by Scott Moeller
So there I was, once again, paralyzed with indecision in the produce aisle of my local supermarket. I must have been there for ten minutes, repeatedly thumping and sniffing every watermelon in the bin. I know there’s no official time limit on melon selection, but I couldn’t help glancing nervously at the stockboy, and wondering when he might decide to assert his limited authority and kick me out. I imagined him snapping at me, confiscating my cart, and roughly escorting me from the building with my arm twisted behind my back. (He was only a high school kid with no facial hair, but I figure watching people steal grapes all day has gotta create a lot of pent-up rage).
Instead, he moved on to another task, and left me with my conundrum. How do you select a quality melon when the senses we have just aren’t adequate? “My kingdom for a sixth, melon-selecting sense!” I muttered dramatically under my breath, causing a nearby mother to keep her two children closer to her than she otherwise might.
After my produce was sufficiently manhandled in the checkout lane, crammed safely into the bottoms of my shopping bags, and heaved into my trunk, I began to think more about this idea. On the drive home, I wondered, “What would it be like to have a sixth sense?”
Fish don’t have to wonder about this. Fish actually do have a sixth sense. And, it’s not something that they only use occasionally like my mythical melon-sense or my treadmill. Fish use their sixth sense constantly.
Of course, we should start by mentioning that humans and other animals actually have lots and lots of different senses, but we typically lump them all into the five major sensory organs, and refer to them loosely as “the five senses.” You probably can recite them with just a bit of thought: Vision, Hearing, Smelling, Tasting, and Touch.
Fish, of course, have all of these plus a sixth: Vibration-sense. Fish can sense tiny vibrations and pressure differences in their environment, and they do this with a sensory organ known as the Lateral Line. If you’ve ever caught or seen a live fish, you’ve probably seen this lateral line. It is displayed as, well, a lateral line running along the side of the fish from the gills to the tail.
The lateral line is actually made up of a series of many individual sensory structures called neuromasts. Each neuromast consists of nerves attached to tiny sensory hairs which are encased in a gelatinous capsule. When even the slightest change in water pressure occurs, the hairs move inside the capsule and transmit the pressure change information to the brain of the fish.
With this information, a fish can determine the precise location and activity of another fish, animal, or object in the area, even without seeing or hearing it. It would be roughly analogous to sensing when another person enters the room simply by perceiving the tiny changes in the air pressure caused by the movement of their body.
Fish also use information obtained with the lateral line to track their own movements. The forward swimming of a fish causes what is called a bow wave which moves out in front of the fish, and is then perceived with the lateral line as the fish swims forward. When a fish swims toward an object, these pressure waves become distorted, and the fish is able to sense the oncoming object without the need of sight or sound.
Much like a bat, which has eyes but does not need them thanks to echolocation, most fish can navigate using only the tiny pressure differences they perceive with their lateral line. The most impressive example of this, with which most of us are familiar, is the remarkable ability of hundreds of schooling fish to move together almost as one large super-organism. Each fish knows the position of the other fish and can mirror the sudden movements of its neighbor, by monitoring the very tiny pressure differences registered by the lateral line.
Not surprisingly, the lateral line is found not just in fish, but also in many aquatic amphibians, as well as some crustaceans and cephalopods. Additionally, there are many different versions of the organs of the lateral line. For example, sharks have similar sensory organs in their nose that, instead of sensing pressure changes, can detect electromagnetic radiation. Even large freshwater predatory fish such as northern pike have additional pores on the head to give them an extra edge in sensing the precise movements of prey animals such as smaller fish.
You can learn more about the lateral line and the other five senses of a fish, by checking out Lesson 2:1 Fish Sense in the Fishing: Get in the Habitat Leader’s Guide.
In the meantime, I’ll keep dreaming of additional senses that I’d like to have. Come to think of it, in addition to a sixth melon-sense, I might also like a seventh sense that allows me to know how much gas is left in my lawnmower without having to look. Ooh, and also an eight sense when I’m in the shower that tells me when one of my kids is about the flush the toilet. And maybe a ninth sense……