Exposure of blue crab (Callinectes sapidus) to modified clay treatment of Karenia brevis as a bloom control strategy.

Harmful algal blooms (HABs) of the toxic marine dinoflagellate Karenia brevis, commonly called red tides, are an ongoing threat to human health and marine ecosystems in Florida. Clay flocculation is a standard control strategy for marine HABs in China and Korea and is currently being assessed for use in the United States. We evaluated the effects of a PAC-modified clay called Modified Clay II on mortality, eyestalk reflexes, and righting reflexes of 48 adult blue crabs (Callinectes sapidus). Crabs were exposed to clay alone (0.5 g L  ), untreated K. brevis (1 × 10 cells L  ), or a combination of K. brevis and clay for eight days. Clay treatment reduced cell concentrations in the water column by 95% after 24 h. We detected no significant differences in mortality, righting reflexes, or eyestalk reflexes between treatments. Our results indicate that the clay alone is not harmful to adult crabs at typical treatment concentrations within the measured time frame, and that treatment of K. brevis with this clay appears to have a negligible impact on crab mortality and the reflex variables we measured. These results suggest that Modified Clay II may be a viable option to treat K. brevis blooms without impacting adult blue crab populations. Additional controlled experiments and field tests are needed to further evaluate the impact of clay on natural benthic communities.