Stonefish are venomous marine fish classified in the genus Synanceja and the family Synancejidae, found in shallow waters of the tropical Indo-Pacific. They are sluggish, bottom-dwelling fish that live among rocks or coral and in mudflats and estuaries. Thickset fish with large heads and mouths, small eyes, and bumpy skins covered with wart-like lumps and, sometimes, fleshy flaps, they rest on the bottom, unmoving, blending almost exactly with their surroundings in form and color. They are dangerous to fish. Difficult to see, they can, when stepped on, inject quantities of venom through grooves in their dorsal-fin spines. Wounds produced by these fish are intensely painful and sometimes fatal. The family Synancejidae includes a few other species of robust, warty fish. They are also venomous, though not as notorious as the stonefish.
Piranha, also called caribe or piraya, are any of more than 60 species of razor-toothed carnivorous fish of South American rivers and lakes, with a somewhat exaggerated reputation for ferocity. In movies such as Piranha (1978), the piranha has been depicted as a ravenous indiscriminate killer. Most species, however, are scavengers or feed on plant material.
Most species of piranha never grow larger than 60 cm (2 feet) long. Colors vary from silvery to orange undersides to almost completely black. These common fishes have deep bodies, saw-edged bellies, and large, generally blunt heads with strong jaws bearing sharp, triangular teeth that meet in a scissor-like bite.
Piranhas range from northern Argentina to Colombia, but they are most diverse in the Amazon River, where 20 different species are found. The most infamous is the red-bellied piranha (Pygocentrus nattereri), with the strongest jaws and sharpest teeth of all. Especially during low water, this species, which can grow up to 50 cm (about 20 inches) in length, hunt in groups that can number more than 100. Several groups can converge in a feeding frenzy if a large animal is attacked, although this is rare. Red-bellied piranhas prefer prey that is only slightly larger than themselves or smaller. Generally, a group of red-bellied piranhas spreads out to look for prey. When located, the attacking scout signals the others. This is probably done acoustically, as piranhas have excellent hearing. Everyone in the group rushes in to take a bite and then swims away to make way for the others.
The lobetoothed piranha (P. denticulate), which is found primarily in the basin of the Orinoco River and the tributaries of the lower Amazon, and the San Francisco piranha (P. piraya), a species native to the San Francisco River in Brazil, are also dangerous to humans. Most species of piranhas, however, never kill large animals, and piranha attacks on people are rare. Although piranhas are attracted to the smell of blood, most species scavenge more than they kill. Some 12 species called wimple piranhas (genus Catoprion) survive solely on morsels nipped from the fins and scales of other fishes, which then swim free to heal completely.