Fish Anatomy

by Karen Paul

External Anatomy

A. caudal fin - B. dorsal fin - C. lateral line - D. nostril - E. barbel
F. operculum - G. pectoral fin - H. pelvic fin - I. vent - J. anal fin

Body

Body shape is the best indicator of the natural environment of a species. Tall narrow species such as Angelfish and Discus usually live in slow moving water. Torpedo-shaped species are adapted for living in faster moving water. Bottom dwelling species tend to have flattened undersides and some have modified swim bladders to keep them on the bottom.

Head

A. terminal - B. superior - C. inferior

The position of the mouth gives an indication of the feeding habits of a species. Mid water feeding species usually have a 'normal' forward pointing (terminal) mouth. Surface feeding species generally have an upturned (superior) mouth. Bottom feeding species usually have a down turned (inferior or sub terminal) mouth and often have barbels (slender, tactile sensory organs used for locating food in dark or muddy water). Fish in the Loricariidae family have a sucker mouth which is adapted for grazing on algae, plants and wood.

Fins

The fins are primarily used for propulsion, steering, stability and balance. In many species the fins are also used during spawning and for nest building.

The fins of bony fish may have spines or rays. Spines are generally bony, stiff and unsegmented. Rays are generally soft, flexible, segmented and are sometimes branched. The main difference between rays and spines is the segmentation of rays, spines may be flexible but they are never segmented.

Fins are either median or paired. Median fins are situated along the center-line (median) of the body while the paired fins correspond to the arms and legs of land animals.

Caudal Fin

A. heterocercal - B. protocercal - C. homocercal - D. diphycercal

The caudal (tail) fin is used for propulsion. The types of caudal fins are:

• Heterocercal: the vertebrae extend into a larger lobe of the tail.

• Epicercal: the upper lobe is larger, as in sturgeons and many sharks.
• Hypocercal: the lower lobe is larger, as in flying fish.

• Protocercal: the tail extends around the end of the vertebral column, as in embryonic fish and hagfish.
• Homocercal: the most common type of caudal fin in bony fish. The tail is symmetrical and extends beyond the end of the vertebral column.
• Diphycercal: the spinal column extends horizontally all the way to the end of the tail fin, dividing it into two equal parts. Characteristic of lungfish, several other primitive fishes, and the juvenile stage of modern bony fishes.

Dorsal Fins

The dorsal fin may consist of one or more fins located along the back of the fish and helps to maintain balance. Some species have dorsal fins with defensive spines, sometimes containing venom. The adipose fin is a small fleshy fin located on the back of some species (Salmonidae, Characidae and catfish families) behind the dorsal fin near the tail.

The adipose fin on a trout (Salmonidae family)

Pectoral Fins

The paired pectoral fins, corresponding to the forelimbs of higher vertebrates, are located on each side usually just behind the operculum and are used for manoeuvring.

Some species have developed highly specialized pectoral fins. They can provide the upward lift (dynamic lifting) that helps some fish, such as sharks, to maintain depth and also enables the flight of flying fish and hatchet fish. The pectoral fins of some species have defensive spines. In other species, such as the mudskipper, the pectoral fins are adapted to be used for "walking".

Pelvic (ventral) Fins

The paired pelvic or ventral fins, corresponding to the hind limbs of higher vertebrates, are located between the pectoral and anal fins. The pelvic fins provide stability while swimming.

Anal Fin

The single median anal fin is located on the ventral surface, usually between the vent and the caudal fin and is primarily used for stability.

Skin, Scales and Scutes

Skin

The skin is composed of two layers, the outer epidermis and the inner dermis. The dermis contains dense fibrous connective tissue, collagen, blood vessels and pigment cells. The scales are embedded in the dermis and are covered by the epidermal layer. The epidermis is a thin layer containing epithelial cells and the unicellular mucus glands.

Colours and patterns are produced by the pigment cells, chromatophores, within the dermis. The cells are named according to the colour pigment they contain:

• Melanophores: Dark brown or black pigment called melanin.
• Erythrophores: Red, orange or brown pigment.
• Xanthophores: Yellow pigment.
• Iridophores: Contain guanine crystals which reflect light, giving the fish a metallic appearance.

Changes in colour are caused by the movement of melanin grains within the cells. The fish appears darker when the grains are dispersed because they absorb more light. When tightened the fish appears pale, as happens when a fish is stressed.

Scales

Cycloid scales on carp. A partially scaled ghost doitsu (mirror) carp and a fully scaled chagoi.

Fish scales are transparent and colourless, the colour comes from skin pigmentation below. Not all species of fish have scales. In some species the scales are so small that the fish appears to be scaleless. In some, such as mirror carp, the scales may be present only on parts of the body. The arrangement of the scales may be either imbricate (overlapping) or mosaic (fitting closely together).

There are four basic types of scales:

• Placoid: The tooth-like placoid scales, also known as dermal dentilcles, consist of a layer of enamel-like substance called vitrodentine, a layer of dentine, a pulp cavity, and a basal plate that is embedded in the skin. Unlike other scales, placoid scales do not increase in size as the fish grows but new scales are added. Sharks and rays have placoid scales.
• Ganoid: Ganoid scales are usually mosaic in arrangment and rhomboid in shape. They consist of a bony layer, a layer of cosmine and a layer of an enamel-like substance called ganoin. Sturgeons and gars have ganoid scales.
• Cycloid: Cycloid and ctenoid scales are the most sommon types of scales found on bony fish. They are imbricate in arrangement and round or oval in shape. They grow with the fish producing concentric growth rings which can be used to determine the age of the fish. Cycloid scales have a smooth outer edge. Carp have cycloid scales.
• Ctenoid: Similar to cycloid scales but with a spiny comb-like outer edge. Perch have ctenoid scales.

Scutes

Scutes are shield-like bony plates that are often seen on sturgeons and armour-plated catfishes.

Scutes on the dorsal ridge and flanks of a Diamond Sturgeon (Acipenser gueldenstaedtii)

Internal Anatomy

A. lateral line - B. spleen - C. kidney - D. swim bladder - E. Weberian ossicles
F. inner ear - G. brain - H. eye - I. gills - J. heart - K. liver - L. stomach
M. gall bladder - N. intestine - O. gonads: testes/ovaries

Lateral line

The lateral line consists of a series of pores along the side of the fish. The pores connect to sensory organs that can detect vibrations and pressure changes in the water.

Immune System

The spleen is an important part of the fishes immune system, producing antibodies to fight infection. White blood cells are produced and redundant red blood cells are destroyed in the spleen.

The kidneys are involved in waste excretion, filtering waste from the blood to produce urine. They regulate water and salt concentration in the body. The kidneys are also an important part of the fish's immune system, producing antibodies to fight infection.

Swim Bladder

The swim bladder is a gas-filled sac used for maintaining neutral buoyancy. The swim bladder in some fishes is connected to the inner ear and functions as a sound amplifier.

Weberian Apparatus

The Weberian apparatus consists of a set of small bones derived from the first four vertebrae, known as Weberian ossicles, that connect the inner ear and swim bladder. It is a characteristic of the superorder Ostariophysi (carps, minnows, loaches, catfish, characins etc) and serves to enhance hearing.

Gills

Gills are a highly efficient way of extracting oxygen from water. Water enters the gill chamber through the mouth and oxygen is absorbed by the blood in the gill filaments. The water then exits through the gill openings under the operculum (gill plate), a flexible bony plate that protects the gills.

Gills are more efficient than lungs. Human lungs are only able to extract about 25% of the oxygen from the air but gills can extract up to 80% of the oxygen in water as it passes over them.

A bony fish can breathe while motionless by opening it's mouth to draw in water and pumping it over the gills by drawing the sides of it's throat together.

Gills usually consist of arches, rakers and filaments

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Gill arches are curved bony or cartilaginous structures that support the gills.

Gill rakers are comb like structures that project from the front edges of the gill arches. They help when feeding by preventing prey from escaping through the gills. They vary in size, shape and density according to the fishes diet.

Gill filaments are where the oxygen is absorbed and carbon dioxide is removed. Each filament has tiny folds in it's surface, giving it a huge surface area. The tiny blood vessles in the filaments give the gills their red colour.

Skelleton

A. hypural - B. neural spine - C. vertebra - D. posteria dorsal fin ray - E. radial cartilage
F. anterior dorsal fin ray - G. opercular - H. skull - I. orbit - J. upper jaw
K. lower jaw - L. clavicle - M. pelvic girdle - N. pectoral fin ray - O. pelvic fin ray
P. rib - Q. radial cartilage - R. anal fin ray - S. hemal spine - T. caudal fin ray

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