Tetrodotoxin is a toxin that affects the nervous system. It binds to nerve cell sodium channels preventing the nerves from transferring sodium ions which prevents the normal transmission of nerve impulses through the body. This leads to rapid paralysis of the nerves controlling voluntary movement and eventually disruption of the nerve impulses that regulate the heart rate, causing tachycardia, and the loss of peripheral nerve sensation. It is extremely potent and 25mg will kill a human weighing about 165 lb. in half of cases.
Although the toxin is found in many types of poisonous sea creatures, most notably the pufferfish and the blue octopus, it is actually created by a symbiotic bacteria within those sea creatures.
The body will eventually rid itself of the toxin over time, and there being no anti-toxin, treatment focuses on supportive care.