### Identification The *Acestrorhynchus heterolepis*, often called the Needle-jawed Characin, is a masterclass in hydrodynamic design. Reaching up to 40 cm (16 inches), its elongated, torpedo-like body is cloaked in shimmering, iridescent silver that reflects the dappled light of the rainforest canopy. Look for two primary field marks: a distinct black humeral spot just behind the gill cover and a prominent black blotch at the base of the caudal fin. Its most defining feature is its namesake—*heterolepis* means "different scales." If you look closely, you’ll notice the scales along the lateral line are significantly larger and more robust than the tiny, velvet-like scales covering the rest of the body. Its snout is remarkably long and pointed, packed with needle-sharp, conical teeth designed for a "grip and gulp" hunting style.
### Habitat & Range This sleek predator is a specialist of the Amazon and Orinoco river basins in South America. You won’t find them in turbulent rapids; instead, they haunt the slow-moving "blackwater" tributaries and the nutrient-rich "whitewater" floodplains. They are particularly fond of *igapó* (flooded forests), where they weave through submerged root systems and overhanging vegetation at low to mid-elevations.
### Behaviour To observe a Needle-jaw in the wild is to witness a silver ghost. They are primarily solitary, diurnal ambush hunters. An observer will see them hanging motionless in the mid-water column, perfectly camouflaged by their reflective sides. They are "sprinters" rather than marathon runners; they remain deathly still until a target enters their strike zone, then vanish in a lightning-fast blur of silver.
### Diet They are strictly piscivorous (fish-eaters). Their diet consists almost entirely of smaller characins and tetras. They use their specialized, protruding jaws to snatch prey mid-body before expertly manipulating the victim to swallow it head-first, ensuring the fins fold down smoothly.
### Fascinating Fact The "different scales" that give this species its name aren't just for show! The enlarged scales along the lateral line act like a high-definition acoustic sensor, allowing the fish to detect the "heartbeat" vibrations of prey through the murky river water before they even see them.