One of the main attractions when building a garden pond is keeping fish. A shoal of small fish will add colour and movement to a smaller planted pond, or ornamental koi can be raised and trained to hand-feed. Not only do they enhance and enrich the pond and garden, fish become loved pets and take the water-gardening hobby to a whole new level. Although the external and internal anatomy of fish can vary greatly between species, there are a few common anatomical characteristics.

External Anatomy

Mouth – The position of the fish’s mouth reflects the area from which they gather their food. A fish with its mouth facing upward feeds from the water’s surface, fish with forward facing mouths feed while swimming and a fish with a downward facing mouth feeds from the bottom.

Some fish, such as Koi, have barbels around their mouths. A barbel is a whisker-like organ which houses taste buds, and are used to search for food in murky water.Mucus – Fish’s bodies are covered in a protective mucus layer, which is excreted from cells in the skin. The mucus contains various chemicals which prevent disease causing organisms from growing on the fish’s body. It also reduces drag when the fish is swimming and offers physical protection to the skin.Skin – A fish’s skin (or epidermis) is a very delicate membrane. It more closely resembles the lining of our mouths than it does our skin. The skin prevents bacteria from entering the body as well as regulating the movement of salt into and out of the fish’s body. The epidermis covers the fish’s scales.Scales – Most fish have scales, which are made of a bony material and grow from deep pockets under the skin. They help to physically protect the body whilst retaining flexibility.

Gill Cover – The gill cover, or operculum, is a hard, bony flap, much thicker than a scale which protects the fish’s delicate gills.Lateral Line – Fish have a sensory organ called the lateral line, which is usually visible as a faint line running down the side of a fish. This is used to detect movement and vibrations in the environment.In mammals, the lateral line has developed into the inner ear.

Fins – Fins consist of bony rays connected by a thin membrane. They are used for propulsion, manoeuvring, and in some species, for display. The different fin types are listed below;

• Pectoral Fins (Breast Fins) – The pectoral fins are paired and are used for side-to-side movement and locomotion.
• Pelvic Fins (Ventral Fins) – The pelvic fins are also paired and are used for moving the fish up and down in the water, turning sharply, and stopping quickly.
• Dorsal Fins - The dorsal fin serves to protect the fish against rolling, and assists in sudden turns and stops.
• Anal Fin – The anal fin is used to stabilise the fish when swimming.
• Adipose Fin – This is a soft, fleshy fin found between the dorsal and caudal fins on most fish. The fin possibly functions as a flow sensor, but this has not been proven.
• Caudal Fin (Tail Fin) – The fish’s tail fin is it's main means of propulsion.

Internal AnatomyThe positioning, shape and appearance of most of the fish’s organs differs from species to species, but their functions are roughly the same.

Central Nervous system – Fish are vertebrates; animals with a backbone through which their spinal chord runs. Electrical signals run along the spinal chord between the brain and the rest of the body. These signals pass information from the sense organs and transmit movement into the muscles, etc.Swim Bladder – The swim-bladder (1.), or gas bladder, is a sack filled with air which aids buoyancy. The fish can regulate the amount of air in the swim-bladder giving neutral buoyancy at different depths, and saving energy.Respiratory and Circulation – The gills (2.) are the fish’s main respiratory organ, they are the equivalent of lungs in a terrestrial animal. They absorb oxygen from the water and excrete carbon dioxide. Also, up to 90% of the ammonia released by a fish as waste is through it's gills. The absorbed oxygen is passed into the fish’s blood which is pumped around its body by the heart. The blood carries oxygen and nutrients to the tissues and organs, and picks up waste materials, which are processed by the kidneys (3.) for excretion.Digestion – Food passes into the fish’s mouth, through the oesophagus and into the stomach (4.) where it is broken down. Enzymes are released by the pyloric caeca (5.) to aid this break down. The food then passes into the intestines (6.) where further enzymes are added from the liver and pancreas (7.). Nutrients are absorbed through the intestinal wall and passed into the blood stream. Waste material, undigested food and old stomach lining are passed out through the anal vent (8.).Reproductive Organs – The reproductive organs (9.) are positioned in similar areas in both male and female fish. Pond fish are egg scatterers. Female fish produce a large number of small eggs in their ovaries. These eggs are released into the water and become attached to vegetation where male fish then release sperm over the eggs to fertilise them.