Shrimp are important to the ecosystem and economy of the southeastern United States. In their natural estuarine habitat and in aquaculture ponds, shrimp are periodically exposed to low oxygen, high carbon dioxide and low pH, together referred to as hypercapnic hypoxia. Graduate student Tina Mikulski tested whether hypercapnic hypoxia increased the susceptibility of grass shrimp and Penaeid shrimp to bacterial disease. Using an in vivo challenge model, Tina injected healthy shrimp with the bacteria, Vibrio parahaemolyticus, and monitored the shrimp's survival under various levels of oxygen, carbon dioxide and pH.. Her results, published in the Journal of Shellfish Research (Mikulski et al., 2000) proved that these poor water quality conditions place shrimp at risk for bacterial disease. Further work to clarify the effects of periodic hypercapnic hypoxia and to identify the life stages that are most sensitive to poor water quality are continuing in our laboratory under funding from the US Department of Agriculture. Meanwhile, Austin Dantzler, hypothesized that hypercapnic hypoxia directly suppressed the normal function of cells responsible for immune defense in shrimp. He has isolated these cells, known as hemocytes, from the aquacultured shrimp, Litopenaeus vannamai. Surprisingly, Austin found that moderately low levels of hypercapnic hypoxia actually enhanced the ability of hemocytes to control the in vitro growth of bacteria.
As a followup to these unexpected results from Austin's in vitro experiments, Joe Burgents developed a set of assays to track the in vivo tissue distribution and inactivation of bacteria that penetrate an intact shrimp. Using standard microbial culture techniques in parallel with quantitative real time PCR, Joe found that despite its very small size, the lymphoid organ of L. vannamei plays an important role in the inactivation of bacteria. In contrast, the gills and hepatopancreas accumulate larger numbers of the pathogen and do not inactivate the accumulated bacteria as efficiently as the lymphoid organ. Joe also found that hypercapnic hypoxia decreased the rate at which culturable bacteria were removed from shrimp hemolymph and reduced the rate at which the lymphoid organ killed bacteria. The increased "hang time" of circulating bacteria led to greater accumulation of culturable bacteria in gills and hepatopancreas, which appear to be the target organs for Vibrio infections. In a recently submitted manuscript, postdoctoral fellow Brett Macey and graduate student Kolo Rathburn provide evidence that exposure to bacteria generates a temporary increase in antibacterial activity in blue crabs; the underlying mechanism is sensitive to low dissolved oxygen and high carbon dioxide concentrations in the environment. In crustaceans prophenoloxidase plays a critical role in sealing wounds and encapsulating invading bacteria to limit infections. Undergraduate researcher Chris Tanner soon documented that hypoxia and low pH independently suppress the activity of prophenoloxidase in another decapod crustacean, the blue crab Callinectes sapidus. Are other factors responsible for the observed effects of hypercapnic hypoxia on the crustacean immune system?
For more information, please go to the descriptions of grants that support this research:
Disease resistance lowered by dissolved oxygen and pH in aquacultured shrimp.
Hypercapnic hypoxia impacts shrimp immune defenses against bacterial pathogens.
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