Fig. 1 Image of a Guineafowl puffer, Pacific Ocean, when fully grown they can reach a length of 50 cm. 
Puffer fish famously contain a poison, the name of which, comes from a particular species- Tetraodontidae. Tetrodotoxin (TTX) is the neurotoxin which can be fatal if ingested. It’s found in a variety of marine creatures including several frog species, tropical fish and octopus.
On the 7th September 1774, Captain James Cook and his crew purchased a tropical fish from a local fisherman whilst on their second voyage of discovery. After tasting the fish, the crew members had trouble breathing and experienced numbness in their arms and legs. 
TTX has the structure below (Fig. 2), the rings containing Oxygen and Nitrogen atoms are what give this molecule a venomous characteristic. As well as occurring naturally in wildlife the poison can be synthesised in the lab in a number of ways. 
Fig. 2 The structure of tetrodotoxin.
The venom works by blocking channels in nerve cells, preventing electrical signals reaching the brain. When the diaphragm and intercostal muscles cannot move, the chest cavity will not enlarge and the lungs do not inflate. It’s easy to see why TTX is so life-threatening.
TTX is around 10,000x more poisonous than potassium cyanide. About 25 mg of TTX will kill the average adult upon ingestion (that’s less than half a teaspoon). Via injection, the required amount is as little as 0.6 mg. 
Symptoms typically occur between 10 and 45 minutes after ingestion and include:
Unfortunately, there is no antidote. A victim can only be sometimes be saved with artificial machine assisted respiration, pumping of the stomach and doses of dopamine to boost heart rate and blood pressure. 
Puffer fish is served to eat in Japan. The dish is known as ‘Fugu’ and a typical serving costs around £25.
Fig. 3 Fugu Sashimi 
Chefs are trained over several years in how to safely remove toxic organs such as the liver before gaining a license. Heating, freezing and marinating have no effect on the poisonous aspect of the meal. Around 100 Japanese people die each year from TTX poisoning.
Why do people eat it then?
Well apparently the potential of death isn’t enough of a downside to a novelty dinner and the way traces of TTX create a tingling sensation on the tongue. 
The Chemistry of TTX makes it poisonous, when it binds to channels between nerves and the brain it essentially blocks the ‘keep breathing’ message as Captain Cook and his crew found out.
Coming into contact with TTX is life-threatening. If you’re brave enough to sample a dish of Fugu I’d double-check the chef’s credentials.