Adorable and Deadly: Puffer Fish Paralysis

Puffer fish, of the family Tetraodontidae, inhabit tropical and subtropical oceans, with some species living in brackish or fresh water. They range in length from one inch to two feet. All species are coated in pointed spines and have beak-like mouths made of four teeth fused together.

The puffer fish gained its name from its practice of inflating its stomach with water when threatened. The fish can triple its body volume through this process. The puffer’s stomach is highly elastic and has lost its digestive function. This defense mechanism prevents the puffer’s predators from swallowing it. In addition, when inflated, the fish’s spines, which lie flat when the fish is unthreatened, protrude outward, making the puffer even less palatable.

Check out this video of the puffer inflating (ignore the first 20 seconds):


However, the real danger of puffer fish is not their sharp spines. A toxin called tetrodotoxin is prevalent in puffer fish, most concentrated in the liver, gonads, and skin. To humans, tetradotoxin is lethal, 1,200 times more toxic than cyanide. Each puffer has enough poison to kill thirty men; there is no known antidote .

Tetrodotoxin is a neurotoxin that blocks the diffusion of sodium through sodium channels, preventing depolarization and discontinuing nerve cell function, causing paralysis. The toxin does not target the nerve cells that control the heart, so rather than dying of a heart attack, the victim slowly suffocates. Victims who survive the first twenty-four hours after ingestion usually recover with no residual effects, but recovery takes days. Symptoms begin fifteen minutes to several hours after ingestion. Death can occur within four to six hours of the initiation of symptoms.

Humans are exposed to tetrodotoxin when they ingest the meat of the puffer fish. In Japan, puffer fish meat is a considered a delicacy. Called fugu, the meat is incredibly difficult for chefs to prepare because one mistake could spell death for a customer.

Scientists believe the puffer fish’s toxicity stems from association with tetrodotoxin-producing dinoflagellates. Through a process called bioamplification, the toxin travels up the food chain in ever increasing concentrations from the plankton to worms and shrimp and later up to the puffer fish. This theory is supported by the fact that farm-raised puffer fish do not contain any tetrodotoxin. Moreover, when the puffer fish were fed small concentrations of tetrodotoxin, it began to concentrate in their livers, while other toxins were not absorbed.
This process of bioamplification is very similar to the path of mercury up the food chain resulting in toxic levels of the metal in apex predators such as tuna and dolphins.

Thus the paralyzing neurotoxin in puffer fish results from their diet rather than chemical processes within the fish. However, because puffer fish target tetrodotoxin specifically and amasses the toxin within their internal organs, this process can be defined as a defense mechanism.