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Attack of the Killer Algae!

The dangers lurking in shellfish may make you wish you were allergic.

Sea Sparkle
Image from Tasmanian Geographic
This lake in Hong Kong might look like a great place to take a dip and go for a swim, but most marine animals would strongly disagree. This light show, dubbed ‘sea sparkle’, is a result of a bioluminescent bloom of algae. The protist responsible is called Noctiluca scintillans—which essentially means “bright night lights” in Latin. Unfortunately, this beauty is shallow; beneath the surface, these waters are malicious. These single-celled protists secrete ammonia into the surrounding waters; thereby, they turn the once neutral water alkaline. This alkalinity puts many marine invertebrates at risk, mostly mollusks and crustaceans. Cephalopods (octopi, squids, etc.) are killed by the ammonia, while the ammonia in bivalves (clams, oysters, etc.) and crustaceans (crab, lobster, etc.) prevents them from making their protective chitinous or calcium carbonate shells. This keeps crustaceans in a soft-shell state, and it prevents newborn bivalves from creating their shells-meaning crustaceans are unable to finish molting, and bivalves are completely exposed1.

While most algae are harmless, there are some which are dangerous due to their excretions from digestion/reproduction or those that create hypoxic conditions when in mass1. These dangerous exceptions are collectively termed ‘Harmful Algal Blooms’ (HABs) and are of increasing concern. Although swimming in blooming waters is unlikely to kill humans, when HABs contaminate aquaculture operations and fisheries, the results can be catastrophic.

The protist Alexandrium excretes the alkaloid toxin ‘Paralytic Shellfish Poisoning toxin’ that can accumulate in shellfish. Eating approximately 3.5 ounces of contaminated shellfish can be fatal to humans1. Since toxins from Alexandrium infect many animals in food webs, large whales and porpoises can also become susceptible to HABs2.

Paralytic Shellfish poisoning Alexandrium
Different varieties of Alexandrium. Image from Phycological Review

Besides creating toxins or suffocating waters, HABs can also manifest mechanically. In aquaculture, caged finfish have no freedom to swim away from noxious HAB outbreaks. This makes caged fish susceptible to certain secretions from HABs that degrade gill structures or can produce hemolytic enzymes that break down gill blood vessels3. This effectively suffocates fish despite the surrounding waters being rich in oxygen content. In 1972 in Japan, a bloom of the raphidophyte Chattonella antiqua killed 500 million dollars worth of caged yellowtail fish in the Seto Island Sea because the fishes’ gills became severely mutilated3.

It is important to note that HABs are not a new occurrence. They are even referenced in the bible; however, there have been more HAB outbreaks this year (2018) than any other year in history (i.e. humans are likely to blame)1-3. This increase is probably from two different sources: eutrophication from fertilizer runoff, which pollutes water sources with nutrients, and overfishing, which disrupts food webs forcing more marine animals to consume dangerous algae, eventually infecting all trophic levels1.

It is easy to see how overfishing increases the prevalence of HABs but eutrophication is a more abstract and convoluted concept. When farmers use fertilizer to condition soil, rain can cause the excess fertilizer to leach from the soil into nearby water sources. These fertilizers contain many macro and micronutrients that are needed for plants to succeed, yet they limit plant growth because they are in short supply. When the fertilizer runoff leaches into water, these nutrients then allow for marine bacteria to experience a population boom. Finally, algae eat the excess bacteria and experience a population boom themselves.

The second most dangerous HAB behind Alexandrium is the domoic acid secreting Pseudonitzchia which cause amnesic shellfish poisoning3. As the name implies, amnesic shellfish poisoning can cause amnesia, but unexpectedly, in certain amphibious mammals such as sea lions, it can cause them to become hostile with such traits as attacking people in coastal communities3. Human amnesic shellfish poisoning was first identified in 1987 at Prince Edward Island, Canada, where it was responsible for 3 deaths and 105 cases of acute human poisoning following the consumption of blue mussels3.

Amnesic Pseudonitzschia
Pseudonitzschia. Image from Phycological Review


The most common vector to get paralytic or amnesic shellfish poisoning is from, well, shellfish. If you have just eaten some shellfish and start to feel numbness, you might have paralytic shellfish poisoning. In moderate cases of paralytic shellfish poisoning, the face and neck will become numb, but, in severe cases, extremities can go numb, you may slur your speech, you may go blind, and you may find breathing can become difficult4. In extreme outbreaks, your medulla oblongata can be affected4. This can cause complete paralysis and, possibly, respiratory failure and death4. Thankfully, the effects don’t last more than 12 hours, and if proper medical intervention is introduced before it progresses, paralytic shellfish poisoning can be treated. However, if left untreated, the fatality rate is about 10% in adults and up to 50% in young children4.

Amnesic shellfish poisoning often has more acute symptoms, but the symptoms are more mild than paralytic shellfish poisoning. The symptoms include vomiting, diarrhea, dizziness, disorientation, and, in severe cases, seizures and comas can occur5. Unfortunately, the long-term effects of amnesic shellfish poisoning are often worse than the paralytic variety5. Long-term cognitive dysfunction and neurologic sequelae occur most often in elderly males and in young people with pre-existing illnesses like diabetes, hypertension, or those with history of stokes5. From a set of 153 poisoned individuals, 3 people died and 12 were hospitalized for seizures, comas, respiratory secretions, or mercurial blood pressure5.

While HAB-related shellfish poisoning may look like food poisoning at first, as time progresses the symptoms can be fatal. It is important to make sure that the shellfish you eat is sold by reputable vendors, and, if you intend to catch your own, make sure you check with state or national health departments for warnings about algal blooms or shellfish poisonings. If you are cautious of where you are getting your shellfish from, you should be fine since HABs are easy to identify via satellite imaging or periodical water tests. All in all before you crack into some shellfish, take a minute to think about HABs.

[1] Heisler, J. “Eutrophication and Harmful Algal Blooms: A Scientific Consensus.” Elseveir, vol. 8, 2008, pp. 3–13.

[2] Geraci, Joseph R., et al. “Humpback Whales (Megaptera Novaeangliae) Fatally Poisoned by Dinoflagellate Toxin.” Canadian Journal of Fisheries and Aquatic Sciences, vol. 46, no. 11, 1989, pp. 1895–1898.

[3] Xu, F.l, et al. “Marine Coastal Ecosystem Health Assessment: a Case Study of the Tolo Harbour, Hong Kong, China.” Ecological Modelling, vol. 173, no. 4, 2004, pp. 355–370.

[4] “Paralytic Shellfish Poisoning.” Oceanus Magazine, WHOI,

[5] “Amnesic Shellfish Poisoning.” Oceanus Magazine, WHOI,

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