The lateral line
Issue 6 (Sep-Oct 2015) Bill Brazier
As anglers we are often guilty of over-complicating our tactics and methods when trying to catch a fish. At the same time, it is remarkable how little thought we give to the inner workings of the creatures we spend an inordinate amount of time pursuing. For example, how does a fish know what direction to face in a river current? How does it avoid being eaten by predators? How does it find food? Many will simply say it’s purely down to “instinct” but there is an awful lot more to it than that… The answer to these questions lies largely (though not entirely) with the lateral line.
All fish species on the planet – that’s a whopping 30,000+ ! – possess a lateral line, with it being visible to us on the flank of the majority of fish. This line really acts like a sense of ‘touch’ for our fishy friends. In its simplest terms, the lateral line is a water-filled canal positioned under the skin. There are openings in this canal located on top of (or as part of) the scales, which allows water to constantly pass in and out. Usually there is one canal on each side of the fish, but some species can have up to 4 or 6 on each flank. Underneath the canal there are individual sense organs called neuromasts. These neuromasts are best described as a jelly-like rod containing thousands of tiny, super-sensitive hairs that jut out from the inner skin and into the canal.
The lateral line doesn’t just run along the body of a fish though, it is also found on the head and tail, depending on the species. In many fish, neuromasts are also found outside of the lateral canals, on the head and even the tail – these are known as “free neuromasts”. Pike, for example, have visible ‘holes’ or pores on their head and chin – these are the same sense organs which are located along the lateral line. In shark and ray species, as well as most catfish, these sensory cells are located all over the body, as well as the head. In these cases the whole body of the fish acts like a giant tongue.
Wherever they are found, the tiny sensory hairs of the neuromasts are connected to sensory cells and nerves which send signals to the fish’s brain – but how does all this help a fish in its day to day life?
The lateral line system is amazingly sensitive and is stimulated by local water movements and vibrations. Primarily, neuromasts allow fish to detect their food. All living things in water generate water currents as they move and breath, and the lateral line allows fish to detect this and even the direction in which it is coming from. Even tiny water fleas (e.g. Daphnia), worms and snails move enough water for fish to detect and home in on. Stationary objects can also be detected by the change in water currents as it passes over or around them. Predators are able to track and follow moving prey by picking up associated water movements (termed the “hydrodynamic trail”), even several minutes after the prey has left the area! Perhaps this is worth bearing in mind when lure or fly fishing, especially?
Being able to detect movement and where it is coming from is what allows shoal fish to move in sync without bumping into each other and other objects. It also helps fish to ‘see’ in darkness, or in very murky water. The blind Mexican cavefish (Astyanax mexicanus), now often found in pet shops, is, as its name suggests, blind, but is able to navigate almost perfectly in its underground, cave environments without eyes – all thanks to its highly developed neuromast system.
Fish are also able to roughly judge distance using their lateral line. Brown trout, for example, pick up the frequency of the waves spreading downward from the surface as an insect drops onto it, allowing them to judge how much energy is needed to get from their resting place to a free meal. Similarly, lateral line pressure information allows fish to conserve energy by moving into areas of lower flow.
In some species (e.g. sharks and rays), some neuromasts have evolved into what are known as “electroreceptors”, able to detect minute electrical impulses generated by the muscle contractions of prey. These cells(known as ampullae of Lorenzini) can also, believe or not, detect the Earth’s electromagnetic field, and many species are thought use these electroreceptors for homing and migration.
River species always face upstream, but how do they know what upstream is? Again, the lateral line (as well as vision) helps them. In simple terms, the sensory hairs in the neuromasts face slightly one direction so water flows over them more easily from an upstream direction than a downstream one, meaning the fish can tell what direction is what based on water pressure, even in rivers and streams under low flow conditions, and even in the dark. Natural changes in lateral line function (hormonal suppression) are linked to downstream migration of many species, such as Atlantic salmon.
Functions of the lateral line
• Detecting food
• Avoiding predators
• Awareness of surroundings
• Judging distance
• Detecting flow direction
• Homing & migration
Above is a very brief, very simplified explanation of a fish’s lateral line system and its functions. In reality, one could spend an entire lifetime studying the topic and still be left with unanswered questions. Although the lateral line is involved in many key functions, it rarely acts alone and often works together with other senses, like sight. However, we hope this article has taught you a few things and given you something to think about!
Fish, as it turns out, are pretty damn complicated…
Bill Brazier