It's late and you're stuck in the office. Suddenly, the power goes out and you're plunged into darkness. How do you navigate in this low-light environment? The reflex response is to stick out your hands and feel your way around. The bumpy edge of your keyboard, the smooth surface of your desk, the cool roundness of the door knob: our fingertips can convey a lot of information about our surroundings. Turns out, we may have our fish cousins to thank for this fine-tuned sense of touch. Fish may not have fingers, but they certainly aren't without feeling!
It's no surprise that fins transmit information like movement and position relative to the body (i.e. proprioception) back to the fish brain. However, Adam Hardy and colleagues from the University of Chicago, USA, wanted to see whether more detailed information about the environment could be sensed by fleshy fin membranes. The difficulty in addressing this question lies in the principal role of most fish fins, which is to get around. As touch is distinct from the proprioception experienced when swimming, the researchers needed a fish in which they could easily distinguish between these two senses. Their best bet was to find a species that might rely on a heightened sense of touch to survive, but which doesn't use all of its fins to swim. Luckily, they came across the pictus catfish, Pimelodus pictus. This bottom-dweller spends its life lurking in the murky Amazon River. The catfish possesses a serrated spine along the front edge of its pectoral, or ‘arm’, fins, which it can lock into place and use for defense. The rigidity of this spine means the catfish pectoral fins are useless for swimming. However, the rest of the fin is made up of soft rays and membranes, which it drags along the riverbed. Hardy wondered whether this combination of ridged front edge and soft rays allows the fish to feel its way around.
The team first used high-speed cameras to film live fish swimming. Once they confirmed that the pectoral fins are not used for movement, they examined the neural architecture of these fins by counting the density and types of sensory cells found in the soft tissues. Finally, they stimulated the fish's pectoral fins with a variety of different motions, speeds and pressures, and measured the activity of neurons sending information back to the brain.
Hardy and his colleagues found that catfish pectoral fins are packed with a dense network of sensory fibers, which resemble the Merkel cells responsible for touch in humans. Interestingly, these neurons transmit information about both the applied pressure and motion of the stimulus back to the fish. The team suggests that catfish pectoral fins function like fingers: by dragging their fins along the riverbed, the fish gain important sensory information about their habitat through touch.
Feeling your way around underwater is probably most helpful if you're living close to the bottom in dim environments, like catfish. The team also proposes that our sense of touch may have evolved much earlier in our evolutionary history than previously thought. In essence, fish and humans might experience similar sensations when touching slippery river rocks or soft clumps of algae. Now, how does that make you feel?