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The Sixth (or seventh or eighth) Sense: the lateral line
If you ever showed a fish the movie The Sixth Sense, it would be mightily confused, and not just because (as far as we know) fish don’t speak English. Fish would be unimpressed at the notion of a mere six senses, because fish already have more than six senses, many of which are unique ways of experiencing the environment that humans do not possess. Here I wanted to talk about an extremely common, important, and easily visible fish sense that humans lack: the lateral line!
To the right we see an example of the lateral line in one of Fantastic Fish’s favorite fish, Cory catfish, but the truth is that we could have picked virtually ANY fish because all fish have this unique structure. Below is a close-up of a section of a goldfish’s lateral line:
Those curious little divots give all fishes a sense for which humans have little context: the ability to detect the behavior of the water around them. The closest parallel that we have is our sense of touch- humans can feel some air movement, like wind blowing, but the lateral line blows our sense of touch out of the water. The lateral line is sort of like our sense of touch injected with Captain America serum, and the information it gives fish is invaluable. For example, you might have wondered how gigantic schools of fish in the ocean manage to move together so gracefully. The answer: the lateral line! Each individual can sense the minute water movements caused by the other fish around them- it can sense the direction and speed of its neighbors’ movements, as well as minute changes in those movements. Lateral lines let a fish feel the tiny eddies and swirls of water from other fish moving nearby, and they let individual fish in gigantic schools coordinate their own swimming accordingly. As these schooling fishes use their lateral lines to escape from predators, predator fish can use their lateral line to figure out whether prey is near and how it’s swimming- lateral lines are a HUGE deal for all kinds of fish trying to do all kinds of things. It’s a fish eat fish world out there a lot of the time, and lateral lines play a role in a great many fishy affairs.
Of course, the way lateral lines work is also hugely fascinating, and has parallels in a completely different human sense: hearing! Human hearing and fish lateral lines both require the same type of specialized cell, confusingly called a hair cell.
Here you can see highlighted the main lateral line.
These specialized sensory cells sit under a fish’s scales, but they have nothing to do with hair (or scales!); they are mechanosensitive, which means they sense the movement and vibrations of the substances they’re wired into. In humans, hair cells sense the mechanical vibrations of the fluid in the ear to create the sensation of sound (fish ears also have hair cells, but some fish also involve their swim bladders in hearing); in the lateral line organ, hair cells sense the mechanical vibrations and movements of the water in the lateral line canal. So better watch out when talking about human hair – which is somewhat similar to fish scales – and hair cells – which are used for hearing in humans and detecting water movements in fish and sit under the scales (not the hair!) – and hearing – which humans do with ears and fish do with swim bladders, but also with ears in which (of course) hair cells are found… it is all very confusing!
What all of this means is that fish can sense changes in the movement and dynamics of the water around them. The lateral line has clusters of hair cells in a canal with a special fluid that allows in water from the environment- the little divots visible in the midline of each fish’s flanks are the pores that allow water into this special canal. When the behavior of the water (such as pressure) is different between pores, this causes the fluid in this canal to flow, which is what the hair cells sense. This gives the fish essential information about the way the water around it is moving.
While all fish have lateral lines, they are particularly crucial to certain fishes. Blind cavefish, for example, can map out their environment by swimming around and collecting information about what the water around them is bouncing off of; the movement of the fish itself causes the water to move, which collides with objects and tell the fish what’s around it. In this way, the fish learns the shape of its surroundings and the location of objects in its vicinity. Cavefish can form entire mental maps this way, and they also rely almost entirely on their lateral lines to catch prey in the complete darkness they live in (don’t worry, they don’t have eyes and therefore don’t know to be scared of the dark). In that sense, the lateral line sense is similar to the echolocation sense. So blind cave fish orientate in ways not unlike bats (both bats and fish rarely bump into things, precisely because they have echolocation and lateral line organs). But they are not bat fish, those are a whole different beast. Again. A lot of potential for confusion!
Look ma, no eyes!
Lateral lines are a uniquely fishy sense that transforms the way fish experience and respond to their environment- I think they’re part of the reason it’s so hard to imagine being a fish. So, if we were to do The Sixth Sense for fish, Bruce Willis would also have a lateral line (necessitating the film being named The Seventh Sense). But he would also have electroreception (many fish do! So now it is The Eighth Sense). And magnetoreception (again, many fish do! We are at The Ninth Sense by now). Not sure that title would have drawn large crowds. Also not sure Bruce Willis’ acting is good enough to pull off nine senses. Took him almost the entire movie to get to grasps with six, I’d say.
Further reading:
1) A great article that goes into further depth of the concepts above: https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1749-4877.2008.00131.x
2) The use of lateral lines by cavefish to navigate and catch prey: https://www.sciencedirect.com/science/article/pii/S0012160618301192
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