Research Area 1. Hypoxia increases the susceptibility of crustaceans to infectious disease

(in collaboration with Dr. Lou Burnett).

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 campbellii, 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.

As a follow-up to these observations, Joe Burgents developed a set of assays to track the in vivo tissue distribution and inactivation of bacteria that penetrate an intact shrimp (Burgents et al., 2005a). 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. Importantly, 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 (Burgents et al., 2005b). Separately, undergraduate researcher Jeremy Holman observed in the blue crab Callinectes sapidus that hypercapnic hypoxia slows the mobilization of hemocytes (crustacean immune cells) out of hemolymph (blood) in response to injected bacteria (Holman et al., 2004). Subsequently postdoctoral fellow Brett Macey and graduate student Kolo Rathburn showed that exposure to Vibrio bacteria generates a temporary increase in antibacterial activity in blue crabs, and that the underlying immune defense mechanism is sensitive to low dissolved oxygen and high carbon dioxide concentrations in the environment (Macey et al., 2008). In crustaceans prophenoloxidase plays a critical role in sealing wounds and encapsulating invading bacteria to limit infections. Separately, undergraduate researcher Chris Tanner (Tanner et al., 2006) 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?