Kailua resident Greg Turnbull sent a picture of a freshly dead, 36-inch-long fish he found on Kailua Beach during a recent dawn walk. Greg thought the fish was an ulua (jack), and I agree. Although it’s unusual for a big jack to wash up on an Oahu beach, that’s not what drove Greg to take the picture. The gaping ulua had an inflated porcupine puffer fish stuck in its throat.

The puffer fish made a fatal mistake. Swelling up in a predator’s throat is a good ploy to prevent being swallowed, but step two is crucial. The puffer must shrink and run before the suffocated fish washes ashore.

People call the 120 or so species of puffer fish several names — balloon fish, blowfish, globefish, toadfish — all referring to their famous defense: an elastic stomach. When threatened, a puffer engorges its stretchy stomach with water or, if a predator drives the puffer to the surface, air.

The idea is to grow too big for a predator’s mouth. If the pursuer has a really big mouth, the puffer sticks in the fish’s throat. Because fish breathe by taking water in the mouth and expelling it out the gills, a throat full of puffer fish is deadly.

Another deadly puffer defense is the ability to store a poison called tetrodotoxin. In the ocean, puffers eat plants and animals that contain naturally occurring bacteria that manufacture tetrodotoxin. By choosing food that doesn’t contain those bacteria, aquaculturists can raise poison-free puffer fish.

Tetrodotoxin is well known for being so potent that a tiny dose can kill a person in minutes. The poison blocks sodium channels in nerves, and when sodium can’t enter a nerve cell, it can’t tell muscles what to do, such as breathe. There’s no antidote to the toxin, but, because it wears off by itself, artificial breathing can save a victim.

So, I wondered, are jacks immune to tetrodotoxin?

Hard to say. Puffer fish have been found in tiger shark stomachs, and the Internet has pictures of a snapper, frogfish, loggerhead turtle and duck eating a puffer fish, but it’s unstated whether the animals lived or died. What I did learn, however, is that the sodium channels in some snails, crabs and presumably tiger sharks are unaffected by tetrodotoxin.

I also learned in my reading that 19th-century warriors from the Gilbert Islands (today Kiribati) wore what a British explorer in 1847 called “an extraordinary looking apology for a helmet.” The illustration shows a porcupine puffer fish skin on the head of a warrior. Apparently the man mistook a fish for a hat.

Thanks, Greg, for sharing your picture. It’s great food for thought.

Reach Susan Scott at susanscott.net.