Clams  are filter feeders they tend to feed at the bottom taking in small particles, during a red tide there is an increase in the population of dinoflagellate the causative  agent of red tide these organism possess chemical substances which are toxic especially when it reacts with acids, this organism are then filtered by clams so it stays within the clam and if the clam with the dinoflagellate is eaten by human it can cause poisoning
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It may cause died and vomiting if you eat a clams that came from a red tide 
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