Dear Biologist,
Do you know which toxin was used in CIA's poisonous dart gun? I have been searching for the answer for a long time now and my patience is wearing thin. The only clue I have is that it's a shellfish toxin, but which one?It should be able to cause a heart attack for all I know. I'm looking forward to hear from you.

assuming the answer is not readily available then you can presumably pick any toxin you want that fits the description. How would anyone be able to prove you wrong!