It is very likely that diseases play a significant and underappreciated role in shaping populations of marine species. Assessing the prevalence and effects of disease in wild populations of aquatic species has been historically extremely difficult. With the application of molecular and biotechnological tools for the discovery and detection of aquatic diseases, it is becoming feasible to assess the influence of specific diseases on populations of fish, shellfish and crustaceans.
Shellfish pathogens in the Chesapeake Bay
Economically and environmentally important species in the Chesapeake Bay are impacted by a spectrum of protozoan, bacterial, and viral diseases. Episodic and large-scale mortalities of clams, oysters, and blue crabs result in multi-million dollar losses for the respective fisheries. Human activity may contribute to disease outbreaks by degrading water quality in estuaries, by introducing new pathogens in ballast water, and by intentional movement of shellfish. The state of Maryland's current oyster management plan emphasizes dual efforts in restoration/sanctuaries and in aquaculture. Both of these endeavors will require tools to understand oyster health and disease, whether that be in hatcheries for producing oyster spat, or in wild populations in sanctuaries.
Diseases in the Blue Crab
For over 30 years, scientists at the Cooperative Oxford Lab on the eastern shore of MD have documented viruses of all types in blue crab (Callinectes sapidus). Application of molecular techniques now offers us the chance to understand more about the prevalence and health effects of these viruses. One virus of particular concern is the reo-like virus (RLV). RLV has been associated with crab mortality in soft-shell crab facilities, and is an agent of concern for any future blue crab aquaculture operation. Characterization of the double-stranded RNA genome of this virus has been accomplished, and sensitive PCR-based assays have been developed to monitor RLV in the environment and in aquaculture.
A second pathogen of significance is Hematodinium sp., a parasitic dinoflagellate that, like RLV, inhabits the hemolymph of blue crabs. Hematodinium sp. produces seasonal crab mortality in Atlantic coastal bays form Delaware to Louisiana. The life cycle of Hematodinium sp. may include stages that reside in biotic or abiotic reservoirs. The development of PCR-based detection methods has enabled us and others to investigate how this pathogen over-winters and causes recurring infections in coastal bays (Nagle et al, 2009).
Molecular detection of estuarine pathogens, molecular characterization of blue crustacean pathogens, viruses in the blue crab
The blue crab is the trademark shellfish of the Chesapeake Bay, and the combined harvest of blue crabs in MD and VA is valued at over $60 million per year. In the 1990s and early 2000s, populations and harvests of blue crab in the Chesapeake Bay were at alarmingly low levels for a decade. While recent harvest restrictions have led to a rebound in crab abundance indicating that fishing pressure was a dominant force driving this decline. However, the role of disease in crab mortality is still potentially significant. We are uncovering evidence that one virus in particular appears to have the potential to impact blue crab populations. It is a reovirus, first described in the 1970s, but little studied or appreciated until recently.
In the face of declining blue crab harvests, one avenue for added value in the industry is soft-shell crab production. Unfortunately, crabs in soft-shell facilities frequently suffer mortalities of over 25%. A combination of state of the art technology with practical field-based and aquaculture-based solutions are needed to predict or reduce virus-related soft shell mortalities. We've shown that the blue crab reovirus is present in over 50% of all soft shell mortalities [Bowers et al, 2010 PDF here]. The virus has the potential to cause major mortality in wild populations as well. We are currently examining the prevalence of reovirus in early life stage crabs, and have completed an analysis of crabs from throughout the Chesapeake Bay, sampled by the NOAA Cooperative Oxford lab.
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