populations spawning naturally in the wild. So for each adult fish that returned to the river from the sea, we counted up the total number of offspring they produced across their lives that themselves survived to spawning age,” says co-lead author Ronan O’Sullivan of the UCC School of BEES and the Environmental Research Institute. “We used a genetic pedigree coupled with four decades of salmon data from the Marine Institute’s research facility on the Burrishoole catchment. The results show that captive-bred fish that are deliberately or inadvertently introduced into the wild contribute fewer offspring to the next generation than wild fish, and therefore are not a substitute for natural wild spawners. Thus, they do not enhance the conservation status of naturally self-sustaining salmon populations”. Dr. Paul Connolly, CEO of the Marine Institute of Ireland, adds: “We welcome the use of Marine Institute data to answer a question of international significance that is relevant to conservation efforts for the culturally iconic Atlantic salmon. This analysis underlines the importance of having long-term biological data to allowmanagement decisions to be based on the best available scientific evidence.” Further research is needed to work out exactly what is happening when the wild and captive salmon mix, but the research ( C O N T I N U E D O N P A G E 7 2 )

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