Koi biology consists of its external and internal anatomy. In terms of its anatomy, the basic structure of a koi fish is made up of a skeleton, muscular system and digestive system.
Koi External Anatomy
The Koi’s skin is called the cuticle. The cuticle has a dermis, in which is found nerves, blood vessels, pigment cells and scales. It is the pigment cells that determine the quality, sharpness and thickness of colour on a koi. The pigment cells in turn are determined by the koi’s genes and environmental factors. As a koi owner, you can do your part by ensuring your koi has the best pond environment to live in with high quality koi food and some color enhancer.
Koi fins are made of layers of specialized skin, supported by fin rays, enclosed in muscle tissue. These fins each have separate functions. For example, the dorsal fin is used to stabile the fish, the tail or caudal fin is mainly used to propel movement. On the other hand, the pectoral fins are the brakes and rudder. They enable the koi to stop, slow down or turn sharply. The pelvic fins are use for horizontal and vertical movements. The anal fins are for stabilizing the koi when it is swimming.
The koi’s mouth and gills make up its external respiratory organs. Koi draw water in through the mouth then it flows into the two opercular cavities, which house the gill arches and a complicated system of capillaries and blood vessels. This blood capillary system delivers oxygen to every cell of the koi’s body.
But the function of the koi’s gills is not limited to respiration. They also excrete waste products which build up in the blood e.g. ammonia and larger compounds like purines.
Koi have good sense organs that help with finding and detecting food. The koi’s mouth have the ability to extend forward like a vacuum cleaner nozzle in foraging for food at the bottom of the pond. They also have prominent barbels, whisker-like structures at the corners of their mouths to help them differentiate between food types.
Sight is very important to the koi in detecting predators and their surroundings. Koi can also see in colour.
Koi have no external genitalia so it is not easy to tell if they are male or female. The male testes and female ovaries and oviducts are within the body cavity.
Koi Internal Anatomy
The cardiovascular system of koi is rather basic. They have a heart that circulates blood through a system of arteries and veins via an oxygenation system (gills) to the tissues. The koi’s heart is quite primitive making koi vulnerable to low oxygen levels. Thus, it is very important to provide good aeration for your koi.
The digestive system of koi is rather interesting because they seem to lack a true stomach. Because of this, they are not able to store a meal. This is why you should feed Koi smaller meals two to three times per day, rather than one large meal. Koi have a very long intestine (around five times the length of its body), a gall bladder and pancreas.
In summer when the temperatures are high, koi can digest and absorb food quickly. But in winter their digestive system slows down tremendously, almost to a halt. That’s why koi reduce the amount of food they consume in winter and get by with only nibbling algae in the pond. In summer it is good to feed your koi a protein-enriched growth food whereas in winter, feed them less frequently and switch to a winter food or Wheatgerm, which is easier for the koi to digest.
The Koi’s nervous system consists of a brain, spinal cod and peripheral nerves. The autonomic part of the nervous system controls vital functions such as breathing, blood circulation and blood pressure.
The Koi’s urinary system comprises of the kidneys. Blood is filtered while excessive water and metabolic wastes form urine, which is then excreted out through a urethra in the anal pore.
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