Probable competitive antagonist of fish muscle acetylcholine receptor. Inhibits postsynaptic nicotinic acetylcholine receptors (nAChRs) from fish (zebrafish and goldfish) and frogs (IC(50)=0.1 uM). Protects these receptors from block by alpha-bungarotoxin and alpha-conotoxin EI. Does not block nAChRs at the neuromuscular junction of Rana pipiens. Shows a weak inhibition on mammalian adult and fetal muscle nAChRs (alpha-1-beta-1-delta-epsilon/CHRNA1-CHRNB1-CHRND-CHRNE and alpha-1 beta-1 gamma delta/CHRNA1-CHRNB1-CHRNG-CHRND) (IC(50)=3-45 uM). In vivo, induces paralysis in goldfish (Carassius auratus) but not mice. Expressed by the venom duct. The cysteine framework is III (CC-C-C-CC). Classified in the M-1 branch, since 1 residue stands between the fourth and the fifth cysteine residues. Does not inhibit mammalian neuronal alpha-3-beta-4/CHRNA3-CHRNB4, alpha-4-beta-2/CHRNA4-CHRNB2 and alpha-9-alpha-10/CHRNA9-CHRNA10 nAChRs. Belongs to the conotoxin M superfamily.