Aquatic snipers of the mangroves
In the brackish waters of Southeast Asia, the archerfish (family Toxotidae) employs a hunting technique that involves complex physics. This fish spits a precise jet of water to knock insects and other small animals from overhanging vegetation into the water. To do this, it must account for the bending of light, or refraction, as it passes from water to air, which distorts the apparent position of the target. The fish positions itself, rotates its eyes to place the target's image on a specific part of its retina, and then fires.
The mechanism for creating this jet is a specialized anatomical feature. The fish presses its tongue against a groove in the roof of its mouth to form a channel, similar to the barrel of a gun. By rapidly compressing its gill covers, it forces water through this channel. The jet can strike prey up to 3 meters above the water's surface. Studies using high-speed video show the initial velocity of the water is around 2 meters per second. The fish modulates the jet's velocity during the spit, firing the water at the end of the stream faster than the water at the beginning. This causes the water to accumulate into a single, massive globule just before impact, delivering a powerful blow to the target. This external power amplification results in an impact six times more powerful than the fish's muscles could generate on their own.
A brain without a cortex
The archerfish's ability is not instinctual; it is a learned skill refined through trial and error. Younger archerfish are less accurate and must practice to correctly account for variables like target distance, angle, and even gravity's effect on the water's parabolic arc. Laboratory studies show they can adapt their aim to new challenges, like a persistent airflow introduced to deflect their water jet. This motor adaptation was previously associated with more complex vertebrate nervous systems.
This learning capacity is important because fish lack a cerebral cortex, the part of the mammalian brain associated with high-level functions like reasoning and thought. Despite this, archerfish demonstrate cognitive abilities that challenge previous assumptions. They can be trained to recognize and shoot at specific images on a screen, and can even distinguish between different human faces in as many as 44 distinct pictures. This shows that complex visual processing and learning can evolve through different neural pathways. The fish's capacity for solving these ballistic puzzles provides a model for understanding visuomotor control in a simpler nervous system.