Anatomy of koi carp: gills, lateral line, swim bladder, and warning signs

One koi stays stuck at the surface, another swims at an angle, and a third has raised scales: these signs mean nothing until you know which organ they point to. Knowing the anatomy of koi carp means learning to connect behavior observed at the water's edge with the organ that is suffering, and then with the cause, very often water quality. Here are the organs that really matter to pond owners and what they help you understand.

Why knowing anatomy helps with treatment

At the edge of a pond, you never see a diseased organ. You see one koi staying at the surface, another rubbing against the bottom, and a third swimming upside down. The task is to work back from the behavior to the organ, then from the organ to the cause, which is almost always water-related. A veterinarian specializing in fish proceeds in exactly this way: observation, a hypothesis about the affected organ, and verification through tests and samples.

This guide is not intended to be a biology course. It reviews the organs that explain the vast majority of visible signs in a pond and ends with a “symptom → organ → action” table that you can keep close at hand. One rule runs throughout the text: in koi carp, almost every vital organ is in direct or indirect contact with the water, so a water problem very quickly becomes a health problem.

A fish does not “fall” ill in the same way as a mammal. Its gills, skin, and intestines are constantly exchanging with the environment. When ammonia rises, oxygen runs low, or pH collapses, these surfaces take the brunt first. Opportunistic germs and parasites then take advantage of the breach.

Skin, mucus, and scales: the first barrier

Contrary to a common belief, scales are not the fish's outer layer. They form in the dermis and remain covered by a living, very thin epidermis, itself coated with mucus. This mucus is continuously secreted by specialized cells in the epidermis. It contains enzymes, antibodies, and antimicrobial peptides that inhibit the establishment of bacteria, fungi, and parasites.

Three things to remember for everyday observation:

• Normal mucus is not visible. The koi appears shiny, without a film. A grayish or bluish film, especially on the back, signals irritation: parasites, acidic or overly polluted water, or recent handling.
• A torn-off scale leaves an opening for infection. The area is covered with cells within a few days if the water is good. In water high in organic matter, it can become infected and develop into an ulcer.
• Scales that lift up like a “pine cone” are not a skin problem: the dermis swells because the fish is retaining water. This sign points to the kidneys and overall condition (see below).

The most telling behavior is rubbing, often called “flashing”: the koi briefly lies on its side and rubs against the bottom or a wall, revealing a white flash. An isolated instance of rubbing is not a cause for concern. Repeated rubbing by several fish, several times an hour, points to external parasites or chemical irritation. Before considering treatment, test the water: a nitrite spike causes exactly the same behavior.

Mucus is also why you should avoid handling koi with dry hands or a rough net. Each capture removes part of the protective layer. When a fish must be removed, an observation basin and a fine-mesh net limit the damage.

Gills: respiration and ammonia excretion

Under each gill cover are four gill arches. Each arch bears a double row of filaments, and each filament is covered with microscopic lamellae where blood flows counter to the water. This arrangement makes it possible to extract a large proportion of the dissolved oxygen, but it comes at a cost: the barrier between the blood and the water is only a few microns thick.

Gills are not used only for breathing. They also handle most nitrogen excretion, in the form of ammonia, and part of salt regulation. A freshwater fish constantly loses ions to its surroundings and actively takes them back up through its gills. This makes it clear why ammonia-laden water is doubly toxic: it prevents the fish from excreting its own ammonia and directly damages the gill filaments.

What to look for:

• Gill-cover movement rate. At rest, in cool water, the gill covers move calmly. Rapid, pronounced movement, especially if several koi gather near the waterfall or water inlet, indicates a lack of oxygen or damaged gills.
• The colour of the gills. During an examination in a tub, you can sometimes gently lift the gill cover. Bright red gills are normal. Pale gills, mottled with white or covered in mucus, indicate anaemia, necrosis or a heavy parasite load. This examination requires experience: at the slightest doubt, leave it to the veterinarian.
• Only one gill cover is moving. A koi that ventilates on only one side often has an injury or a parasite localized on the other side.

Temperature plays a direct role. The warmer the water, the less oxygen it dissolves, and the more oxygen koi metabolism requires. Summer stormy nights, when algae consume oxygen without producing any, are the riskiest times. Independent aeration for the waterfall, using an air pump and diffusers, remains the best safeguard. We explain this choice in detail in our guide to choosing an aerator for koi carp.

The lateral line and the fish's senses

On each side of the body, a row of perforated scales forms a more or less visible line: the lateral line. Beneath these pores, sensory cells detect water movements and low-frequency pressure variations. Thanks to it, a school of koi can turn as one, a fish can avoid an obstacle in murky water, or sense a heron entering the water.

Carp also have an unusual system that connects the swim bladder to the inner ear through a chain of small ossicles. The bladder acts as a resonating chamber and amplifies sounds: cyprinids hear better than many other fish. The vibrations from your footsteps or the noise of the food container are enough to bring the group over.

Taste is not limited to the mouth. Koi have two pairs of barbels at the corners of their lips, covered with taste buds, and also have them on their lips. They therefore search through the bottom for food, which also explains why they turn over plant pots.

What does this have to do with health? A koi that no longer reacts to familiar stimuli, stays apart, and no longer comes to feed is showing general malaise, not a sensory disorder. Loss of appetite is one of the earliest and most reliable warning signs. Note it, especially if it affects a fish that is usually greedy.

Swim bladder and swimming disorders

The swim bladder is a gas-filled sac that allows the fish to float at different depths without effort. In koi, it has two chambers, an anterior and a posterior one, and remains connected to the esophagus by a small duct. The fish can therefore swallow air at the surface to fill it and release air from it.

When this organ functions poorly, the sign is dramatic: a koi leaning on its side, head down, tail up, a fish that sinks as soon as it stops swimming, or floats belly-up without being able to descend. The causes vary:

• a bacterial infection of the swim bladder itself;
• compression by a neighboring organ, such as highly developed ovaries, a tumor, or a distended intestine;
• a sudden drop in water temperature, which slows down the entire metabolism;
• a malformation, common in some varieties with very round bodies, particularly « butterfly » koi or very stocky specimens.

The prognosis varies widely. Some temporary disorders disappear with slightly warmer water, increased aeration, and a period of fasting. Others, caused by an injury or malformation, cannot be corrected. One simple measure often helps: isolate the fish in a shallow, well-oxygenated quarantine tank so it does not have to struggle against the water column. An accurate diagnosis (X-ray, ultrasound) should be made by a veterinarian.

Digestive system: why koi have no stomach

This is undoubtedly the most useful anatomical feature for an owner to know. Koi carp have no stomach. The esophagus opens directly into a long, coiled intestine, significantly longer than the fish's body. There is therefore neither a storage pouch nor any prior acid digestion. Swallowed food moves continuously and must be digested as it goes.

Koi also have no teeth in their jaws. They grind their food with pharyngeal teeth, located at the back of the throat, which rub against a hard plate. You can sometimes hear them crack when the fish eat hard pellets.

Practical consequences:

• Several small meals are better than one large one. Without a stomach, a large portion is partly poorly digested and ends up in the water as waste.
• Digestion depends on temperature. Digestive enzymes and intestinal flora slow down sharply as the water cools. Below around ten degrees, switch to a highly digestible food, in small quantities. Below approximately 6 to 8 °C, most breeders stop feeding. These are rough guidelines, to be adjusted according to the condition of the fish.
• A swollen belly is not always a full belly. In a female during spring, it is often eggs. Asymmetrical swelling, or swelling accompanied by raised scales, bulging eyes, or loss of appetite, calls for veterinary advice.
• Excrement provides clues. Long, whitish, stringy feces in several fish may indicate an intestinal problem or an unsuitable diet.

Kidneys, heart, and internal organs: what the exterior reveals

The internal organs can never be observed directly, but certain external signs can provide clues about them. The koi's heart is simple: an atrium and a ventricle that send blood to the gills and then to the rest of the body. The kidneys, located along the spine, have a dual function. The front part contributes to the production of blood cells and immunity. The rear part filters the blood and produces abundant, very dilute urine: a freshwater fish constantly absorbs water through osmosis and must excrete it.

When this excretory system is overwhelmed by an infection, kidney damage, or severely degraded water quality, the fish swells. The scales stand up like a pine cone, and the eyes may become bulging. This condition, often called dropsy, is a sign of severe systemic disease. The prognosis is guarded, and immediate isolation is essential, if only to monitor the rest of the group.

Red streaks in the fins or on the belly indicate dilation of small blood vessels. They are seen in generalized bacterial infections, but also after exposure to ammonia or nitrites. Once again, the first response is to test the water.

Reproduction: distinguishing males and females

Knowing how to sex your koi helps interpret a swollen belly or chasing behavior in spring. Before 25 to 30 cm, or before two to three years of age, the distinction remains uncertain. After that, several signs can be considered together:

Small white bumps on males during the breeding season are sometimes mistaken for a disease, particularly white spots caused by a parasite. The tubercles are hard and aligned along the rays of the pectoral fins and on the operculum. A parasite's spots are spread across the entire body and accompanied by rubbing.

In females, spawning may not occur if conditions are unsuitable. The eggs are generally reabsorbed, but prolonged retention can lead to complications. A female that remains very round and lethargic in late summer warrants an examination.

Symptom → organ → action table

This table summarizes the most common correspondences. It does not replace a diagnosis, but it gives the right order of response: water first, isolation next, specialist advice if the condition does not improve.

Most of these situations have one thing in common: a water test performed within the hour provides decisive clues. Keep a kit close at hand and record the values. A simple log, with the date, temperature, and results, is a great help when you need to describe the situation to a professional.

Common pond cases, understood through anatomy

Common case: at the end of May, with water at 20 °C, several koi rub themselves and one remains at the surface. The gills and skin are the first affected. Rising temperatures reactivate parasites and increase oxygen consumption. The right sequence: additional aeration, test the water, then observe a fish in a tub. If the tests are good and the rubbing persists, a mucus and gill sample examined under a microscope will settle the question.

Common case: in November, a koi swims crookedly after a cold night. The swim bladder and slowed metabolism are to blame. No food, stable water, and possibly a sheltered, slightly warmer tank. Many of these seasonal disorders improve, but not all.

Common case: one fish swells over a few days while the others are fine. This suggests a general condition with water retention. Isolate it, check the main pond water, and seek advice. The rest of the group should be monitored closely for several weeks.

Also read

To identify the main diseases and their treatments: Koi carp diseases - How to treat them?.

READ THE ARTICLE

For more information on routine care, our article How to care for koi carp? covers feeding, water, and filtration throughout the seasons.

Frequently asked questions

— Why is my koi swimming head-down?

This sign most often points to the swim bladder: infection, compression by a nearby organ, sudden cooling, or a deformity. Isolate the fish in a shallow, well-oxygenated tank, stop feeding it for a few days, and seek veterinary advice if nothing changes.

— Is it true that koi carp have no stomach?

Yes. The esophagus leads directly into a long intestine. The koi digests food as it passes through, which requires small, frequent meals and highly digestible food when the water cools.

— How can I tell if my koi's gills are damaged?

Rapid, pronounced operculum movements, fish gathering near water inlets or at the surface, and pale or mottled gills during an examination are warning signs. Test the water and increase aeration without delay.

— How can you tell a male koi from a female?

After approximately 25 to 30 cm: the male is more slender, with larger pectoral fins and white breeding tubercles in season; the female is broader when viewed from above, with a rounder genital opening.

In summary

• The skin and its mucus are the first line of defense: a grayish film or rubbing indicate irritation.
• The gills breathe and eliminate ammonia: poor water quality affects them first.
• The lateral line and highly sensitive hearing explain koi responsiveness; loss of appetite is an early warning sign.
• The swim bladder explains tilted swimming; the prognosis varies depending on the cause.
• Without a stomach, the koi digests as food passes through: small meals and easily digestible food in cool weather.
• For any symptom: test the water, aerate, isolate if necessary, then seek specialist advice.

Last updated: October 3, 2026.