The effects of prolonged exposure to hypoxia and Florida red tide (Karenia brevis) on the survival and activity of stone crabs

Florida red tides are harmful algae blooms caused by the dinoflagellate Karenia brevis, which occur along Florida's gulf coast almost annually. In recent years Florida red tide blooms have become more common, frequent, and intense. Florida's southwest coast, from Manatee to Collier County, has experienced repeated and prolonged K. brevis blooms since 2011 with the most recent bloom in 2017 lasting 17 months and resulting in both hypoxic and anoxic events. We therefore determined the survival and level of lethargy (e.g., lack of responsiveness or reduction in behavioral reactions) of sublegal stone crabs to K. brevis and hypoxia as both singular and simultaneous stressors. Crabs were randomly assigned to one of six treatments that included: 1) high concentration of toxic K. brevis (> 1 million cells L-1) maintained at normoxic levels (7.2 mg L-1 +/- S.D. 0.47 dissolved oxygen), 2) moderate hypoxia (1.6 mg L-1 +/- S.D. 0.42 dissolved oxygen) with no K. brevis, 3) moderate hypoxia (1.5 mg L-1 +/- S.D. 0.43 dissolved oxygen) with a high concentration of K. brevis, 4) severe hypoxia with no K. brevis (0.69 mg L-1 +/- S.D. 0.36 dissolved oxygen), 5) severe hypoxia (0.63 mg L-1 +/- S.D. 0.40 dissolved oxygen) with a high concentration of K. brevis, and 6) a normoxic control (7.3 mg L-1 +/- S.D. 0.61 dissolved oxygen) with no K. brevis. Survival and stone crab lethargy or responsiveness was monitored every 10-12 h for six days. Crabs simultaneously exposed to K. brevis and severe hypoxia exhibited a 43% decrease in survival and experienced increased lethargy within 24 h relative to the control (7% decrease in survival, no increase in lethargy). The increase in stress level and sluggish behavior during exposure to hypoxia was evident by a general lack of responsiveness or movement which indicates that nearshore populations of stone crabs are unlikely to emigrate away from such conditions suggesting that future harvests may be reduced following prolonged K. brevis blooms and hypoxic events.