Brevetoxin B is a polyketide neurotoxin produced by Karenia species and other dinoflagellates. It binds to site 5 on the alpha subunit of voltage-gated sodium channels (IC50 = 15 nM) on neurons at the neuromuscular junction, causing the channel to open irreversibly at potentials more negative than normal, discharging action potentials repetitively. Brevetoxin B is ichthyotoxic at nanomolar concentrations and is responsible for an illness described as neurotoxic shellfish poisoning.