Shellfish: Nutrition and Toxins

This section seeks to embolden the harvest of shellfish, particularly mussels and chocolate clams along the California and Baja coasts. Both are exceptional in flavor, nutrition, and abundance. With precise knowledge of the risks, you may reap incredible rewards.

There is a rule in Fish Lore: “Only eat shellfish in months with an ‘R’”. If you were to take one thing away from this section it is that the window is more like September to April, and don’t you dare break this rule. Additionally, before even thinking about packing your gear, call the Shellfish Biotoxin Information Hotline maintained by CDPH at 1-800-553-4133, listen to the recording twice. They provide up-to-date measurements of toxins in various species and regions of California.

For spearfishermen, sickness from shellfish is associated with warm weather which cultures prolific blooms of toxin producing phytoplankton. However, a full definition of “shellfish poisoning” should include bacteria and virus transmission from a shellfish harvested from a fecal-contaminated water supply. I present three classifications of Shellfish Poisoning: paralytic, amnesic, and diarrheic.

(1) Domoic acid causes amnesic shellfish poisoning (ASP); strongly associated but not exclusive to mussels.

(2) Saxitoxin causes paralytic shellfish poisoning (PSP); strongly associated but not exclusive to chocolate clams.

(3) Norovirus, vibrio spp. and hepatitis A cause diarrheic illnesses; strongly associated but not exclusive to farmed oysters.

Domoic Acid

What is it: Domoic acid is a neurotoxin produced by the red algae, Chondria armata, and the phytoplankton, Pseudo-nitzchia. Eaten in excess, this toxin accumulates in the memory centers of the brain leaving the victim with memory loss. In warm weather months (May-October), these algae proliferate. Filter feeding shellfish consume these species, which then secrete domoic acid inside the shellfish’s viscera. For humans, mussels are the most common vector for domoic acid, however, it is frequently found in razor clams, viscera of Dungeness crab, anchovies, and sardines. Unique to the Razor clam (found often in Humboldt and Del Norte counties), domoic acid accumulates significantly in the foot as well as its viscera.

This toxin takes down mammals bigger than humans too. Sea lions have been increasingly subject to fatal domoic acid poisoning, and most reliable research on domoic acid poisoning has come from dissections of poisoned, dead sea lions in the wild. For the horror movie buff, Alfred Hitchcock’s film, The Birds, was inspired by a massive die-off of the Sooty Shearwater bird in 1961 in Monterey Bay, which coincided with a very large pseudo-nitzchia bloom.

Mechanism: The domoic acid molecule binds to neurons and causes them to fire in rapid succession known as excitotoxicity, draining them of all their energy, leading to neuron death.

Presentation: Within 24 hours of eating contaminated shellfish, you will experience abdominal pain, diarrhea, and vomiting, followed shortly by headache, memory loss and disorientation. Severe poisoning leads cardiac instability, seizures, coma, and death. In survivors, it is common to have anterograde memory loss, where you cannot store new memories. The symptoms described above correlate to the organ system which domoic acid accumulates: hippocampus (memory loss), retina (vision loss), area postrema (vomiting reflex center), dorsal root ganglia and ventral spinal chord at all levels (extremity paralysis and tremor), and heart (arrhythmias and heart failure). There is no known antidote to domoic acid.

Saxitoxin

What is it: Saxitoxin is a neurotoxin secreted commonly by the ocean-borne dinoflagellate, Catenella spp., and the fresh-water cyanobacteria, Anabaena. Consumed in excess, this toxin induces paralysis.

Geography: Spearfisherman will most likely encounter saxitoxin in chocolate clams harvested on the pacific or gulf coasts of Baja. Saxitoxin accumulates in the viscera of these clams just like domoic acid in California mussels. Unlike domoic acid, very low levels of saxitoxin create risk for human toxicity, and so outbreaks are common without an obvious red tide.

In a Mexican study, researchers harvested and analyzed chocolate clams from Bahia de la Paz monthly, and detected saxitoxin in December, March (highest), April, June, and August. This schedule does not fit the shellfish “R” rule, exemplifying how there is year to year variability in algae proliferation. This variability is true for domoic acid as well but likely more significant human impact with saxitoxin chocolate clams due to the lower dosage required for toxicity. To hammer home again: ALWAYS call the Bivalve hotline or consult locals before attempting these shellfish.

Saxitoxin reversibly inhibits sodium channels in neurons, inhibiting signals from being transmitted neuron to neuron. Within 1-2 hours of eating a chocolate clam with saxitoxin, it is common to have loss of sensation around the lips and face, odd feelings of floating, tingling of the extremities, and rapidly progressive weakness, imbalance, and paralysis. Saxitoxin does not affect the memory centers of the brain. Diarrhea, abdominal pain and high blood pressure are also common symptoms. Unlike domoic acid poisoning, due to the reversible blockage of neuron firing, most victims fully recover within 1-2 days without medical care. However, general fatigue has been reported for a week after poisoning. Currently there are no known antidotes to saxitoxin and treatment is supportive, generally targeting gastrointestinal symptoms.

Norovirus, Vibrio spp., and Hepatitis A – diarrheic shellfish poisoning

 Oysters are the typical culprit for bacterial and viral shellfish illnesses.

Norovirus is caused by human fecal contamination in water sources where shellfish are grown. Within a few hours of consumption, the virus causes fever, vomiting and watery diarrhea, resolving within 3 days. Norovirus is highly contagious, requiring only 10-100 viral particles to initiate symptoms. It is likely your whole family will get sick even if it was only you who ate the bad oysters. It is essential to maintain hydration during infection due to profound water loss in diarrhea.

Vibrio vulnificus and V. parahaemolyticus are two bacteria which also commonly result from human fecal contamination of water sources where shellfish are grown. In the United States, V. parahaemolyticus is more common, but V. vulnificus is the most common cause of death from seafood consumption. Symptom onset is typically within 24 hours and includes low-grade fever, characteristic abdominal cramping, and vomiting and diarrhea. V. vulnificus has additional more severe symptoms including blisters on the skin which can progress to cellulitis and even necrotizing fasciitis. If you have chronic liver disease, like cirrhosis, you have an increased risk for infection. Treatment for vibrio infection is typically a 1-2 week course of hydration and doxycycline.

Nutrition of Mussels and Chocolate Clams

 Shellfish are a nutritional superfood. The previous sections described everything that could go wrong, but let’s conclude with everything that can go right. 

Mussels are a gusher of essential minerals and B vitamins within a high protein, low fat bite.  In a 3 ounce (85 gram) serving of mussels you consume 150 calories, 20 grams of protein, 4 grams of fat, 50mg of good cholesterol. The manganese found in 3 ounces of mussels accounts for 251% the daily recommended value - essential for wound healing, bone strength, sex hormone synthesis, carbohydrate metabolism and much more. Selenium (138% daily value) is an antioxidant relied upon by your immune system, thyroid, and reproductive system. Iron (32% daily value), energy, blood cell formation, growth and development. Vitamin B12 (850% daily value), growth and development of red blood cells, nervous system and structure of DNA. Mussles are rich in other B vitamins including riboflavin, Niacin, B5 and folate.