Sarasota Marine Lab
Red tides occur throughout the world, drastically affecting Scandinavian and Japanese fisheries, Caribbean and South Pacific reef fishes, and shellfishing along U.S. coasts. Most recently, it has been implicated in the deaths of hundreds of whales, dolphins, and manatees in North American waters.
These red tides are caused by several species of marine phytoplankton, microscopic plant-like cells that produce potent chemical toxins. Red tide is the result of a massive multiplication (or bloom) of tiny, single-celled algae called Gymnodinium breve, usually found in warm saltwater, but which can exist a lower temperatures. Some red tides have covered up to several hundred square miles of water.
G. breve produces potent neurotoxins. Filter-feeding shellfish, such as oysters, clams, mussels and other bivalve mollusks, that consume G. breve concentrate the toxin in various organs. Whereas red tide toxins are deadly to finfish, shellfish are uneffected. Shrimp, Crab, Scallops and Lobsters in red tides are safe to harvest and eat, since these shellfish do not accumulate the red tide toxin in the meaty or hard muscle tissue which we normally consume.
It is not a good idea to eat liver, organs, or other soft tissue of shellfish. The muscle or hard meat of freshly caught finfish in red tides are safe to eat, provided the fish behave normally. Although no evidence of harmful effects in humans from contaminated fish have been reported, it is not a good idea to eat liver, organs or other soft tissues. Oysters, Clams, Mussels, Mollusks, Whelks are unsafe to harvest and eat since they may accumulate red tide toxins in their tissues.