Phytoplankton are microscopic, single-celled algae and are functionally the "plants of the sea" because they photosynthesize and cycle nutrients in much the same capacity as land plants. Pseudo-nitzschia refers to a particular genus of diatom, one of many types of phytoplankton. Diatoms are responsible for over 40% of the ocean's primary production and are different from many other phytoplankton types in that they have a siliceous shell (or frustule). Think of diatomaceous earth, silica, or even opal - all are connected back to diatom life cycles in the ocean. Several species of Pseudo-nitzschia bloom seasonally as natural members of the phytoplankton community off the U.S. West Coast. In California these blooms have been growing in frequency, size, and toxicity since about 1998, particularly in the Santa Barbara Channel and Monterey Bay. For reasons scientists still don't entirely understand, some species of Pseudo-nitzschia will produce the neurotoxin domoic acid, a molecule that looks a lot like one of our non-essential amino acids, glutamate or gluatamic acid. It may be that the diatom produces it simply as a metabolic byproduct or that domoic acid serves some function, such as to aid competition with other phytoplankton or to ward off predation by zooplankton. In humans and all other vertebrates, domoic acid has the potential to overexcite neurons at glutamate receptor sites since it is such a similar molecule. We call this a glutamate agonist. This leads to many neurological problems and is more of a risk to animals higher up the "food chain" since domoic acid bioaccumulates as it moves from smaller to larger animals. When humans eat shellfish (or any fish containing high enough levels of domoic acid), there is a high risk of developing Amnesic Shellfish Poisoning, which presents first with gastrointestinal distress and can be followed by dizziness, respiratory distress, short-term memory loss, and even death. Sea lions are often seen bobbing and weaving their heads with disorientation or seizing on beaches during a domoic acid event. These symptoms almost always mean the animal is suffering from either acute or chronic domoic acid poisoning.
If an animal such as a sea lion has an acute case of domoic acid poisoning, it will end up on a beach, pier, parking lot, or somewhere completely unexpected and require immediate attention. These animals are often euthanized since rehabilitation is difficult. In the ocean, dolphins and other cetaceans exposed to DA may simply die before washing ashore. Seabirds are at high risk of perishing from DA poisoning during extreme events since they acquire it through small bait fish. These smaller vertebrates such as anchovies and sardines can sometimes appear disoriented but are not thought to die from DA. This year, however, a large number of anchovies washed ashore in Monterey and had very high full-body loads of DA, which may have led to their eventual stranding on the beach. "Shellfish" such as mussels, oysters, crabs are invertebrates and are not susceptible to the neurotoxic effects of DA.