Koi and Pond Fish Anatomy: What You Need to Know

January 31, 2023

Did you know your koi and pond fish have a simple heart with only two chambers? Should that even matter to you as an owner of pond with fish? Probably not, but you should be aware of koi and pond fish anatomy that actually does matter to you.

The Fins

Let’s start with the obvious. Koi and goldfish have the usual dorsal (top) fin, an assortment of bottom fins, and the caudal (tail) fin, as well as a few others.

The dorsal fin in koi is extremely vulnerable to ulceration and it’s not uncommon for the mildest of bacterial infections to erode a hole right in the middle of it! Once the ulcer has healed, the fin can remain incomplete. When purchasing a new fish, inspect the dorsal fin to make sure it’s whole.

The two pairs of pelvic fins are the pectorals and the ventrals. These are the fins that define the landmarks for injection of sick fish because it is the safest site with good absorption.

The Scales

Next up in learning about pond fish anatomy are the scales. The scales cover the body of the fish and are of variable size, depending on the location, and they overlap at five points. People tend to think the scales overlap on the obvious four sides, but the center of the scale is underlapped by the scale behind and in front of it.

When a fish gets a deep wound, they may dislodge or shed their scales. The dead tissue and loose scales that surround a wound can provide bad bacteria with fuel to continue to infect the fish. To avoid this, gently scrub the wound with a piece of gauze soaked in grocery store hydrogen peroxide. The term for this process is debridement. It’s not meant for all wounds and should not be overdone. Over-cleaning a wound removes cells that are trying to heal, so usually only one debridement is needed.

Fish do a good job of replacing lost scales over the course of several weeks after their loss. However, the replacement scale is of a finer (thinner) quality and sometimes unevenly marked compared to the original scales.

The Gills and Operculum

The gills are to a fish what your lungs are to you – a delicate organ system that exposes all your blood to oxygen. The gills are the same structure only they’re pushing the blood through capillaries that exchange with water, not air.

What most people don’t know is that the gills perform another important function – the excretion of fish waste in the form of ammonia. The gills are the most important waste excretion organ in the fish’s body. Damage to the gills by way of infection with bacteria, destruction via viruses, infestation with parasites, or just damage from medications or poor water quality, can impact the fish far more than just breathing.

The operculum is the thick boney covering on either side of the head and protects the gills. You may need to lift the operculum to examine the gills, but don’t lift it too far or it might tear. The gills of any sick or dead fish can be photographed to provide important forensic information after a fish disease outbreak.

The Eyes

Koi and goldfish don’t need eyes to live. In fact, there’s a genetic mutation that causes one out of ten thousand fish to hatch without any at all. If a fish loses one or both eyes through trauma, it can still find food perfectly well with their barbels, lips, and mouths which are loaded with sensory structures like taste buds.

There’s a fat cushiony blood supply to and from the eye called the choroid plexus, designed to protect the eye from any trauma. Sometimes the choroid swells due to a blow to the eye. This is temporary and the eye may sink back into the socket over the coming week. A popped eye for an indefinite period could be the sign of another problem that may require the help of a fish professional.

An Overview of the Guts

You’ll find koi and pond fish anatomy shows a very simple digestive tract. They have an esophagus that comes from the mouth and goes to the “stomach.” Their “stomach” is actually just a stretchy wide spot in the top of the intestine for food storage before it’s passed into the intestine.

Koi tummies hate to be full in cold water. When this occurs, the lining of the intestine is damaged and bad bacteria can get through the damaged lining into the blood stream. The fish show no obvious signs of this, but may die later in the spring when the water warms and the bacteria go to work on the poor fish. This is one reason we recommend that you not feed your fish when water temperature reaches 50-degrees Fahrenheit or below.

The Air Bladder

The air bladder is an amazingly delicate structure. It fills with air via a thin veil of capillaries extending over its surface, and air is released by way of a thin tube that comes from the caudal sac. The air is burped into the esophagus, which then escapes through the mouth. The air bladder is balanced to the weight of the fish against the water, which is the primary means by which the fish can hang in the water without paddling the whole time.

If a fish is floating upside down on the surface, something has happened to the air bladder’s ability to let air out, so it is then too large and too buoyant for the weight of the fish. A problem with the air bladder can also cause the fish to sink to the pond bottom. This usually occurs when the spinal cord is damaged near the point where the nerves that regulate the air bladder emerge. Sometimes the air bladder can be removed or surgically corrected, enabling the fish to swim normally.

The Skeleton

Fish are boney, and ornamental pond fish are no exception, as you’ll see in the pond fish anatomy diagram. Unlike the bones of sharks and stingrays, their bones are truly bones and not cartilage. The bones of a fish are not meant for bearing weight because, in water, the fish is pretty much weightless. The two principal stresses on the fishes’ bones are hydrostatic pressure from the water, and the push and pull of the fishes’ mighty muscles on those bones.

That’s why, when you net a fish and carry it in that net, you’re putting a unique force on their skeleton which can damage them. The fish is bent into a u-shape and its full weight torques the skeletal bones. Broken backs are a common result. Instead, use your net to catch the fish and then slide a big plastic bowl under the fish to carry it.

If a fish suffers from a broken back, the cure is simply time. The fish may compensate for the injury – even if crooked from that day on – or it may simply starve to death.

The Muscles

Fish muscles are different from your muscles. If you’ve ever seen a fish filet, you remember that there are red and white lines in the meat. The muscles of a fish are oriented in thick bands called somites. These bands are stacked all down the sides of the fish in thick, orderly rows. Let’s go back to carrying fish in nets to find out why this matters. When the process causes damage to the back, it also destroys at least some of the nerves to at least one of the bands of muscle.

Whenever a somite dies, the muscle gets smaller and a kink in the fish will be seen. The concave side of the bend is the side with the dead somite. If you don’t carry fish around in nets, this is unlikely to ever have been seen in your collection.

Koi Teeth

In the back of the koi throat, emerging from the lower gill arch in the back, there are three to four molars. These molars have serrations on the top like your own molars. They’re broad, crowned teeth used to destroy shells and pulverize insects and crustaceans scavenged from the pond bottom. These teeth are shed and replaced continually through the life of the koi. They are too far back for you to ever be bitten by a koi.

Hopefully you now feel a little more confident in understanding koi and pond fish anatomy and how it relates to your care for them. Finned friends are just as important as furry friends but are often less understood since you can’t hold and cuddle them. Enjoy watching your fish and familiarize yourself with their behavior. That way, it will be easier for you to identify when something might be amiss.

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