There may be a chance that last year’s hit movie, starring Bruce Willis was inspired the extraordinary sensory physiology that is shared by all fish (including koi). That is, their lateral line, often referred to as their sixth sense.
A koi’s sixth sense capitalises on the physical properties of its aquatic environment and works in partnership with two other key senses, (hearing and sight), to detect, locate and identify features of their dynamic environment.
Where possible, a koi’s senses should not be discussed independently of the other 5, as this can oversimplify the way koi sense their environment and overlook how each sense is a fully integrated member of a koi’s means of detection. Yet time and space only allows these 3 senses to be described and understood in a way that is both for the practical use and greater appreciation of us who keep and interact with our koi. It is logical to choose these 3 senses (eyes, ears and lateral line) because as we will see later, they work in close association with each other.
The reason why fish have two methods of gathering information about the ‘sound’ that is being made below the surface is because their environment can produce and transmit two different types of sound. Koi are no different from other fish in that they utilise an acoustico-lateralis system (which consists of the inner ears and the lateral line) to detect the two different types of sound.
Because water is 14,000 times less compressible than air, sound consequently travels 5 times faster in water as opposed to air (1500m/s vs 300 m/s).
When an air stone is dropped into a pond, koi will detect this in two ways using their acoustico-lateralis system as the action of the air-stone hitting the water produces two different types of sound effect.
1). Particle Displacement.
Picture your pond as being a mass of billions of relatively tightly packed water ‘particles’. Because there is little space between each particle (compared with the very diffuse air), if physical energy is released onto the water particles at one end of the pond, the ‘message’ will soon reach the other end by virtue of the fact that these particles bombard their neighbour in a chain reaction. A good way of picturing it is to imagine the water behaving like an executive toy that consists of ball bearings strung on a cradle.
As you pull back and release 1, the ‘message’ of 1 ball bearing is sent through to the end one. This effect, known as particle displacement, has a greater effect closer to the air stone, soon fading with distance and is detected by koi using their lateral line system. In a way, the lateral line could be thought of as a koi’s ears for particle displacement.
How it works:- A koi’s lateral line runs the length of its body on either flank. Although it is referred to as a line, it actually consists of a row of perforated scales which when viewed from a distance, appear to form a line (in the same way as a dot to dot drawing). Each hole through a scale indicates the position of a pore which connects the water in the pond with the nervous system located within the lateral line canal beneath the koi’s skin.
Situated within the canal are highly sensitive cells called neuromasts which flex and bend in response to particle displacement. This allows koi to detect both the direction and the force of water movement. Because these delicate nerve cells are so sensitive, they need to be protected from potential hazards that may come into contact with them in pond water and are retained in a gelatinous coating. Koi combine the information they gain from their lateral line with another type of sound that is detected by their inner ear.
2). The Inner Ear.
The inner ear detects higher frequency sound (up to 8000Hz) that is transmitted through the water via ‘sound waves’. Such sound waves travel further distances compared to particle displacement. Fish, of course, have no external opening to their ear, and sound is relayed from the dense water through fish tissues into their ear. If you have ever snorkelled or spent time diving underwater, you would have noticed this effect. I remember being underwater near a coral reef and being startled by the high pitched whirring sound of a propeller from a speed boat. When I got to the surface for my safety, I was surprised to see the boat on the distant horizon (and yet it sounded very close), showing how well higher frequency sound is transmitted through water. I have often wondered what overall effect the constant ‘humming’ or ‘whirring’ noise created by an underwater must have on a koi, when it’s physiology is used to a dead quiet environment of a still or slow moving lowland lake.
How it works:
The inner ear is also equipped with sensory hair cells (neuromasts) that are situated adjacent to bony structures called otoliths. These calcified structures increase in size each year (in the same way as scales), and can also be used to age fish. When sound waves are transmitted through the fish’s head to the denser bony otoliths, they vibrate, and in doing so trigger the neuromasts which in turn is interpreted by the brain as ‘sound’.
Adjacent to the otoliths in the inner ear are 3 bony semi-circular canals that are fluid filled and also lined with nerve cells. Each semi-circular canal relates to one dimension in which koi can move (up and down, left and right, backwards and forwards), and they are used by koi to orientate themselves. I.e. The semi-circular canals in the inner ear confirm whether a koi is swimming the right way up.
Koi hearing is also enhanced by their own in-built amplifier. The first few vertebrae of the back bone link the bony structure of the inner ear to the dual-chambered swim bladder. Being gas-filled, the swimbladder vibrates and amplifies any sound waves it receives, transmitting them through the vertebrae to the inner ear, increasing the chances of it hearing over even further distances.
Koi possess monocular vision in that each eye works independently of the other to produce 2 images. In doing so, they can virtually cover 360o of vision, – something that is very important for a scavenging omnivore, keeping an eye out for predators. (You will notice, that predatory fish will generally have binocular vision, where both eyes are used to focus on the same object, giving a better perception of distance for an effective kill).
Koi are able to use their sight far more effectively in a well filtered, crystal clear koi pond compared to their natural lowland water environment that would typically be very turbid as a result of a combination of suspended clay, silt and algae. Compared to their sense of hearing that is able to detect sound over large distances, a koi’s vision is more limited. Even in relatively clear ponds, the density of water reduces the resolution and consequently sharp focus and it is consequently felt that koi are able only to detect light/dark contrasts and movement.
Koi use their sight to good effect at feeding time, locating floating pellets, but even this is not an accurate exercise because of their monocular view, making them clumsy and imprecise feeders. – Something we can all testify to.
In summary, these 3 senses work in collaboration, where the sounds produced by a new air stone being dropped into a pond are detected using their acoustico-lateralis system. Having had their interest aroused and not being too spooked, koi will swim to the direction of the original sound source to confirm the identity of the new object by sight.l sound source to confirm the identity of the new object by sight.