Beware the Red! The Red Tide!

By Brian Tse

When you think of the colour red, thoughts of love and passion are often evoked. However, in the world of marine microorganisms, the colour red, can have potentially more deadly connotations. As a person from Vancouver, BC, I am used to being near the ocean and enjoying delicious seafood and wonderful ocean breezes. However, a phenomenon that can occur in the coastal waters off BC known as Red Tide has heightened my fears of eating the delicacy that is fresh shellfish from our waters.

Red Tide effect at Thermaikos Bay, Greece. Photo by Anthony Sigalas. Used under the Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic license.

Red tide occurs in waters when there are giant eruptions of growth of certain marine microorganisms. Known in the scientific world as harmful algal blooms or HABs, the species responsible for these events are dinoflagellates, taxonomically classified as a Phylum of protozoans, usually in the genera Alexandrium, Gymnodinium, or Karenia (Mudie et al., 2002). A clarification of what dinoflagellates are for those not savvy with the biological jargon: they are tiny prokaryotic, and sometimes photosynthetic, organisms that have two flagella protruding from them (Lackey, 2014). Photosynthetic dinoflagellates of the genus Symbiodinium, also called zooxanthellae, form symbiotic relations with corals and other multicellular organisms (Pechenik, 2010). Some genera also have bioluminescent properties (Pechenik, 2010) and look quite beautiful if seen at night producing a bright blue glow.

Breaking waves at La Jolla, California, creating bioluminescense from dinoflagellates. Photo by Kevin Baird. Used under the Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license.

Back to the HABs. Bi-valve shellfish such as clams or mussels, feed by letting water pass through them and essentially filter the water by catching any tiny microorganisms that unfortunately get caught inside it. When there are large amounts of dinoflagellates in the water, the shellfish can’t help to pick up one or two along the way. However, the dinoflagellates produce a toxin that gets picked up by the shellfish as it doesn’t get filtered (Setala et al., 2014). Under normal conditions, the amount of toxin is negligible, but when HABs occur, the amount of toxins collected by a shellfish is high enough that eating said shellfish, is potentially fatal. While the shellfish are not harmed for just accumulating these toxins, other marine species such as fish and crustaceans are killed. (Pechenik, 2010).

The toxins produced by the various dinoflagellates are neurotoxins, which can cause paralysis at varying severity. The condition for people who suffer paralysis after consuming toxic shellfish is aptly named “Paralytic Shellfish Poisoning” (Setala et al., 2014). The different genera of dinoflagellates produce different types of neurotoxins such as brevetoxin from Karenia (Pierce and Henry, 2008) or saxitoxin from Alexandrium (Setala et al., 2014). What makes things worse is that cooking infected shellfish does not get rid of the toxins and some shellfish can store toxins within their body for years afterwards (Pierce and Henry, 2008). Less severe cases can result in symptoms of vomiting, nausea, and diarrhea (Pechenik, 2010).

Now that you’ve learned about paralytic shellfish poisoning and HABs and all the dangers that surround them, aren’t you just craving for a fresh oyster at the local seafood bar? While the threat of toxins and deadly shellfish do loom, especially for those who live in coastal cities, it is thankful that many countries like Canada and the United States have good laws on commercial farming as well as food inspectors which alleviates a lot of the risk from eating shellfish. Personally, I won’t be stopping anytime soon, but if you’re out by ocean with some friends, and find some shellfish in a pool of beautiful red water, and proceed to eat it, don’t say I didn’t warn you.

Works Cited

Lackey, James B. (2014). Dinoflagellida. In AccessScience. McGraw-Hill Education. Retrieved from

Mudie, P.J., Rochon, A., and Levac, E. (2002). Palynological records of red tide-producing species in Canada: Past trends and implications for the future. Palaeogeography, Palaeoclimatology, Palaeoecology, 180, 159-186.

Pechenik, J.A. (2010). Biology of the Invertebrates. New York: McGraw-Hill.

Pierce, R. H., and Henry M.S. (2008). Harmful algal toxins of the Florida red tide (Karenia brevis): Natural chemical stressors in south Florida coastal ecosystems. Ecotoxicology, 17(7), 623-631.

Setala, O., Lehtinen, S., Kremp, A., Hakanen, P., Kankaanpaa, H., Erler, K., and Suikkanen, S. (2014). Bioaccumulation of PSTs produced by Alexandrium ostenfeldii in the northern Baltic Sea. Hydrobiologia, 726, 143-154.