Pfiesteria’s Effects on Shellfish

Preliminary research indicates that Pfiesteria adversely affects a number of shellfish species. Motile shellfish (scallops, crabs) exhibit a strong escape response when exposed to toxic zoospores of Pfiesteria.   During fish kill events in the field, “crab walks” often occur in which animals attempt to leave the toxic water.   Crabs collected during these events often have been damaged to the point that large areas of the carapace are absent and by all appearances have been “eaten” away.


In experimental bioassays, shellfish have exhibited a range of responses to the presence of P. piscicida at sublethal densities (100-1,000 cells/mL). These responses range from lethargia (blue crabs), to loss of valve control (bay scallops), and significantly decreased filtration rates (clams, oysters, scallops). Concentrations of P. piscicida zoospores greater than 1,000 cells/mL have been lethal to all shellfish assayed.

Note: Cell concentrations of Pfiesteria piscicida during kill/disease events range from 150-35,000 cells/mL.

Table 1. Shellfish which are susceptible to Pfiesteria’s toxin(s) under field conditions (active fishkill events) or in culture bioassays (containing toxic stages of Pfiesteria sp. >1,000 cells/mL).

Species Affected life stages to date Laboratory Bioassay/Field
Argopecten irradians (Bay Scallop) Larvae, juveniles, adults Laboratory
Callinectes sapidus (Blue Crab) Adults Field, Laboratory
Crassostrea virginica (Eastern Oyster) Juveniles, adults Field, Laboratory
Mercenaria mercenaria (Northern Quahoq) Juveniles, adults Laboratory
Mytilus edulis (Ribbed mussel) Adults Laboratory
Venus cancellata (Venus Clam) Adults Laboratory


Overall, the scientific community currently has little knowledge of Pfiesteria’s effects on shellfish.   Our lab under the direction of Dr. Shumway and Jeff Springer is currently undertaking a major research effort to characterize the response(s) of three commercially important species of bivalve shellfish to toxic stages of Pfiesteria.   One of our most pressing needs is to assess whether shellfish harvested from areas with active Pfiesteria-related fishkills are safe for consumers to eat.   Laboratory experiments in progress will determine if shellfish accumulate toxin(s) in the presence of Pfiesteria zoospores and, if they do, how long these toxin(s) are sequestered in the shellfish tissues.   We also plan to examine a number of physiological parameters as well, including clearance rate, filtration rate, and heart rate. Findings on these research efforts should be complete by late summer 1999. Please check back for up to date advances in Pfiesteria research.