Gene silencing technology alters sex of prawns

Original article available at: ScienceDaily

Israeli scientists have developed a novel method for generating single-sex populations of prawns. This could be used to boost the productivity of aquaculture farms and even as a biocontrol measure against invasive species and pests.

Male sexual differentiation in crustaceans is regulated by the androgenic gland. It is thought that this overrides a default programme of female differentiation, allowing male features to develop. This comes from the observation that removing the androgenic gland in juvenile males causes them to become “feminised” and capable of producing eggs. Curiously, when these “neo-females” are mated with normal males, the entire offspring is male. For commercial prawn farms, all-male populations are highly desirable as males are considerably larger than females. Furthermore, an absence of females prevents energy from being used in reproductive efforts, causing the males to grow faster and reach a greater overall size. However, surgically removing the androgenic gland is not a feasible option for large farms.

Professor Amir Sagi (Ben Gurion University of the Negev) and his colleagues recently found that prawn androgenic glands produce specific molecules known as insulin-like androgenic peptides (IAGs). Professor Sagi believes that these activate “a cascade of genes regulating masculine differentiation.” Working on giant freshwater prawns (Macrobrachium rosenbergii), they used a technique known as RNA interference (RNAi) to generate populations of neo-females. This method uses genetic molecules, similar to naturally occurring ones, which temporarily bind to the target sequence, causing it to be recognised by enzymes which suppress the gene’s function. The prawns were injected with a sequence designed to silence an IAG-encoding gene. This caused the males to undergo complete sexual reversal until they were indistinguishable from normal females. The key difference, however, is that these neo-females only produced male offspring.

“This is the first RNAi based biotechnology ever to be applied in aquaculture and indicates future opportunities” Professor Sagi said. Besides boosting yields in commercial farms, this technology could also be used to control invasive crustaceans by altering population ratios. Professor Sagi also adds that non-reproducing prawns could be used “as sustainable bio-control agents” against freshwater snails that carry diseases (including shistosomiasis) or damage rice paddy fields. As these prawns cannot form reproductive populations, there would be no risk of them becoming invasive species themselves.

Reference
Society for Experimental Biology. “It’s a girl! Gene silencing technology alters sex of prawns.” ScienceDaily. http://www.sciencedaily.com/releases/2014/06/140630193415.htm (accessed June 30, 2014).

ImageMacrobrachium rosenbergii (Credit: Citron / CC-BY-SA-3.0 Own work)

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EMS Continuing to Take its Toll but Farming Practices, Knowledge Improving

Original article available at: thefishsite.com

In his May presentation to members of the National Fisheries Institute Shrimp Council, Mr Chamberlain updated the EMS status of primary production areas and passed on recommendations for management methods to reduce the impacts of EMS. The update comes about two months after GAA launched a case study to identify the shrimp-farming practices that prevent the manifestation of EMS. The case study will act as a foundation for recommendations to the aquaculture community for better shrimp-farming practices.

EMS Status

Mr Chamberlain said the incidence of EMS in China varies among regions. In early 2014, EMS appeared at a low rate in eastern and southwestern Guangdong, and Guangxi provinces, but manifested at moderate to high rates in the Pearl River and Zhangjiang regions. Farms in northern China had not been stocked.

Some operations have engineered deep ponds with self-cleaning bottoms that also use heavy water exchange to eliminate sludge. The presence of covert mortality nodavirus and microsporidean parasites, other pathogens that affect the hepatopancreas organs of shrimp and cause slow growth and size variation, have made clear identification of EMS more challenging in the country.

Despite the presence of EMS, recent high shrimp prices continued to fuel rapid farm expansion in Viet Nam, Mr Chamberlain said. Now that prices have dropped to US $5.00/kg, fewer farms are stocking, and postlarvae sales are dropping.

Thailand was again hit hard by EMS. First quarter estimates showed only 30,000 mt of production versus 100,000 mt last year. Coming off three months of low temperatures and facing continued failure rates of over 30 per cent in the first 40 days, many farms are not expected to restock for a time.

In Malaysia, production remained down overall, although concerted efforts at the large-scale farm run by Agrobest are yielding rising results.

Mr Chamberlain said Mexico’s first outbreaks of EMS came at Nayarit in the center of Sinaloa and a couple of farms in Sonora. New farms starting in the south and on the Gulf of Mexico seem to have escaped EMS so far. Head-on shrimp production of 55,000 to 60,000 mt is projected for the country.

India faces an unsure prognosis regarding EMS. Test results have been inconsistent and inconclusive, so the country’s producers should be considered free of EMS at the present time.

Expert Observations

Mr Chamberlain shared a number of observations and recommendations made by members of an expert committee on EMS formed by GAA. For example, those working with EMS have identified varying toxicity in the multiple strains of the Vibrio bacteria that cause the disease. Although test methods are improving, EMS losses continue to be confused with mortalities caused by viral diseases such as white spot syndrome and Taura syndrome.

EMS is thought to be transmitted vertically (on the outside of eggs) from broodstock to postlarvae, and can also transfer to shrimp via water, cannibalism, feces, plankton, macro-organisms, birds and biofilms. Once established in the environment, EMS is difficult to control, Mr Chamberlain said.

Importantly, it has been found in China that antibiotics are not effective against EMS. Sensitivity tests have shown the bacteria responsible for EMS outbreaks have already developed resistance to the full range of antibiotics. Research in Mexico shows that pathogenic Vibrio strains have nearly the same resistance profiles as non pathogenic strains. However, Mexican researchers agree that antibiotics are not effective, because they do not effectively reach the pathogen which colonizes chitinous surfaces.

Recommendations

• EMS-free broodstock are needed. Selective breeding for resistance to EMS would involve challenging families and selecting those with best survival as parents for the next generation.

• Improve farm practices. To eliminate EMS and other pathogens from water, establish a balanced microbial population, stock with healthy postlarvae and closely manage water and bottom quality. Disinfection with chlorine or ozone eliminates multiple pathogens. To create a mature microbial community, probiotics and polyculture can help condition water. Other suggestions were to maintain light to moderate bioflocs, avoid overfeeding and remove sludge regularly.

• Employ a nursery phase. Nurseries hold young postlarvae until they are larger and more robust, while also confirming they are free of EMS. Ten to 20 days in a nursery raceway, tank or net pen allows important physical and medical evaluations.

• Improve farm infrastructure with biosecure intensive ponds. Small, deep ponds covered with plastic or bird nets allow more manageable disinfection and feeding, as well as better control of water quality, bioflocs and sludge. Higher yields (30-50 mt/ha) can cover the extra investment.

• Identify feed additives that reduce the incidence of EMS. These could include quorum-sensing inhibitors, essential oils or immunostimulants.

• Integrated farm management is needed. Fragmented systems do not provide needed controls at each step in the production process. Zone management would avoid farm sitings with shared inlet and discharge canals, and consider the carrying capacity of ecosystems. Access to well-equipped local labs would provide better detection of EMS.

Global EMS Survey

Building on earlier studies coordinated by the Global Aquaculture Alliance that helped identify the cause of EMS and other elements of the disease, GAA is launching a new online survey to collect additional information on the EMS status of farms in affected areas, as well as the practices they apply to combat the disease.

The comprehensive survey — funded by the World Bank Allfish project, the Seafood Industry Research Fund of the National Fisheries Institute and C.P. Prima of Indonesia — will initially be available in English at http://www.gaalliance.org. Versions in other languages will be added to better allow those in Asian and Latin American countries to participate.

Based on responses to the survey, a select number of farms will be chosen to receive in-depth site audits and diagnostic testing to clarify what practices are most effective in managing EMS. In combination with the survey results, these findings will be distributed by GAA to help identify the common denominators of proper management and promote the adoption of better shrimp-farming practices industry wide.

In his May presentation to members of the National Fisheries Institute Shrimp Council, Mr Chamberlain updated the EMS status of primary production areas and passed on recommendations for management methods to reduce the impacts of EMS. The update comes about two months after GAA launched a case study to identify the shrimp-farming practices that prevent the manifestation of EMS. The case study will act as a foundation for recommendations to the aquaculture community for better shrimp-farming practices.

EMS Status

Mr Chamberlain said the incidence of EMS in China varies among regions. In early 2014, EMS appeared at a low rate in eastern and southwestern Guangdong, and Guangxi provinces, but manifested at moderate to high rates in the Pearl River and Zhangjiang regions. Farms in northern China had not been stocked.

Some operations have engineered deep ponds with self-cleaning bottoms that also use heavy water exchange to eliminate sludge. The presence of covert mortality nodavirus and microsporidean parasites, other pathogens that affect the hepatopancreas organs of shrimp and cause slow growth and size variation, have made clear identification of EMS more challenging in the country.

Despite the presence of EMS, recent high shrimp prices continued to fuel rapid farm expansion in Viet Nam, Mr Chamberlain said. Now that prices have dropped to US $5.00/kg, fewer farms are stocking, and postlarvae sales are dropping.

Thailand was again hit hard by EMS. First quarter estimates showed only 30,000 mt of production versus 100,000 mt last year. Coming off three months of low temperatures and facing continued failure rates of over 30 per cent in the first 40 days, many farms are not expected to restock for a time.

In Malaysia, production remained down overall, although concerted efforts at the large-scale farm run by Agrobest are yielding rising results.

Mr Chamberlain said Mexico’s first outbreaks of EMS came at Nayarit in the center of Sinaloa and a couple of farms in Sonora. New farms starting in the south and on the Gulf of Mexico seem to have escaped EMS so far. Head-on shrimp production of 55,000 to 60,000 mt is projected for the country.

India faces an unsure prognosis regarding EMS. Test results have been inconsistent and inconclusive, so the country’s producers should be considered free of EMS at the present time.

Expert Observations

Mr Chamberlain shared a number of observations and recommendations made by members of an expert committee on EMS formed by GAA. For example, those working with EMS have identified varying toxicity in the multiple strains of the Vibrio bacteria that cause the disease. Although test methods are improving, EMS losses continue to be confused with mortalities caused by viral diseases such as white spot syndrome and Taura syndrome.

EMS is thought to be transmitted vertically (on the outside of eggs) from broodstock to postlarvae, and can also transfer to shrimp via water, cannibalism, feces, plankton, macro-organisms, birds and biofilms. Once established in the environment, EMS is difficult to control, Mr Chamberlain said.

Importantly, it has been found in China that antibiotics are not effective against EMS. Sensitivity tests have shown the bacteria responsible for EMS outbreaks have already developed resistance to the full range of antibiotics. Research in Mexico shows that pathogenic Vibrio strains have nearly the same resistance profiles as non pathogenic strains. However, Mexican researchers agree that antibiotics are not effective, because they do not effectively reach the pathogen which colonizes chitinous surfaces.

Recommendations

• EMS-free broodstock are needed. Selective breeding for resistance to EMS would involve challenging families and selecting those with best survival as parents for the next generation.

• Improve farm practices. To eliminate EMS and other pathogens from water, establish a balanced microbial population, stock with healthy postlarvae and closely manage water and bottom quality. Disinfection with chlorine or ozone eliminates multiple pathogens. To create a mature microbial community, probiotics and polyculture can help condition water. Other suggestions were to maintain light to moderate bioflocs, avoid overfeeding and remove sludge regularly.

• Employ a nursery phase. Nurseries hold young postlarvae until they are larger and more robust, while also confirming they are free of EMS. Ten to 20 days in a nursery raceway, tank or net pen allows important physical and medical evaluations.

• Improve farm infrastructure with biosecure intensive ponds. Small, deep ponds covered with plastic or bird nets allow more manageable disinfection and feeding, as well as better control of water quality, bioflocs and sludge. Higher yields (30-50 mt/ha) can cover the extra investment.

• Identify feed additives that reduce the incidence of EMS. These could include quorum-sensing inhibitors, essential oils or immunostimulants.

• Integrated farm management is needed. Fragmented systems do not provide needed controls at each step in the production process. Zone management would avoid farm sitings with shared inlet and discharge canals, and consider the carrying capacity of ecosystems. Access to well-equipped local labs would provide better detection of EMS.

Global EMS Survey

Building on earlier studies coordinated by the Global Aquaculture Alliance that helped identify the cause of EMS and other elements of the disease, GAA is launching a new online survey to collect additional information on the EMS status of farms in affected areas, as well as the practices they apply to combat the disease.

The comprehensive survey — funded by the World Bank Allfish project, the Seafood Industry Research Fund of the National Fisheries Institute and C.P. Prima of Indonesia — will initially be available in English at http://www.gaalliance.org. Versions in other languages will be added to better allow those in Asian and Latin American countries to participate.

Based on responses to the survey, a select number of farms will be chosen to receive in-depth site audits and diagnostic testing to clarify what practices are most effective in managing EMS. In combination with the survey results, these findings will be distributed by GAA to help identify the common denominators of proper management and promote the adoption of better shrimp-farming practices industry wide.

– See more at: http://www.thefishsite.com/fishnews/23278/ems-continuing-to-take-its-toll-but-farming-practices-knowledge-improving#sthash.gLxki9oY.dpuf

In his May presentation to members of the National Fisheries Institute Shrimp Council, Mr Chamberlain updated the EMS status of primary production areas and passed on recommendations for management methods to reduce the impacts of EMS. The update comes about two months after GAA launched a case study to identify the shrimp-farming practices that prevent the manifestation of EMS. The case study will act as a foundation for recommendations to the aquaculture community for better shrimp-farming practices.

EMS Status

Mr Chamberlain said the incidence of EMS in China varies among regions. In early 2014, EMS appeared at a low rate in eastern and southwestern Guangdong, and Guangxi provinces, but manifested at moderate to high rates in the Pearl River and Zhangjiang regions. Farms in northern China had not been stocked.

Some operations have engineered deep ponds with self-cleaning bottoms that also use heavy water exchange to eliminate sludge. The presence of covert mortality nodavirus and microsporidean parasites, other pathogens that affect the hepatopancreas organs of shrimp and cause slow growth and size variation, have made clear identification of EMS more challenging in the country.

Despite the presence of EMS, recent high shrimp prices continued to fuel rapid farm expansion in Viet Nam, Mr Chamberlain said. Now that prices have dropped to US $5.00/kg, fewer farms are stocking, and postlarvae sales are dropping.

Thailand was again hit hard by EMS. First quarter estimates showed only 30,000 mt of production versus 100,000 mt last year. Coming off three months of low temperatures and facing continued failure rates of over 30 per cent in the first 40 days, many farms are not expected to restock for a time.

In Malaysia, production remained down overall, although concerted efforts at the large-scale farm run by Agrobest are yielding rising results.

Mr Chamberlain said Mexico’s first outbreaks of EMS came at Nayarit in the center of Sinaloa and a couple of farms in Sonora. New farms starting in the south and on the Gulf of Mexico seem to have escaped EMS so far. Head-on shrimp production of 55,000 to 60,000 mt is projected for the country.

India faces an unsure prognosis regarding EMS. Test results have been inconsistent and inconclusive, so the country’s producers should be considered free of EMS at the present time.

Expert Observations

Mr Chamberlain shared a number of observations and recommendations made by members of an expert committee on EMS formed by GAA. For example, those working with EMS have identified varying toxicity in the multiple strains of the Vibrio bacteria that cause the disease. Although test methods are improving, EMS losses continue to be confused with mortalities caused by viral diseases such as white spot syndrome and Taura syndrome.

EMS is thought to be transmitted vertically (on the outside of eggs) from broodstock to postlarvae, and can also transfer to shrimp via water, cannibalism, feces, plankton, macro-organisms, birds and biofilms. Once established in the environment, EMS is difficult to control, Mr Chamberlain said.

Importantly, it has been found in China that antibiotics are not effective against EMS. Sensitivity tests have shown the bacteria responsible for EMS outbreaks have already developed resistance to the full range of antibiotics. Research in Mexico shows that pathogenic Vibrio strains have nearly the same resistance profiles as non pathogenic strains. However, Mexican researchers agree that antibiotics are not effective, because they do not effectively reach the pathogen which colonizes chitinous surfaces.

Recommendations

• EMS-free broodstock are needed. Selective breeding for resistance to EMS would involve challenging families and selecting those with best survival as parents for the next generation.

• Improve farm practices. To eliminate EMS and other pathogens from water, establish a balanced microbial population, stock with healthy postlarvae and closely manage water and bottom quality. Disinfection with chlorine or ozone eliminates multiple pathogens. To create a mature microbial community, probiotics and polyculture can help condition water. Other suggestions were to maintain light to moderate bioflocs, avoid overfeeding and remove sludge regularly.

• Employ a nursery phase. Nurseries hold young postlarvae until they are larger and more robust, while also confirming they are free of EMS. Ten to 20 days in a nursery raceway, tank or net pen allows important physical and medical evaluations.

• Improve farm infrastructure with biosecure intensive ponds. Small, deep ponds covered with plastic or bird nets allow more manageable disinfection and feeding, as well as better control of water quality, bioflocs and sludge. Higher yields (30-50 mt/ha) can cover the extra investment.

• Identify feed additives that reduce the incidence of EMS. These could include quorum-sensing inhibitors, essential oils or immunostimulants.

• Integrated farm management is needed. Fragmented systems do not provide needed controls at each step in the production process. Zone management would avoid farm sitings with shared inlet and discharge canals, and consider the carrying capacity of ecosystems. Access to well-equipped local labs would provide better detection of EMS.

Global EMS Survey

Building on earlier studies coordinated by the Global Aquaculture Alliance that helped identify the cause of EMS and other elements of the disease, GAA is launching a new online survey to collect additional information on the EMS status of farms in affected areas, as well as the practices they apply to combat the disease.

The comprehensive survey — funded by the World Bank Allfish project, the Seafood Industry Research Fund of the National Fisheries Institute and C.P. Prima of Indonesia — will initially be available in English at http://www.gaalliance.org. Versions in other languages will be added to better allow those in Asian and Latin American countries to participate.

Based on responses to the survey, a select number of farms will be chosen to receive in-depth site audits and diagnostic testing to clarify what practices are most effective in managing EMS. In combination with the survey results, these findings will be distributed by GAA to help identify the common denominators of proper management and promote the adoption of better shrimp-farming practices industry wide.

– See more at: http://www.thefishsite.com/fishnews/23278/ems-continuing-to-take-its-toll-but-farming-practices-knowledge-improving#sthash.gLxki9oY.dpuf

Phytoplankton and zooplankton biomass are expected to decrease by 6% and 11% respectively by the end of century due to climate change

Original article available at: Basque Research

It is estimated that ocean temperature warming will cause phytoplankton and zooplankton biomass to decrease by 6% and 11% respectively by the end of the century. A lower amount of these two main elements in the marine food web could reduce fish biomass in certain regions. These are some of the main conclusions drawn by research led by Azti-Tecnalia within the European MEECE project and recently published in the prestigious Global Change Biology Journal.

Sea surface temperature is expected to increase 2 ºC on average globally by 2080-2100. Some of the consequences of this increase include changes in ocean circulation and higher water column stratification, thus affecting the nutrient availability for the growth of marine phytoplankton.

The research team led by Azti-Tecnalia points out the effects to primary production (phytoplankton mass produced annually by photosynthetic single-celled organisms that are suspended in the ocean), and to secondary production (zooplankton biomass, made up of small animal organisms that feed mainly on phytoplankton and which fish feed on).

Globally, it is estimated that the sea temperature rise will cause phytoplankton and zooplankton biomass to decrease by 6% and 11% respectively. This suggests that there will be a negative amplification of climate change, which will spread through the marine food web, i.e. zooplankton biomass will decrease more than phytoplankton. This process will take place mainly in tropical oceans, which cover 47% of the global ocean surface.

Differences by region

Phytoplankton and zooplankton reduction, however, will affect different regions in different ways. In the seas in Central and Southern Europe (North Sea and temperate Northeast Atlantic), higher thermal stratification of the ocean water layers and, consequently, a lower presence of nutrients for phytoplankton to grow, will reduce primary production; and in the Baltic, Barents and Black Sea phytoplankton production is expected to increase.

Azti-Tecnalia researcher Guillem Chust, leader of the scientific work and main author of the paper, says that “in the ocean regions that lose more phytoplankton and zooplankton biomass, that is, with a negative amplification, fish biomass may also decrease dramatically, especially pelagic species (i.e. those living the water column, excluding the seabed)”.

“Climate regulation will also be affected negatively by the primary and secondary production decrease globally,” Chust explains, “because, as there will be less phytoplankton, absorption of CO2 from the atmosphere by the oceans will be lower, as plankton is responsible for half of the planet’s photosynthetic activity. This in turn will reduce the ocean’s capacity to regulate the climate”.

 

Publication reference:

Guillem Chust, J. Icarus Allen, Laurent Bopp, Corinna Schrum, Jason Holt, Kostas Tsiaras, Marco Zavatarelli, Marina Chifflet, Heather Cannaby, Isabelle Dadou, Ute Daewel, Sarah L. Wakelin, Eric Machu, Dhanya Pushpadas, Momme Butenschon, Yuri Artioli, George Petihakis, Chris Smith, Veronique Garçon, Katerina Goubanova, Briac Le Vu, Bettina A. Fach, Baris Salihoglu, Emanuela Clementi, Xabier Irigoien. Biomass changes and trophic amplification of plankton in a warmer ocean. Global Change Biology, 2014; DOI: 10.1111/gcb.12562

Suspended Solids Effects In Shrimp Biofloc Systems

Original article available at: thefishsite.com

When a production cycle begins with a well-stocked biofloc inoculum, the concentrations of ammonia and nitrite are relatively low, but concentrations of suspended solids tend to be higher in this phase. Generally, nitrite concentrations rise as suspended solids levels increase. At dissolved-oxygen concentrations above 5 mg/L, excess suspended solids were not a problem for shrimp respiration.

Marine shrimp aquaculture systems that incorporate biofloc technology often experience a high density of suspended solids within the systems. The injection of air at the bottoms of culture tanks promotes both the diffusion of oxygen in the water column and mixing of suspended material. The use of lined tanks increases the confinement of biofloc in the culture environment and restricts the organic particles in the water column from being recycled completely.

Suspended Solids

Suspended solids consist mainly of organic matter, which is comprised of microbial forms that when decomposing exert a high demand for oxygen. This demand can decrease the dissolved-oxygen concentrations in the culture system to reach levels below the recommended concentration for the cultivated species. An increase in suspended solids can also reduce the water quality within the system. Overall, these less-than-optimal conditions reduce system performance.

These conditions can occur throughout the different stages of the marine shrimp production cycle, depending on whether the system starts a cycle with mature biofloc inoculum. However, the use of biofloc inoculum relates to the suspended solids concentration in the water column.

At the beginning of a cycle, interactions among the nitrogenous compounds, ammonia and nitrite are most notable when biofloc is forming in the culture. The reduction of ammonia occurs with the establishment of ammonium-oxidizing bacteria, which does not require the use of organic carbon, and with the absorption of heterotrophic bacteria. Therefore, the nitrite accumulation occurs due to the slow growth of nitrite-oxidizing bacteria.

Total suspended solids levels constantly increase, and consequently nitrite concentrations increase. In contrast, when a cycle begins with a well-stocked biofloc inoculum, the concentrations of ammonia and nitrite are relatively low. However, the concentrations of suspended solids are higher in this phase. Both situations require management of suspended particulate matter within the culture system.

Experimental Work

In experiments conducted with Litopenaeus vannamei shrimp in biofloc systems at the Marine Aquaculture Station of the Federal University of Rio Grande in southern Brazil, the best growth performance occurred in a study where suspended solids were removed for the maintenance and control of the total suspended solids concentrations in culture inoculated with biofloc.

In another experiment, concentrations of nitrogen compounds differed when compared to different total suspended solids (TSS) concentrations during the formation of biofloc. Higher TSS concentrations resulted in higher concentrations of nitrite.

In another experiment that used the biofloc inoculum, it was observed that when the concentration of dissolved oxygen was maintained above 5 mg/L, the suspended solids excess was not a problem for the respiration of the reared shrimp. Considering the interactions of water quality parameters listed above, maintenance of the suspended solids levels is required for better water quality.

Monitoring, Intervention

Measurements of suspended solids were made using the gravimetric method, which measures total suspended solids and settleable solids within an Imhoff cone. Various techniques can be applied to reduce and maintain suspended solids concentrations, such as the use of a settling chamber or clarifier to remove solids. Set up in straightforward settling chambers, clarifiers rely on gravity to move particles to the bottom.

The radial water flow in the settling chamber can be adjusted based on prior analysis of biofloc sedimentation in an Imhoff cone, increasing the efficiency of the method. This method allows the control of TSS and keeps the concentrations near the recommended values. Another advantage of this method is maintaining a constant flow during the application of settling, so a small amount of water is sufficient for removing suspended solids.

Mouth Vision: Blind Fish Suctions Water to Navigate

Original article available at: livescience.com

By Laura Poppick

The Mexican blind cavefish does not have eyes, but it can “see” obstacles in dark caves by puckering its mouth and producing bursts of suction, according to a new study. The research describes this unique form of navigation for the first time.

Scientists previously thought the eye-less Mexican cavefish navigated by sensing changes in water pressure produced by waves sent off from the fish’s own body. But when the researchers examined the fish, they found some problems with this explanation. For example, larger fish, which would presumably produce larger waves, should be able to identify objects from farther away than smaller fish. In fact, larger fish detected objects at about the same distance as smaller fish did.

Researchers at Tel Aviv University in Israel decided to investigate the sightless navigation further, conducting an experiment in which they counted the number of times the fish opened and closed their mouths when near objects the fish were familiar with. The researchers then moved the objects and observed changes in the fishes’ mouth movement in the unfamiliar environment.

The fish opened and closed their mouths more than twice as frequently in unfamiliar surroundings, and more frequently when approaching an object than in the open, with no objects nearby, suggesting this behavior plays a role in detecting the fish’s environment.

Through further analyses, the team determined the suction sent off by this mouth motion produces a signal similar to echolocation — a system in which animals, like bats and dolphins, emit sound waves and detect the distance of an object based on how long the sound takes to bounce back. Instead of measuring time, however, the cavefish appear to measure the magnitude of the pressure change produced by their mouth suction, study co-author Roi Holzman told Live Science.

“In this sense, it is different from echolocation, but it is similar because you have an animal that is purposefully emitting pressure waves to locate obstacles,” Holzman said.

The team does not know if other fish use this form of navigation. But some likely do, since all fish have the ability to produce suction waves with their mouths, and all have receptive organs along the sides of their bodies that can detect changes in water pressure. Both adaptations can be traced far back in the evolutionary history of fish, said Holzman.

“It’s a [newly discovered] mechanism made out of ancient material, and it just makes sense that other fish would have it,” Holzman said. “We haven’t tested it yet, but I’d really like to.”

The fish may also passively gather information produced by body waves when they swim through water, as previous studies have suggested, the team speculates. But the cavefish likely use both navigation methods in tandem, similar to how submarines rely on both active and passive sonar, Holzman said.

The researchers are now using a hydrophone to study how the fish modulate the suction signal depending on their distance from an obstacle, Holzman said.

Surface water is a key factor in the transmission of pancreas disease in salmon

Original article available at: Norwegian school of veterinary science

Anne Stene’s PhD thesis explains how environmental factors affect the outbreak and transmission of pancreas disease (PD) in farmed salmon.

Both infected and dead salmon can shed the salmonid pancreas disease virus into the sea and the virus particles can be spread by the wind and ocean currents from one fish farm to the next along the coast.

Pancreas disease (PD) is currently the most serious of the viral infections affecting Norwegian farmed salmon. The disease leads to increased mortality, weight loss and low fish product quality. It therefore has a significant influence on fish welfare and on profitability in the aquaculture industry.

Coastal currents are a key factor in disease transmission
The PD virus can survive for long periods of time outside the salmon host in cold, clean seawater and it therefore has a strong infective potential along the Norwegian coast. Using a hydrodynamic model developed by SINTEF (http://www.sintef.no/home/), Stene was able to demonstrate that the transmission of the disease between fish farms at different locations is primarily caused by the direction of ocean currents near the surface of the water. Her findings also show that fish farms located in close proximity to infected/diseased salmon and fish farms owned by companies with many other infected farms have an increased risk of their stocks becoming infected with PD.

In addition to identifying risk factors for the transmission of PD, Stene focused on risk factors for outbreaks of the disease. Her doctoral project, which was carried out at the Norwegian Veterinary Institute, shows that salmon become infected with PD when the sea’s temperature rises over a period of time. The reason for this may be that the increase in water temperature leads to a state of chronic stress in the fish, which in turn has a negative effect on their immune defence system. An outbreak of the disease usually occurs when there is a high concentration of the virus in the fish. This can result in extensive shedding of the virus, which in turn leads to a high infection pressure in the sea.

An important finding in Stene’s study is that fat from infected and dead fish at the bottom of the cages also contains the virus. Some of this fat will float to the surface and can potentially infect salmon that come into contact with it. This floating layer of fat can spread to other fish farms by means of ocean currents near the surface. This underlines the need to remove dead fish as quickly and efficiently as possible.

Important knowledge to prevent infection
Stene’s thesis shows that the virus does not pose a problem during the smoltification of salmon in fresh water. Rather, the most important factor is the transmission of infection during the growth phase in salt water. If it is possible to slaughter infected fish before the temperature rises, outbreaks of PD can be limited and this will reduce concentrations of the virus in the sea. And when fewer viruses are carried by the currents, the risk of infection will decrease. Knowledge about water-born transmission and the risk of outbreaks is an important tool in when it comes to production planning with a view to preventing infection.

Stene’s research therefore provides fish farmers with new information, when they are considering locations for releasing young salmon, as regards the direction of predominant surface currents in relation to farms containing infected fish. Similarly, the slaughter of fish can be planned in relation to the location’s infection status, outbreak risk and the probability of disease transmission to neighbouring farms with fresh fish. Such measures must of course be weighed up against concerns regarding commercial viability for the individual farm and for the industry as a whole in each area. The costs can be high in the short term but must also be appraised in a more long-term perspective.

Anne Stene defended her doctoral research on 23rd October 2013 at the Norwegian School of Veterinary Science with a thesis entitled “Transmission of Pancreas Disease in marine salmon farming in Norway”.

A new species of the fish pathogenic bacterium Edwardsiella

The original article available at: Norwegian school of veterinary science

Takele Abayneh Tefera’s doctoral research project has uncovered a genetic divergence between the fish pathogen Edwardsiella tarda and Edwardsiella tarda type strain.

He has also identified phenotypic markers that distinguish one from the other. The fish pathogenic strain is now classified as a separate species: Edwardsiella piscicida.

Edwardsiella tarda is a bacterium that can infect a number of animal species and also humans. Edwardsiellosis is one of the most serious systemic bacterial diseases in fish, resulting in substantial losses in the fish farming industry all over the world.

Takele Abayneh Tefera has developed effective molecular tools for the identification and characterization of different strains of Edwardsiella. He developed a new TaqMan real-time and conventional PCR analysis for this purpose and then evaluated it in relation to the Loop Mediated Isothermal Amplification (LAMP) analysis. For the first time, he also used the Multi-locus Sequence Analysis (MLSA) for the typing and characterization of E. tarda, isolated from different sources. This is a useful tool for detecting sources of infection and for understanding the epidemiological relationship between isolates from the environment, fish, livestock and humans.

The MLSA analysis showed that there exist geographic and host-specific genotypes of the species E. tarda. Bacterial strains from fish were found to be only distantly related to E. tarda type strain and other strains from humans. Furthermore, genetic and phenotypic analyses also confirmed a genetic divergence between these strains of bacteria.

The findings of this study indicate that the fish pathogenic strain has been wrongly classified as E. tarda. Phenotypic characterization by means of a number of biochemical tests and pathogenity studies on zebra fish identified phenotypic markers which also differentiate fish pathogenic strains from the reference bacterium.

Based on genetic and phenotypic differences, Tefera and his colleagues have proposed that fish pathogenic Edwardsiella strains previously classified as Edwardsiella tarda should now be classified as a new species: Edwardsiella piscicida. The new species has now been approved and incorporated into prokaryotic nomenclature.

Tefera’s work also led to the introduction of a new Multi-locus Variable Number Tandem Repeat Analysis (MLVA) for the further typing of different isolates of the new species of bacteria Edwardsiella piscicida. Employing this method proved to be more sensitive than the MLSA-analysis when it came to investigating outbreaks of E. piscicida.

Takele Abayneh Tefera defended his doctoral research on 26th November 2013 at the Norwegian School of Veterinary Science with a thesis entitled: Fish pathogenic Edwardsiella tarda: Evaluation of molecular identification methods and characterization of a novel species.”

 

ImagePicture: Electronic micrograph of E. Tarda (now E. piscicida) LTB4 strain after negative staining

(Source: Courtesy of Lan et al. 2008)