The genetic response of Pacific razor clams to domoic acid

Razor clams can hold on to low levels of toxic domoic acid. Being able to test for the presence of harmful toxins can serve as an early warning to monitor the health of clams, their ecosystem, and species--like us--who may eat them.

National Oceanic and Atmospheric Administration

As ocean conditions change, harmful algal blooms are expected to become more common. One type of algae, Pseudo-nitzschia, can produce a toxin called domoic acid (DA). At certain levels, DA is harmful to humans and animals. Regulatory limits have been set for safe consumption by humans, however, some clams, like Pacific razor clams, can hold onto low levels of DA for extended periods of time and there are no regulatory limits for low-level, chronic exposure.

We expect that chronic exposure to DA may disrupt important cellular processes, leading to concerns about effects on their overall health and potential population- and ecosystem-level impacts to clams. To study this, we looked at differentially expressed genes in razor clams before, during, and after a DA-producing bloom. We found changes in the genes that show signs of DA exposure, even in clams with low toxin levels. Targeting these genes in real-time using PCR (polymerase chain reaction) assays may provide an early warning system for routine monitoring of DA in clams. (This is the same kind of testing used to detect or diagnose some infectious diseases or genetic changes.) In addition, this research helps us understand how DA exposure (even at low levels) may affect clam health, which could have ramifications to clam populations and overall ecosystem health.

Phytoplankton blooms can create toxins, such as domoic acid (these are called harmful algal blooms). The toxins have harmful effects on animals that ingest the plankton directly, such as clams, and further up the food chain, such as people and walruses that eat the clams. We’re still learning about the impacts of the toxin, especially at lower levels. Developing an early warning test can provide for added safety before more harmful levels are reached.

Transcriptomics as an Early Warning of Domoic Acid Exposure in Pacific Razor Clams (Siliqua patula)

Abstract

As oceans warm, harmful algal blooms (HABs) are expected to increase, including blooms of Pseudo-nitzschia, a diatom that produces domoic acid (DA), which is a potent neurotoxin. Regulatory limits for human consumption (0.075–0.1 mg/kg/day; acute exposure) exist for the Pacific razor clam; however, fisheries currently do not have regulatory limits for chronic low-level exposure to DA even though razor clams can retain DA for over a year after an algal bloom. For bivalves, exposure to marine toxins may disrupt important cellular processes, leading to concerns about effects on their overall health and potential population- and ecosystem-level impacts. Transcriptomics was used to identify differentially expressed genes in razor clams (N = 30) from Long Beach, WA, collected prior to, during, and after a DA-producing bloom. Differentially expressed genes were identified that may indicate exposure of razor clams to DA, including clams with tissue DA concentrations that fall below regulatory limits for human consumption. Targeting these genes in real-time PCR assays may provide an early warning system for routine monitoring of DA in clams. Our results suggest DA exposure is associated with physiological responses ranging from decreased immune function to the potential disruption of cell communication, including retinoic acid catabolic processes, cell adhesion, collagen fibril organization, and immune effector processes. This work may also allow us to examine potential drivers of population-level change and whether chronic lower-level exposure to DA negatively impacts razor clam function, consequently affecting individual and population health.

Bowen, L., S. Waters, B. Ballachey, H. Coletti, Z. Forster, J. Li, and B. Jenner. Transcriptomics as an early warning of domoic acid exposure in Pacific razor clams (Siliqua patula). Toxins 17(4): 194.