External Anatomy

Every hobbyist should try to acquaint themselves with the external anatomy of the fish they keep. The names of various fins and parts of the body are constantly occurring in the description of the species. I will try to help with the identification and definition of the various fish parts in the following section.

Typical Characin
Typical Cichlid



(N) Nose or nares (L) Lateral line (D) Dorsal fin
(S) Adipose fin (O) Gill cover or operculum (C) Caudal peduncle
(T) Tail or Caudal fin (A) Anal fin (V) paired Ventral or pelvic fin
(P) paired Pectoral or breast fin


    1. The fins are made up of stiff rays covered by skin. Some may be jointed and some separate near the edge of the fin. In certain fish some of the rays supporting the find are bony, stiff and unjointed. They are referred to as spines. Almost half the fin rays in the Dorsal fin of Cichlids are bony spines. So the front (Anterior) portion of such a fin is called the Spiney Dorsal and the rear (Posterior) portion is called the Soft Dorsal. In Gobies and some other species the Spiny Dorsal and the Soft Dorsal are completely separated and form two distinct Dorsal Fins. The number of rays in the fin is also used in classification.

Fin Functions

    1. Each fin on a fish is designed to perform a specific function. I will list them here.
    • Dorsal fin. Lends stability in swimming.
    • Ventral fin. Serves to provide stability in swimming.
    • Caudal fin. In most fish, the Caudal or tail fin is the main propelling fin.
    • Anal fin. Also lends stability in swimming.
    • Pectoral fins. Locomotion and side to side movement.
    • Adipose fin. Stability.


    1. The skin of fish is divided into two layers, the Epidermis (outer) layer and the Dermis. The Epidermis is made up of Epithelial cells, arranged one above the other. These cells are constantly shed and replaced with new ones. Inter-spaced between the Epithelial cells are slime cells which produce Mucoid secretions that form the very important protective covering, we know as the slime coat. The Dermis lies under the Epidermis and many important functions happen there.


    1. The deeper place Dermis of the skin is made up of connective Fibroblasts, Collagen and blood vessels. The scales of a fish lie in pockets in the Dermis and come out of the connective tissue. Scales do not stick out of a fish but are covered by the Epithelial layer. The scales overlap and so form a protective flexible armor capable of withstanding blows and bumping.

There are four types of fish scales.

      • Placoid scales, also called dermal denticles, are similar to teeth in that they are made of dentin covered by enamel. They are typical of sharks and rays.
      • Ganoid scales are flat, basal-looking scales that cover a fish body with little overlapping. They are typical of gar and bichirs.
      • Cycloid scales are small oval-shaped scales with growth rings. Carps and similar fishs have this type of scale.
      • Ctenoid scales are similar to the cycloid scales, with growth rings. They are distinguished by spines that cover one edge. Cichlids have this type of scale.
    1. In the hobby most fish will have two main types of scales, Cycloid and Ctenoid. In the

Mailed Catfish

    1. the scales are replaced by bony plates. In some other species there are no scales at all.

Pigment (color) cells


      1. The many pretty colors and patterns seen in fish are produced by cells in the Dermis. The cells are named for the pigment they contain.
        • Melanophores Brownish-Black pigment called Melanin.
        • Erythrophores Red pigment.
        • Xanthophores Yellow pigment.
        • Iridophores Contain crystals which refract and reflect light, given many fish their metallic look.

Fish can change color from one moment to the next. This is caused by the movement of Melanin grains within each cell. When dispersed, they Absorb more light and the area of the fish darkens. When tightened the fish goes pale.


    1. Respiration is carried out by means of gills located under the gill covers. The walls of the Pharynx is perforated by five slit-like openings. The tissue between the slits is called the Gill arch, so on each side of the fish there are five Gill Slits and Four Gill Arches. On the Gill Arches are mounted the actual Gills, a delicate system of blood vessels covered by a very thin Epithelium through which the gaseous exchange takes place.

Lateral line

    1. The lateral line consists of a series of scales, each modified by a pore, which connects with a system of canals containing sensory cells and nerve fibers. It runs in a semi line from the gills to the tail fin. It can easily seen in fish as a band of darker looking scales running along the side. The Lateral line has shown to be a very important sensory organ in fish. It can detect minute electrical currents in the aquarium water. It can also function as a kind of echo location process that helps the fish identify its surroundings.

Other senses in fish

    1. Fish have the five senses man has as well as the Lateral line. In fish the importance of each sense is different than us. I will briefly describe them here.
    • Sight Vision underwater poses many special problems. The most significant is the small am out of light available in all bur the uppermost layers of water. Vision under water is limited to a few yards at best and fish do not use this as one of their primary senses.
    • Smell In most fish the sense of smell is highly developed and is probably used more in the location of food than sight.
    • Hearing It has been shown that fish can hear, but its full function is still not understood.
    • Taste Taste buds in fish are located in the mouth and also in the skin covering the head, body fins, Barbels and lips. Its entirely probable that fish can taste food well before it enters their mouth.
    • Touch Fish also have elevated tactile sense and is shown none better than in certain catfish who use their Barbels as extensions of their body.

Internal Anatomy

    1. The internal anatomy of a fish rarely comes into play for the average hobbyist. The most important item to have a basic knowledge of is the swim blad
    der, this organ helps keeps the fish stable in the water. Various diseases can affect this and in the process affect the health of your fish.