Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Toxin Resistance in Clams

Soft shell clams (Mya arenaria) are bivalves native to the east coast of North America.  They have become invasive species elsewhere throughout the northern hemisphere, inhabiting brackish waters, estuaries and marine environments.

Soft shell clams are susceptible to toxins that are associated with certain algal bloom events. These events are characterized by an upwelling of dinoflagellates, microscopic algal protists that naturally produce toxins harmful to marine organisms. Clams that are not resistant to the toxin often die. However, toxin resistance can occur if there is a mutation in the clam's voltage gated sodium channel gene.

This case will examine the evolution of toxin resistance in clams.

photo of a soft shell clam

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Question Guide


Soft shell clams that are resistant to saxitoxin can build up residual toxin concentrations when they consume dinoflagellates. This toxin can be passed on to organisms at higher trophic levels.

Cell Biology

Saxitoxin affects the function of the neurons in soft shell clams and other organisms that might eat them. The toxin binds to and inhibits the function of voltage gated sodium channels in the axon of the nerve fiber axons.

Molecular Genetics

Some soft shell clams are resistant to saxitoxin because they have a mutated voltage gated sodium channel gene. This mutation leads to a voltage gated sodium channel protein that does not bind saxitoxin, because it has an altered extra-cellular receptor site.

Population Genetics

Resistance to saxitoxin is more common in individual soft shell clams living in native populations in waters that have a history of frequent algal bloom events.