This toxin increases the excitability of nerves by delaying the inactivation of the voltage-gated sodium channel (Nav). Only acts on some crustacean. Is more abundant, but 15-fold less toxic than neurotoxin B-II. Has the structural arrangement of two alpha-helices stabilized by disulfide bonds (CSalpha/alpha 4(S-S)). Belongs to the worm B-toxin family.