Pacific razor clams are an integral part of Washington’s ocean beaches that are typically dug during the fall, winter, and spring as permitted by Washington Department of Fish and Wildlife (1). Their importance to the region dates back thousands of years as they have provided sustenance to the Quinault tribe and are prominent in tribal spiritual beliefs (2). The popularity of clam digging is undeniable as it is commonplace to have a thousand people per mile when the conditions are just right (1). However, there is a limiting factor to the harvesting of razor clams-domoic acid levels- and for good reason. Consumption of high enough concentrations of domoic acid causes incurable poisoning in humans and some marine animals.
What is domoic acid?
Domoic acid is a potent neurotoxin that is produced by certain marine organisms, mainly by planktonic diatoms of genus Pseudo-nitzschia but also by red alga Chondria armata. It was first identified in Canada in 1987 and was initially detected on the west coast in 1991 (1,3). Domoic acid levels are correlated to harmful algal blooms (HABs), which occur when algae and diatoms grow out of control. There are many factors contributing to the occurrence of HABs, but two contributors of concern are nutrient pollution and warming water temperature (4). In 2015, the Pacific Coast experienced a major HAB event that was believed to have cost the Quinault tribal fisheries over $2.4 million in losses due to unsafe toxin levels, which forced the closure of fisheries and crabbing seasons all along the west coast (5).
Razor clams and other shellfish are filter feeders who consume these toxin-producing algae and diatoms. To monitor domoic acid levels, WDFW samples clams on a regular basis and sends them to the Washington State Department of Health for testing (6). While it is not entirely clear why species of Pseudo-nitzschia produce domoic acid, the genes responsible for its production were identified in 2018 by scientists from Scripps Institution of Oceanography and J. Craig Venter Institute (7). Understanding how these genes are turned on and off is imperative for predicting HAB events and monitoring of early stages of domoic acid production (7).
Why is domoic acid so dangerous?
Domoic acid, when consumed by humans, elicits a wide range of symptoms affecting multiple organ systems. This includes nausea, vomiting, abdominal cramps, unstable blood pressure, cardiac arrhythmias, neurological dysfunction, coma, memory loss, and possible death (3). Collectively, an individual experiences what is now called “Amnesiac Shellfish Poisoning” (ASP), named as such due to severe cases causing permanent short-term memory loss (8). There is currently no antidote for ASP and the effects of the toxin on the neurological system are long-lasting. For cases with high concentrations of domoic acid in a victim’s system, the only medical option is life support until the toxin is passed out of their system (8).
Humans are not the only species affected by ASP. Domoic acid experiences a stable transfer throughout the marine food web and has been measured in harbor seals, fish, sea lions, whales, sea otters and sea birds (9). Studies have shown that many populations are more vulnerable to ASP including pregnant women, elderly people, infants and children (10).
As with most toxins, the danger of domoic acid truly lies in its ability to bind certain receptors on the surfaces of brain cells (neurons). The structure of domoic acid has similarities to glutamate, a molecule that is used in signaling from one neuron to another to provoke a response. When glutamate binds to its receptor, it triggers an excitatory response. Amounts of glutamate are tightly regulated as to not continuously activate an excitatory response. Domoic acid, however, has 100x stronger affinity for the receptors that glutamate normally binds to. It outcompetes glutamate for binding sites and stays bound to the receptor longer. This causes excitotoxicity- starting a cascade of signaling within a cell that ultimately leads to loss of neuronal function and cell death (9).
How can we get rid of domoic acid?
The short answer to this question is that we don’t. Domoic acid floating in marine water is not the main issue due to biodegradation and the ability of the sun to degrade the acid, also called photodegradation (9). Domoic acid also becomes diluted in the water column and therefore doesn’t reach concentrations that are even close to the limit for toxicity. The real problem lies in domoic acid that is absorbed into sediments on the ocean floor where they enter the benthic food chain. Once it begins accumulating in tissues of organisms such as zooplankton, shellfish, crustaceans, echinoderms, and worms, it can take up to a few days for the domoic acid to be purged from their systems (9). Notably, the retention of domoic acid is prolonged in razor clams as well as scallops (11). Overall, WDFW in collaboration with the Washington State Department of Health do a great, thorough job of monitoring this potent neurotoxin and protecting clam diggers from ASP. Looking forward, scientists continue to research more accurate ways to predict HABs and hope to learn more about how to mitigate the effects of domoic acid on fisheries, crabbing and clamming seasons.
References
- Razor clam seasons and beaches. Washington Department of Fish & Wildlife. Retrieved February 24, 2023, from https://wdfw.wa.gov/fishing/shellfishing-regulations/razor-clams#current
- Quinault Nation. Quinault Shellfish Recovery Program. Quinault Division of Natural Resources. Retrieved February 24, 2023, from http://qlandandwater.org/departments/fisheries/shellfish/
- Pulido, O. M. (2008). Domoic acid toxicologic pathology: A Review. Marine Drugs, 6(2), 180–219. https://doi.org/10.3390/md20080010
- Environmental Protection Agency. (n.d.). Climate Change and Harmful Algal Blooms. EPA. Retrieved February 24, 2023, from https://www.epa.gov/nutrientpollution/climate-change-and-harmful-algal-blooms#:~:text=Harmful%20algae%20usually%20bloom%20during,green%20algae%20prefer%20warmer%20water.
- US Department of Commerce. What is a harmful algal bloom? National Oceanic and Atmospheric Administration. Retrieved February 24, 2023, from https://www.noaa.gov/what-is-harmful-algal-bloom
- Domoic acid. Washington Department of Fish & Wildlife. Retrieved February 24, 2023, from https://wdfw.wa.gov/fishing/basics/domoic-acid
- 7. Wood, L. F. (2018, September 27). Domoic acid decoded: Scientists discover genetic basis for how harmful algal blooms become toxic. UC San Diego Today. Retrieved February 24, 2023, from https://today.ucsd.edu/story/domoic_acid_decoded_scientists_discover_genetic_basis_for_how_harmful_algal_blooms_become_toxic
- Amnesic shellfish poisoning (ASP) from Domoic acid. Washington State Department of Health. Retrieved February 24, 2023, from https://doh.wa.gov/community-and-environment/shellfish/recreational-shellfish/illnesses/biotoxins/amnesic-shellfish-poisoning
- Zabaglo, K., Chrapusta, E., Bober, B., Kaminski, A., Adamski, M., & Bialczyk, J. (2016). Environmental roles and biological activity of Domoic Acid: A Review. Algal Research, 13, 94–101. https://doi.org/10.1016/j.algal.2015.11.020
- Toyofuku, H. (2006). Joint FAO/WHO/IOC activities to provide scientific advice on marine biotoxins (research report). Marine Pollution Bulletin, 52(12), 1735–1745. https://doi.org/10.1016/j.marpolbul.2006.07.007
- Mafra, L. L., Bricelj, V. M., & Fennel, K. (2010). Domoic acid uptake and elimination kinetics in oysters and mussels in relation to body size and anatomical distribution of toxin. Aquatic Toxicology, 100(1), 17–29. https://doi.org/10.1016/j.aquatox.2010.07.002
© Rachel Davey, February 2023