The basic structure of a koi fish is similar to man, in that it has a skeleton, muscular system and digestive system. However several of these systems are specially adapted to survival in water.

The Koi’s external covering or skin is called the cuticle. The cuticle has a dermis, which accommodates nerves, blood vessels, pigment cells and scales. The pigment cells are very important to koi hobbyist as it determines the quality and thickness of colour on a koi. The pigment cells often mature as they age and this can be seen most often with black and red colours. The black colour is slow to rise to the surface and often the colour on younger koi looks grey. The red pigment cells often appear orange and as they mature become redder. If you look at younger koi you will notice this and remember to buy young koi that do not look finished (perfect in colour) as it will fade faster as they grow older. How the koi finally turns out in colour is determined by genetics and environmental factors. How you look after your koi by providing excellent water conditions and feeding a good high quality koi food with some colour enhancer will give you good looking koi.

Koi have various fins that are used to make the koi move under water and to stabilize the koi to control rolling. Fins are made of layers of specialized skin, supported by fin rays, which are enclosed in muscle to provide fin movement. The dorsal fin is used to stabiles the fish, the tail or caudal fin is mainly used as a source for movement. The anal fin helps to stabilize the koi when it is swimming. The pectoral fins act like brakes and help the koi with rapid turns and the pelvic fins are use for horizontal and vertical movements.
The cardiovascular system of koi is more basic than man. They have a heart that circulates blood through a system of arteries and veins via an oxygenation system (gills) to the tissues. Due to the heart being primitive, koi are vulnerable to low oxygen levels and it is very important to provide good aeration to you koi. You can provide good oxygen buy using air pumps with multiple Airstones.
Koi have no external genitalia that you can see with the eye to tell if they are male or female. The male testes and female ovaries and oviducts are within the body cavity. When koi are young it is harder to tell there sex, once they mature the female gets a rounder tummy and the male koi look long and lean as well as getting a rough feeling if you touch them during the spawning season.

Part two of this article can be found here.