On an Indo-Pacific beach at low tide, a pitted gray rock near the water’s edge can turn out to be a stonefish, the most venomous fish in the world. Members of the genus Synanceia are relatives of scorpionfish and lionfish that have traded flash for invisibility. Their cratered heads and mottled skin copy dead coral, a sticky secretion traps algae and sediment on their bodies, and they can survive out of water for up to 24 hours as long as they stay damp. That puts them on exposed sand where bare feet land.

This episode explains the machinery behind the sting and the science behind surviving it. The fish hunts by ambush and uses venom only for defense, delivered through 13 rigid dorsal spines whose glands are squeezed by the weight of whatever presses down. The toxin, verrucotoxin, jams calcium channels open, flooding nerves with pain signals and throwing the heart out of rhythm. Yet because it is a protein, heat unfolds it, which is why hot water is first aid and why the same animal is served as a prized dish in southern Japan, Hong Kong, Vietnam, and parts of China.

• Each dorsal spine works like a pressure activated syringe: the harder a victim steps, the deeper it goes and the more venom is forced into the wound.
• Beyond its spines, the stonefish carries lachrymal sabers, bony cheek blades it can flick outward to deter anything trying to swallow it head first.
• Deaths are rare but real, including one on Thursday Island, Australia, in 1915 and one in Okinawa, Japan, in 2010. Doctors dose the antivenom by counting puncture wounds.
• Soaking the limb in water at about 45 degrees Celsius denatures the toxin, much as heat sets an egg white, though severe stings still need medical care.
• Unlike pufferfish, stonefish flesh is not toxic, so chefs who cleanly remove the dorsal apparatus can serve it in ginger soup or even raw as sashimi.