Seafood lovers got a glimpse of that threat in 2015, when record high ocean temperatures and lingering toxic algae blooms raised the domoic acid in shellfish to unsafe levels, shutting down the West Coast Dungeness crab fishery from Alaska to Southern California for several months. Though less dramatic, the problem emerged again this season, when harvesting was again delayed for portions of the coasts.
Domoic acid is a toxin produced by Pseudo-nitzschia, a micro algae which can accumulate in species like Dungeness crab, clams, mussels and anchovy. It can be harmful to both humans and wildlife, including sea lions and birds. Remember the famous Alfred Hitchcock movie, The Birds? It was inspired by a real-life incident of California seabirds driven into a frenzy by the neurotoxin.
Bioaccumulation at work brought on by the mechanics of the food chain. When one organism eats another, about 90% of the energy involved in the transaction is lost. That means that the higher up the food chain you are, the more you have to proportionally eat in order to supply your energy demands (this is why it's greener to be a vegetarian than a regular meat-eater, you're getting 10% of the energy from the veg directly instead of 10% of the energy from the cow that was already getting just 10% of the energy from its feed. Basically, 10% vs. 1% energy gained). Because you're consuming more, you're exposing yourself to more of these toxins that can accumulate in the body, so you wind up with an overall higher concentration than organisms below you in the chain. Same reason people worry about mercury in fish.