Anatomy of a Pond Fish
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 The Goldfish
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For illustrative purposes a goldfish is being demonstrated, but the positioning and terminology for the organs of most fish that are likely to be kept in a garden pond are the same as for the goldfish.
Adipose Fin Present in some fish, but not goldfish, and situated between the dorsal and caudal fins. It is small and fleshy, and believed to contribute to the overall stability of the fish.
Anal Fin This is the short fin which is positioned just above the vent. It assists in maintaining the stability of the fish.
Barbel Pendent appendages adjacent the mouth of certain fishes like carp. Sensory organs used for detecting food, but also very sensitive to chemicals in the water.
Caudal Fin The tail fin. This is used primarily for propulsion.
Caudal Peduncle The section of body between the vent and the caudal fin which provides support for the caudal fin.
Dorsal Fin The dorsal fin runs down the back of the fish and assists in maintaining its balance.
Eye The eyes are more rounded in fish than in mammals because of refraction in the water. Focus is achieved by moving the lens in and out.
Gall Bladder This organ is situated close to the liver and stores bile which it release to help with digestion.
Gills These are located beneath the operculum. Water is drawn through the mouth and distributed over the gills and then out through the operculum. As water flows across the gills, oxygen is absorbed by the capillaries.
Gonads These are the reproductive organs, which in the male produce the milt and within the females the eggs or roe. Both are clearly visible in a dissected fish.
Heart As with all creatures, this functions by circulating the blood through the body, thereby delivering oxygen and nutrients to the various organs where they are required.
Kidney The kidney filters fluid wastes from the blood, which are then passed out of the body. The kidney also regulates water and salt concentrations within the fish. Its structure is unlike that of a mammal, being long and extending the length of the abdominal cavity of the fish.
Lateral Line A sensory organ comprising fluid filled sacs and a series of pores which appear as a line along each flank of a fish. Its function is to sense water pressure, currents and movement.
Mouth The mouth, its size and position, give an indication of the manner of feeding and the food that a fish is likely to naturally eat. Fishes have a tongue, but this is attached firmly to the floor of the mouth and cannot protrude.
Mucus (also referred to as Cuticle or Slime Coat). This is a slimy substance which is secreted by mucus glands scattered across the body. It reduces drag or friction when the fish is swimming and protects the fish to some degree from fungal and bacterial infections.
Nostrils (also referred to as Nares) The nostrils and used to detect odours. They are blind sensory pouches and do not connect with the mouth.
Operculum The gill cover. A flexible bony plate that protects the sensitive gills.
Pectoral Fins These are fins that appear on the “chest” of the fish and assist with controlling the direction of travel. They also enable the fish to effectively “tread water” and maintain a hovering position. They are also used for “braking”.
Pelvic Fins These are positioned below the pectoral fins and assist with controlling the direction of the fish and also help it to maintain a hovering position. They are also used for “braking”.
Pyloric Caeca An organ which is still little understood, but is believed to assist with digestion. It is positioned close to the stomach and comprises several finger-like projections.
Ray An internal fin support structure.
Scales These are the protection for the body of the fish. They are composed of connective tissue covered with calcium. They are only developed during the first year of life and get progressively larger. They are mostly arranged like tiles on a roof. Scales can be used for discovering the age of a fish.*
Scale Colours These are very important, especially with goldfish and koi. The absence or presence of various pigments or combinations of pigments produce the colour. An absence of pigments results in a white fish. The coloured pigments cells are called; melanophores-black; xanthophores-yellow; erythrophores-red. Fish with metallic scales also contain iridocytes. These are cells which contain crystalline guanin, a reflective and usually silvery substance.
Skin The skin of a fish comprises three distinct layers which are not all necessarily present over the entire fish. The soft and shallow epidermis contains the mucous glands. The dermis comprises the scales, while the hypodermis is a fatty layer between the epidermis and the muscles.
Spine As with other creatures this is the principle support structure within the body. It connects the head with the tail and is composed of numerous hollow vertebrae through which the spinal cord passes.
Spinal Cord The spinal cord is the central part of the nervous system and passes through the hollow vertebrae of the spine, connecting the brain with the rest of the sensory organs.
Spleen An organ which produces lymph cells and stores red blood cells.
Swim Bladder A gas-filled organ that assists with the buoyancy of the fish. Without a properly functioning swim bladder a fish will sink.
Vent This is situated next to the anal fin and is where the fish excretes and also passes milt or eggs during fertilisation.
* Scales can provide the age of a fish. The annulis or growth rings are like those of trees and counted outwards from the focus (the point where the growth starts). By measuring the distance between the annulii, along a radius, the growth rate of a fish can be estimated.
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