28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 Effects of Stressors on Mortality Rates in Lobsters (Homarus americanus) Infected With a Bacterial Pathogen Richard A. Robohm1 and Andrew F. J. Draxler2 1NOAA, NMFS, Northeast Fisheries Science Center, Milford Laboratory, 212 Rogers Ave., Milford CT 06460; 2James J. Howard Laboratory, Magruder Road, Highlands NJ 07732 The cause of unprecedented lobster mortalities in western Long Island Sound (LIS) in 1998 and 1999 was not established.   A parasitic amoeba found in both moribund and non-moribund lobsters may, in association with environmental conditions, contribute to deaths; however, controlled laboratory studies with this organism cannot be done because the amoeba could not be cultured in vitro.  We report here the use of a surrogate, Aerococcus viridans (a bacterial pathogen of lobsters), to help reveal whether increased, but environmentally realistic, levels of temperature, hypoxia, sulfide, and ammonia can heighten susceptibility to microbial infection.  Lobsters from commercial harvesters were acclimated for seven days to conditions of temperature and bottom light that exist in western LIS in September and to the salinity at the Howard Laboratory.  Each lobster was injected in the ventral sinus with 1x103 or 1x106 A. viridans or saline.   Experimental conditions for temperature and concentrations of oxygen, sulfide, and ammonia were generated in a flow-through system of sealed tanks.  Lobsters were monitored twice daily for behavioral response and viability.   At appropriate intervals, lobsters were transported to the Milford Laboratory to enumerate bacteria in the hepatopancreas and hemolymph. At adequate oxygen levels (200 mM), exposure to 6 mM sulfide and 24 mM ammonia accelerated the 50% mortality rate in lobsters from 12 days to 6.6 days; at lower oxygen levels (80>mM) mortalities were further accelerated to about 3 days.  The lower oxygen levels alone were sufficient to accelerate lobster deaths regardless of the presence of sulfide and ammonia.  Temperature (14.5°C vs. 19.5 °C) and bacterial doses had moderate effects on mortality.  Bacteria in the hemolymph and hepatopancreas of non-stressed lobsters reached 1x109 g-1 within 10 days; whereas counts in dying, stressed lobsters were 1x103 to 1x106 g-1  (lower than counts in non-stressed, infected-but-living animals at the same time interval).  Additional experiments are in progress to separate the effects of sulfide and ammonia and to examine whether the same effects are seen in lobsters infected with Vibrio fluvialis. Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page