Blowfish are known for their incredible defense mechanism: they can puff out into a spiky ball when threatened, as shown in the video. Some also have a spine, reinforcing their armor.
But they also have a backup system for battling predators. Some carry a neurotoxin in their organs and on their skin; that molecule can be very dangerous to other animals. The toxin is made by any of several species of symbiotic bacteria that live within the fish, like Pseudomonas, Vibrio, and others.
In Japan, some people eat the fish that harbor the bacterial poison. Chefs that prepare blowfish as part of a meal need to ensure that the liver, which harbors the toxin, is properly removed before it is served. Poisonings still happen on occasion, but the symptoms are well-known, and fatalities are rare.