Updated: Nov 14, 2021
Have you ever wondered how your schooling fish are able to keep such a tight and precise grouping, or how species with limited vision hunt in the dark or murky waters? The answer lies in a row of pores along each flank or side of most fish called the lateral line system. These pores contain micro hair-like cells which are exposed to the water and sense micro changes in water pressure, much like the hairs in our ears react to sound waves. These changes are then relayed to the brain. This organ system is used in schooling, predation, and orientation within the water column.
Just as a prey animal's hearing can sense a predator, and the same predator uses hearing to sense the prey, the lateral line is used both by schooling prey to confuse the predator's senses and by the predator to sense the prey. The predator will attempt to sense individual prey by their movement before using its sight to make the final lunge toward its meal. However, the final lunge can be inaccurate and unsuccessful, as the predator's lateral line cannot pinpoint any individuals amongst the sensory overload that is the "noise" of the school. However, if the prey become separated, predators can again sense the movement in the water directly behind fleeing prey, allowing them to hone in once more. This is why you see marlins, swordfish and sharks swim at breakneck speeds through schools of prey. They are trying to separate enough individuals to accurately locate one.
Lateral line sensing in predators is not just for open water species like sharks and swordfish. Anyone who has kept mormyrids (elephant fish, clown, ghost knives), loaches, or other bottom feeders has probably observed their unique hunting behavior. In addition to chemoreception (smelling), these predators will use their lateral lines to sense the micromovements of prey such as small insects and crustaceans in the muddy sand beds where they live. Because mormyrids have poor eyesight, their lateral lines are highly developed and can even sense the electrical signals of their prey, allowing for accurate hunting without the aid of sharp sight.
Similar to how we balance ourselves with the aid of our inner ears, fish also balance themselves in the water column with the aid of their lateral lines. Unlike humans, fish consistently move around in three dimensions: forward, backward, side to side, and up and down. This requires a more developed organ system for balance and orientation. Next time you are relaxing in front of your aquarium, pay particular attention to the smallest movements of your fish. Watch closely and you'll notice they can precisely hold their exact position with seemingly no effort.
Every change in orientation when schooling, feeding, hunting, or just exploring, is due to the lateral line organ system. Knowing why and how the fish we keep do what they do, makes their behaviors all the more interesting. For more information on fish biology and behavior, scroll down to contact Brian at Boodleshire Aquatics.
Larsson, M. "Why do fish school?". Current Zoology. 58 (1): 116–128. 2012.
Bleckmann, Horst; Zelick, Randy. "Lateral line system of fish". Integrative Zoology. 4 (1): 13–25. 2009.