Irukandji

A microscopic killer, the Carukia barnesi is the size of your thumbnail and has enough toxin in its' tentacles to paralyze, and in some cases kill, a person. It is the bearer of the deadly Irukandji Syndrome, to which there is no known antivenom. Only recently found offshore on the coast of Australia, the box jellyfish is only 25 mm across it's bell, and 35 mm long. They do not have eyes, but simple sensory organs to which they can see light with. C. barnesi feeds mostly on crustaceans and small fish. their tentacles are loaded with stinging cells called nematocysts which are concentrated in rings along the tentacles. These cells are like miniature harpoons and are released when it comes in contact with its' prey.
WC: 126

Tetrodotoxin

 (http://employees.csbsju.edu/hjakubowski/classes/ch331/signaltrans/pufferfish.jpg)

Tetrodotoxin is a powerful poison derived from the puffer fish which blocks conduction of nerve and muscle through a selective inhibation of sodium carrying mechanisms. It is a neurotoxin with no known antidote. It is roughly 100 times more poisonous than potassium cyanide. Originally, it was thought to be an ingredient in Haitian voodooism and would manifest zombieism in the patient, but this was later dismissed in the 1980's as the symptoms are not related.

(http://analogmedium.com/blog/2006/09/I_Walked_With_A_Zombie-2.jpg)