It has always been the case that big fish eat small fish, but in nature there are small fish that eat big fish, and they specialize in eating big fish such as ferocious sharks. The largest shark is more than 20 meters long, and can swallow dozens to hundreds of small fish in one bite. But it has a nemesis, the small hard-jawed poisonous fish. This fish has a short and thick body, a flat back and a round belly, and a loose outer skin. Except for the edge of the mouth and the tail, it is covered with sharp spines. After it inhales enough air, its body can swell into a ball, and the originally lying spines immediately stand straight up and become sharp spikes. When the big shark swallows the fish, the hard-jawed poisonous fish will sneak into the shark's big belly like Sun Wukong getting into the belly of Princess Iron Fan. After that, it will use all its strength, swell its body, and stab the shark's stomach with all its spines. The big shark is so painful that it rolls in the sea and can't do anything. After a short while, the shark's stomach was pierced, and then the meat on its ribs was gnawed into a bloody mess by the hard-jawed fish. When the hard-jawed poisonous fish came out, the shark was dead.

There is a kind of spiral fish in the Konai River in Greece. It moves in the water in an "S" shape like a spiral. It has a sharp and hard mouth. If a small fish encounters it, it will be spun into pieces and immediately become its delicious meal. If a big fish encounters it, it will be a bigger target and will also be spun into pieces by its hard mouth and die miserably. If a big fish swallows it, it will be even more disastrous. The spiral fish will drill and spin around in the fish's belly, eat a lot of the big fish's internal organs and kill the big fish. But the spiral fish is not invincible. It is most afraid of river clams. If its hard and sharp mouth is clamped by the shell of the river clam, even if it desperately rotates its mouth, it cannot get out and eventually becomes the food of the river clam.

There is also a small fish called hagfish in the sea near Qingdao, my country, which specializes in eating big fish. Because it has lived in the belly of a big fish for a long time, its eyes have degenerated and become blind. It looks like an eel, with a round stick-shaped front and a flat round tail at the back. It is gray-black in color, and gray-white below the belly. It is about 20 to 25 cm long. There is a small suction cup on its mouth, and sharp teeth like a file on its mouth cover. Its tongue is also strong and flexible. It first attaches itself to the big fish, then drills into the belly of the big fish through the gills of the big fish, swallows the viscera and muscles of the big fish, and excretes while eating until the big fish is eaten up. What it swallows every hour is equivalent to two and a half times its own body weight.

There is also a small fierce salmon that can eat fierce big crocodiles. This is a fish that grows in South America, and its body length is only more than 30 cm. Crocodiles can swallow a piglet, but they have to admit defeat when they encounter this fierce salmon. It turns out that the jaws of fierce salmon are so powerful that they can bite off steel fish hooks in one bite. People call it "sawtooth fish". They often travel in groups to look for food. If they encounter a large crocodile, they will rush over and bite the crocodile with their sharp teeth. No matter how strong the crocodile's skin is, it will be useless. In an instant, hundreds of salmon can eat the giant crocodile completely, leaving no bones. Therefore, wherever there are salmon, it is difficult for other fish to survive in the river.

Hagfish with its head drilled into the body of a large fish