Koi are vertebrates, clearly benefiting from having an internal backbone and skeleton. Koi belong to the most advanced group of fish called teleosts which describes their bony skeleton (compared to a less rigid cartilaginous skeletons found in sharks and rays). We should not think of the skeleton as simply being a collection of bones (the type of which are notorious for getting stuck in our teeth) but their skeleton is like ours in that it is diverse in its structure from head to tail.
The head is made up of a collection of bony structures, such as the skull that protects the delicate brain tissue and the opercula which are used to generate a respiratory current and protect the delicate gill tissue. The anterior end of the skeleton behind the head is joined on to the back bone (or vertical column) which ultimately branches out into the tail where we find some of the koi’s finest bones.
Why have a skeleton?
Aquatic animals have a skeleton for similar but slightly different reasons than terrestrial animals (such as you and me). Over the first 18 years or so of our life, we invest considerable amounts of energy into building our skeleton, in the anticipation that such an investment will reap energy-saving rewards in later life. Our skeleton allows us to take on and beat gravity, where other terrestrial animals that do not invest in a back bone have to put up with a life confined to the ground.
Consequently, probably the largest terrestrial invertebrate is a giant African land snail, confined in its size and ground-hugging lifestyle through its lack of a skeleton.
A skeleton allows us to rest in a standing or sitting position (even as you read this article) without having to spend much energy counteracting the downward force of gravity. Imagine life without a skeleton. We would break into an exhaustive sweat simply by sitting down as we would need to exert energy to remain in a sitting position.
Koi have a skeleton, but not for all the same reasons as we do, as they can use the supportive properties of water to counteract gravity, by making themselves less dense than water by employing a gas-filled swim bladder. Where we do share common ground with koi is for the other functions a skeleton serves, namely protection, support and muscle attachment.
Protection. The protective influences of a koi’s skeleton ranges from its bony skull and jaw area, where the bone is very close to the body surface, through to the ribcage, buried deep in muscle, encasing the vital organs such as the intestine, the reproductive organs and swim bladder in a protective cage. Koi also benefit from a form of exoskeleton where a flexible layer of bony scales provides them with a rugged first line of defence.
Support. Even though koi do not rely on their skeletons as much as we do for support, the skeleton provides koi with a means of supporting and defining the body structure. If koi did not have a skeleton, they would be confined to a less well defined body shape, certainly without the means of locomotion and direction as afforded to them by their fins.
Muscle attachment. A rigid, yet flexible bony skeleton is the essential ingredient for providing koi (and other vertebrates) with a deliberate and accurate means of locomotion. Muscles need a site of attachment that is fixed and against which they can pull. The bulk of a koi’s bodyweight is made up of the muscle that sits either side of the spine. These muscles are able to pull against each other rhythmically to produce a rapid tail movement and body wave, generating considerable forward thrust. The bony skull is also a site of muscle attachment, allowing the jaws to open and close in conjunction with the opercula to generate a flow of oxygenated water over the gills. Similarly, fin movement ranging from erecting the dorsal fin to the manoeuvring adjustments to the paired fins are made using muscles.
Koi are classified into a group of fish called the Ostariophysi, members of which share an ingenious amplification system to enhance their sound detection. When sound waves travel through the water, they cause objects in their path to vibrate. The vibration of the bony otoliths within the inner ear is the means by which fish detect higher pitched sound waves. The Ostariophysi have additional amplification, courtesy of the first four bones of their back bone that are fused together. These form a telegraph line style connection between the gas-filled balloon-like swim bladder and the inner ears, situated within the skull. As sound waves travel through the water, they cause the gas-filled and swim bladder to vibrate, which in turn acts as an involuntary amplifier, transmitting this sound up the fused vertebrae into the skull. These fused vertebrae were first discovered by Ernst Heinrich Weber, and are still called the Weberian apparatus and today.
A koi’s skeleton starts to develop as soon as it starts life as an embryo within the egg. The skeleton in an adult koi represents a huge store of calcium and magnesium (as well as other minerals) and to get from the weak skeleton of an embryo to that of a large koi requires a considerable input of calcium and magnesium from their diet. Fortunately, koi also have an option to absorb these vital minerals from the pond water. This is why koi ponds should have an alkaline pH and a high GH.
Other environmental factors can have a detrimental affect on skeletal development, leading to irreversible skeletal deformities.
Scoliosis (a kink in the spine when the koi is viewed from above) and lordosis (a similar deformity, but evident when the koi is viewed from the side) can be caused by a range of environmental factors.
Diets deficient in vitamin C or specific amino acids when koi are experiencing rapid development and bone deposition, can result in skeletal deformities and kinks in the spine. This is also true if organophosphate treatments are used to treat parasites, but where does rates are unusually high.
Orfe and Rudd are particularly prone to these side effects, but koi require a far higher dose to have a similar effect (and hence OP-caused deformities are quite rare in koi). Organophosphates are a set of complex nerve toxins and their use can lead to the disruption of the muscle patterns, causing fish vertebrae to fracture, leaving the affected fish with an irreversible deformity.
The skeleton represents a huge investment in energy from a koi’s point of view and is required to enable koi to enjoy their specific lifestyle. It is laid down to protect vital internal organs and allow agile locomotion through the muscle attachment between the fins and the skeleton. The skeleton however, is fragile, and its development should be safeguarded through the provision of a complete diet and a stable, healthy pond environment. ealthy pond environment.