Fish Feel With Their Fins

Fish fins are touch sensors, and they work in a way that's similar to our fingertips. According to new findings published in Proceedings of the Royal Society B, the pectoral fins located behind the gills of one catfish species have neurons and cell structures that can sense light pressure and motion.

The paired pectoral fins of fish (imagine Nemo's lucky fin) are homologous to our arms and the forelimbs of other four-legged animals. They help propel and balance the fish as they swim, and they also help flying fish fly and mudskippers crawl. Seems pretty obvious that fish would use their fins for feeling around, but little is actually known about the sensory roles of fins. Bottom-dwelling fishes in particular often come into contact with the substrate, and feedback from the floor, objects, and movement of the water could modulate behaviors related to their locomotion, orientation, and finding food. It would be especially helpful for those with nocturnal lifestyles and those living in low-visibility environments or the deep sea.

To investigate, University of Chicago's Adam Hardy and colleagues measured the activity of nerve fibers in the pectoral fins of the catfish Pimelodus pictus, a small fish that lives at the muddy bottom of the Amazon River. Their pectoral fins contain a hardened, serrated spine along the edges for protection, and because their fins don't generate a propulsive force, they're likely not used for locomotion. While it seems their fins serve a defensive role, they still retain the soft, bony rays connected by a membrane that's typical of all ray-finned fishes - likely as touch sensors.

Using the blunt head of a pin and the eye of a needle, the researchers applied a variety of different stimuli to these highly innervated pectoral fins, including a light surface brushing. The neurons responded to touch and relayed information about pressure and motion to the brain. And the ray fin nerves also respond to bending of the rays.

The researchers also discovered the presence of cells that resemble what's called Merkel cells - which are associated with nerve endings in mammal skin and are essential for touch sensation. These findings, Hardy explains in a statement, suggest that the underlying sensory morphology may be evolutionarily conserved.