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Posts Tagged ‘L. Vannamei’

Aquatic “Crop Rotation” in Shrimp Farming

Friday, January 27th, 2012

As the grandson of Ohio farmers, I grew up occasionally overhearing–and sometimes learning–about the business of agriculture. I can still recall conversations between my grandparents discussing the previous season’s surpluses or shortages and how this would effect their choice of which crops to grow in following seasons. But little did I expect decades later to find such an uncanny parallel between their agriculture and the aquaculture TransGenada will support. When I learned about shrimp farmers in the Ben Tre province of Vietnam and how they utilized their own type of “crop rotation” my curiosity was piqued.

As detailed in the September/October 2011 Edition of Aqua Culture Asia Pacific magazine, these shrimp farmers seasonally rotate the species of shrimp they stock. One farmer details how he stocks giant tiger shrimp (P. monodon) for the first two thirds of the season and then Pacific White Shrimp (L. vannamei) during the remaining 75 days of the season. He explains that he knows the chances of a Whitespot (WSSV) outbreak increase from about 20-30% early on in the season in April to about 70-80% later in the season in September and during the dry season (December through February). In fact, the chances for disease are so probable that the authorities have made it illegal for farmers to grow shrimp and for hatcheries to produce postlarvae during the dry season.

What I wasn’t able to discern exactly is why they rotate their types of shrimp like this. Agriculture farmers have several reasons for this ranging from increasing the quality of their soil to helping prevent crop-specific pests from feeling like they’ve found a good consistent field to retire to. But why do the aquaculture farmers of Ben Tre rotate species? My original belief was this was implemented because L. vannamei were less susceptible than P. monodon to WSSV. This made sense afterall because I knew after L. vannamei became more popular after large WSSV and TSV outbreaks. But, I also know WSSV does indeed impact L. vannamei, so perhaps there is another reason, like L. Vannamei have some characteristic, like being cheaper or faster to grow, that naturally makes them more advantageous to grow as farmers approach the end of their season. Or, as is often the way of things, I wondered if there was perhaps a combination of reasons why shrimp farmers would rotate their species of shrimp. I decided to ask TransGenada’s chief scientist, Dr. Jeremy Ellis and what he shared with me was very insightful. He said, “There are a variety of forces that could dictate when crop rotation is an advantageous strategy.  Market preference and prices in some regions make P. monodon a more attractive crop, while L. vannamei has been selected for disease resistance for many years.  By splitting time between these two they are tapping into the strengths of each species.”

While I know farmers, both agriculturists and aquaculturists, are always aware of the impact of disease, I hope sharing this example will help outsiders realize the depth this problem poses for the pressures farmers face. The industry plays a constant game of tug of war. On the one hand we are pushed annually for greater production to supply an ever-growing populace and to compensate for less and less abundant natural resources. But, on the other hand, they have to take steps like the ones mentioned in this post to keep from being pulled back by outbreaks and the like. Just today I was reading about how China, Japan and South Korea’s demand for Vietnamese shrimp is expected to increase this year (despite last year’s 13.7% increase) and about how Sonora, Mexico’s production was halved from 90,000 tonnes to 41,000 tonnes due to WSSV. TransGenada aims to innovate aquaculture and provide better solutions for its farmers and in so doing for our future generations’ welfare. A goal I’m certain my grandparents could appreciate.

The Shrimp Immune System

Thursday, January 19th, 2012

The immune system of crustaceans has become and area of increasing interest for the aquaculture community.  It has been long known that crustaceans, and also arthropods in general, may often harbor pathogenic infections without displaying overt signs of disease.  This fact has been partly responsible for the widespread increase in the adoption of advanced pathogen detection in broodstock in addition to the rapid application of SPF (specific pathogen free) stocks where feasible.  These and related technologies have allowed some level of control of disease in shrimp aquaculture and has undoubtedly resulted in a reduction of catastrophic losses and regional pandemics.  However, the fact remains that even these populations of cultured shrimp are still susceptible to disease.  Facilities near wild populations, that have open circulation systems, or are in close geographic proximity to other shrimp farming facilities have increased risk of disease.

Closed, recirculating, or in-land facilities are often considered the next best method to control the introduction of pathogens; however, when/if an outbreak occurs it is often more catestrophic as the experience and technology to deal with the disease and a lack of natural resistance of the shrimp stocks results in a highly susceptible environment.  Here in, enters the current research into the shrimp immune system.  By studying how the immune system functions in arthropods, or shrimp specifically, molecular interventions may be engineered to block or alter the transmission or virulence of viruses and other pathogens.

A central feature of innate immunity in a wide range of animal species is the Toll-receptor family.  These receptors detect conserved structures found in a wide range of viral and bacterial pathogens.  Once the receptors detect a hint of invading pathogens further immune responses follow.  These receptors were long suspected to be present in shrimp as well and a 2007 report confirmed the existence of Toll-receptors in shrimp, ultimately culminating with the cloning of LvToll1 [1].   More recently two additional receptors, LvToll2 and LvToll3, have been discovered as outlined Developmental & Comparative Immunology [2].  In this paper the authors observed the levels of the various Toll receptors responded distinctly different to different pathogenic agents.  This is not entirely unexpected as other organisms show a similar “combinatorial code” in response to pathogens.

A case in point to help demonstrate the utility of this research is how viruses display tissue specific tropism (simply, which tissues viruses preferentially target).  You need look no further than your own experiences with the common cold and flu viruses.  These viruses target various tissues (mucosal membranes, digestive system, etc), while leaving others (skin, muscles, etc) generally untouched.  Viruses often require specific sets of proteins to be present on a cell to allow for successful invasion.  This selection criteria for tropism is partly what allows viruses to adapt increased virulence properties and efficient transmission strategies.

Just as we see tropism with the cold virus in humans, studies of IMNV in L. vannamei have identified a similar phenomenon.  IMNV is found by quantitative measure to be enriched in the muscle and hemolymph, while being reduced in more peripheral tissues [3].  In light of the Toll receptor research it is clear that some components of the shrimp innate immune system display varying levels of expression in different tissues.  Furthermore, it may be possible that the “natural” pattern of toll gene expression or other innate immune system genes is not sufficient to provide an effective defense against the virus.  In a sense IMNV might have found a “hole” and is exploiting it.  Therefore, in the specific case of IMNV, modulating the expression of some of the toll pathway genes in the muscle may confer additional resistance to IMNV.

By understanding how these receptors respond to infections it could be possible to identify these “holes” that pathogens exploit.  Just like a software patch fixes a vulnerability in a computer operating system, it may be possible to “patch” these organisms to help their immune system more completely fend off viruses and bacteria.  One of TransGenada’s main charges is to identify these exploits and upgrade our shrimp for the betterment of the industry.

For those interested in additional information the following link has more details on the basics of the Toll mediated innate immunity [4].