Paralytic shellfish poisoning (PSP) toxins and domoic acid are naturally occurring marine toxins. Single-celled marine plants (phytoplankton) produce these toxins. Marine animals that filter their food from seawater may accumulate these toxins. The toxins do not appear to directly harm the animals, but people or some predatory animals eating toxic seafoods may become poisoned. PSP toxins and domoic acid are powerful nerve poisons. PSP toxins and domoic acid have no taste or odor. There is no visible difference between toxic and safe seafoods. Cleaning seafoods in many cases will not remove the toxins. Cooking does not destroy the toxins.
Paralytic Shellfish Poisoning Toxins
PSP symptoms begin within a few minutes to a few hours after eating toxic shellfish. Symptoms begin with tingling and numbness of the lips, tongue and fingertips. Later symptoms are lack of balance, lack of muscle coordination, slurred speech and problems in swallowing. Complete paralysis and death can occur in severe cases.
Domoic acid poisoning symptoms begin within 30 minutes to 24 hours after eating toxic seafood. In mild cases, symptoms may include vomiting, diarrhea, stomach cramps, headache, dizziness and confusion. Difficulty breathing, seizures, coma and death can occur in severe cases. Survivors of severe cases have suffered permanent loss of short-term memory, a condition known as amnesic shellfish poisoning (ASP).
California's Mussel Quarantine
Local health officers enforce the quarantine. They post signs advising people of the quarantine. These signs also warn people that clams and scallops at times may contain toxins. During the quarantine period, sport harvesters should remove the viscera from clams and scallops and remove the siphons from Washington clams, and eat only the remaining white meat. Missing or destroyed signs do not change the quarantine period.
Marine Toxin Monitoring Program
Unsafe toxin levels sometimes occur outside the annual quarantine period. CDHS declares special local quarantines whenever unsafe levels of toxins appear. Almost all (over 99%) of PSP cases in California have occurred during the annual quarantine months. Domoic acid poisoning has never been reported in humans in California.
Commercially Harvested Shellfish
Oysters are the principal bivalve shellfish harvested commercially in California. Other shellfish are available in California retail food stores and restaurants throughout the year. These products come from both local and out-of-state sources. State and federal agencies regulate all commercially harvested shellfish in the United States. These regulations make certain that only safe and wholesome shellfish are available to the consumer.
Mussels may be harvested and sold for fish bait at any time. Labels on
containers of mussels for fish bait state:
"MUSSELS FOR BAIT
Shellfish Information Line
510/412-4643 (callers in 510 area code, or out of state)
800/553-4133 (Calif. callers outside the 510 area code)
CDHS updates the message as needed. Callers may leave a message at this
number to request more detailed information. Annual reports on the shellfish
monitoring program are available from:
Myths About Red Tides
Many people believe that red tides cause shellfish to be toxic. Most red tides are harmless. Toxin-producing organisms do not generally cause red tides. Although the presence of a red tide is a warning that shellfish may be toxic, the absence of a red tide does not mean they are safe to eat. Shellfish can consume enough toxic organisms to become toxic even when there is no visible red tide.
Safety Tips For Sport Harvesters
During a quarantine:
During nonquarantine periods:
Gregg W. Langlois, Senior Environmental Scientist, California Department of Health Services, Environmental Management Branch, 850 Marina Bay Parkway, Rm. #G165, Richmond, CA 94804
Pamela D. Tom, Seafood Extension Program Manager, Sea Grant Extension Program Manager, University of California, Food Science and Technology Department, One Shields Avenue, Davis, CA 95616
The information presented here is
revised and expanded from the 1991 California Sea Grant publication, "Natural
This work is sponsored in part by NOAA, National Sea Grant College Program, Department of Commerce, under grant number NA04OAR4170038, project number A/EA-1, through the California Sea Grant College Program, and in part by the California State Resources Agency. The U.S. Government is authorized to reproduce and distribute reprints for governmental purposes.
University of California, the United States Department of Agriculture, and the United States Department of Commerce cooperating.