<\/a>The lateral line on a Dolly Varden char houses sensory cells (Courtesy Photo \/ Bob Armstrong)<\/p><\/div>\n\t\t\t\t
Fishes (and some amphibians) have lateral lines containing sensitive structures allowing the fish to ‘read’ water currents. As a fish swims, it displaces water that flows back along the body. The lateral line is a row of pits containing sensory cells that detect pressure and distortions of that flow, created by an obstacle or an approaching predator. The lateral line also allows schooling fish keep track of each other and to coordinate their movements.<\/p>\n\t\t\t\t
We see shorebirds foraging out on the intertidal zone, poking their bills into the sand and mud. The tips of their stiff little bills are loaded with touch receptors that can sense the movement of water around pebbles, sand grains, and little clams or worms. As the birds probe down into the sediments, their bills set up tiny currents of water that flow around objects that distort the currents—distortions that can be detected by the bill-tip sensors. (Then the bird has to discriminate pebbles from food…)<\/p>\n\t\t\t\t
Mice and rats (and opossums) sweep the spaces ahead of themselves with long, mobile whiskers, each whisker registering contacts in a different part of the brain, so the critter can form a sort of map of what’s in front of it. But cats and dogs don’t whisk their mobile whiskers back and forth like that (Why?).<\/p>\n\t\t\t\t
Harbor seals have a face full of whiskers, which the seals use to follow hydrodynamic wakes left by prey fish; even a blind seal can track a fish this way, discriminating victims by size and shape and direction of movement. And here is the fascinating part of this story: As a seal swims, its whiskers create their own little turbulences, which might get in the way of detecting wakes produced by prey. But it turns out that seal whiskers have a special shape: they are elliptical (not round) in cross-section and variable in thickness and that somehow reduces the whisker’s own turbulence, allowing better reception of prey wakes. These modified whiskers are not found in walruses and seals that are bottom-feeders nor in sea lions that are reported to rely chiefly on vision for hunting.<\/p>\n\t\t\t\t
Crocodiles and alligators have little pressure-sensitive bumps along the edges of their jaws. These predators often lurk quietly in the water, with the snout at the water’s surface. Those bumps can detect surface vibrations created when an animal falls or walks into the water (and can even note the disturbance made by a drop of water falling onto the surface). Then the predator responds quickly!<\/p>\n\t\t\t\t