A number of genera of marine phytoplankton produce an array of chemically similar neurotoxins responsible for PSP¹. Neurotoxins are so-named because they disrupt nerve impulses. These toxins act together to produce PSP. Symptoms may progress from numbness and tingling of lips, and loss of muscular coordination, to respiratory arrest and, ultimately, death. There is no known antidote. In extreme cases, survival depends on immediate availability of life-support systems. Death results in about 15 percent of cases worldwide² . Action level for PSP at no more than 80 µg of PSP toxin in 100 g of shellfish tissue³. The Alexandrium catenella toxins are extremely potent nerve poisons. In fact, as little as one milligram can be enough to kill an adult.


Bivalve shellfish concentrate biotoxins when they filter toxic phytoplankton out of the water
while feeding. The shellfish do not appear to be affected by the toxin. PSP toxin accumulates in marine animals that feed either directly on toxic phytoplankton or on consumers of toxic
phytoplankton. These consumers include zooplankton, bivalve shellfish (oysters, mussels,
clams, etc.), predatory marine snails (moon snails and whelks), crabs, fish, birds and marine
mammals⁴. Mass mortalities among other shellfish-eating animals, including
birds, fur seals, foxes, sea otters⁵, and humpback whales have been
traced to PSP⁶. People are poisoned when they eat shellfish or other marine life that contain excessive levels of PSP toxin.

Phytoplankton Ecology and Biotoxins

Phytoplankton are one-celled marine plants that can conduct photosynthesis. Photosynthesis
enables plants to use the energy of the sun to make food from carbon dioxide and nutrients taken from the water. Oxygen is produced as a by-product. Phytoplankton and other photosynthesizers are the basic source of energy for all components of the marine food web.


During wintertime rain storms, rivers and stormwater runoff carry nutrients into water from nearby uplands and watersheds. Strong winds mix the freshwater with nutrient-rich water coming in from the open sea. The dim sunlight limits growth. Phytoplankton is continually cycled from the surface to the dimly lit depths and back up again, awaiting the arrival of favorable conditions for growth.


In early spring, strong winds subside, and the sun warms the water's surface. The water column stabilizes and vertical mixing stops. A second influx of nutrients occurs when summer snowmelt increases flow to the water. The abundant dissolved nutrients in a stable water column lead to blooms of many phytoplankton species in sunlit surface waters. Blooms can be so dense that the phytoplankton colors the water. The condition is sometimes called a red tide. However, the term is misleading because the colors (produced by photosynthetic pigments) may range from brown to orange to purple. Blooms are sometimes associated with a group of phytoplankton termed dinoflagellates. About 60 dinoflagellate species (of over 2,000) produce biotoxins. Alexandrium catenella, which causes PSP , is a dinoflagellate. It is important to note that PSP may reach dangerous levels in shellfish long before the density of Alexandrium catenella becomes numerous enough to change colore of the water.

A brief description of Alexandrium catenella, the poison-producing organism, will help to explain the seasonal occurrence of shellfish containing PSP toxins and the differences in timing of toxicity in different shellfish. Alexandrium catenella is a microscopic singlecelled type of plankton. Plankton are plants and animals that are carried along by the currents.

Alexandrium catenella belongs to a group called dinoflagellates, which have both plant and animal characteristics, can manufacture their own food, and can swim. Each cell of A. catenella can live independently, but during periods of rapid growth and division, the cells remain attached to each other in a chain-hence the name catenella or "little chain." Each cell is equipped with two tiny whips used in swimming. At times, some of the swimming cells drop their whips and form non-swimming, heavy-walled resting cells or cysts. These settle to the bottom and can lie dormant in the bottom sediment through low winter temperatures. Many of the cysts are eaten by animals, including bivalve shellfish, or become buried. Some germinate when environmental conditions are favorable, giving rise to another population of swimming cells.

Two whip-like flagella enable dinoflagellates to move vertically in the water column. By mid to late summer, surface waters are frequently depleted of nutrients and many species of phytoplankton die back. But dinoflagellates journey to deeper water where nutrients remain plentiful. They return to the sunlit surface to carry on photosynthesis. Vertical migration may give dinoflagellates an advantage over other phytoplankton species. As a result, dinoflagellate blooms often extend into late autumn. By that time, falling water temperatures and the onset of wind-induced vertical mixing lead to a breakdown of the stratification of the water column, and sunlight becomes too dim to continue production. Under these conditions dinoflagellates, such as Alexandrium catenella, may form resting cysts that settle to the bottom to await the return of favorable growth conditions⁷. A resting cyst may be ten times more toxic than its free-swimming form⁸.

METHOD:

DISCUSSION:

None: PSP level was less than 80 µg per 100 grams of shellfish tissue ;

Low: PSP level ranged from 80-499 µg per 100 grams of shellfish tissue;

Moderate: PSP level ranged from 500-999 µg per 100 grams of shellfish tissue;

High: PSP level was greater than 1000 µg per 100 grams of shellfish tissue.