Biology of Fish

Biology of Fish


Written by: Robert B. Moeller Jr., DVM
March 22, 2000 part I

The author of this lecture wishes to thank Drs. Floyd, Bowser, Plumb, Herman, Wolke, Migaki, Harshbarger, Schmale, Smith, Colorni Leard, Chen, Muench, Hedrick and Wedemyer for supplying photographs of the various fish diseases to the Registry of Veterinary Pathology.


Fish have some unique anatomical and physical characteristics that are different from mammals, however, they still possess the same organ systems that are present in other animals. All fish are poikilothermic and must be able to adapt to changes in water temperature. Fish live in a variety of temperatures ranging from less than 0C to hot geothermal springs. Yet, each species of fish must live in its particular specific temperature range. Abrupt temperature changes in the water can be lethal to fish.

Organ systems of fish vary to some extent from that of mammals due to the aquatic environment they live in. The following are some of the important differences.


Fish do not have a keratin layer over the epidermis. These animals are covered by a cuticle composed of mucus, mucopoly-
saccharides, immunoglobulins and free fatty acids. The epidermis is composed of a stratified squamous epithelium of variable thickness (4-20 cells thick). The outermost epidermal cells (Malpighian cell layer) retain the capacity to divide. Other cells present in the epidermis are goblet cells (responsible for secreting the cuticle), large eosinophilic club cells or alarm cells (present in most species of fish), eosinophilic granular cells (unknown function), leukocytes and macrophages.

The dermis is composed of an upper stratum spongiosum and a deeper stratum compactum. Numerous melanophores, xanthophores, and iridophores (give fish their silvery color) are observed scattered throughout the dermis. Scales are calcified plates originating in the dermis and covered by the epidermis. There are two types of scales: ctenoid scales and cycloid scales. Ctenoid scales of elasmobranchs have spicules extending from the external surface giving these fish a rough sandpaper-like texture. Cycloid scales of teleost fish have a smooth outer surface and are laid down in concentric rings which makes them useful in determining the age of some fish. Scales also represent a source of calcium for fish; some fish will utilize the calcium in the scales in preference to the calcium in their skeleton during times of starvation or prespawning activity.


The gills consist of four holobranchs which form the sides of the pharynx. Each holobranch has two hemibranchs projecting from the gill arch. The hemibranch are composed of rows of long thin filament called primary lamella. The primary lamella have their surface area increased further by the secondary lamella that are semilunar folds over the dorsal and ventral surface. Gas exchange takes place at the level of the secondary lamella. These are lined by epithelial cells bounded by pillar cells. A thin endothelial lined vascular channel lies between the pillar cells and is the site of gas exchange, removal of nitrogenous waste and some electrolyte exchange.

The pseudobranch lies under the dorsal operculum. This organ is a gill arch with a single row of filaments. The function of the pseudobranch is unknown, however it is believed that this structure supplies highly oxygenated blood to the optic choroid and retina and may have thermoregulation and baroreceptor functions.


Adrenal Gland

There is no true adrenal gland present in most fish (exception is sculpins). The adrenal cortical tissue is represented by the interrenal cells. These cells are pale eosinophilic cuboidal cells associated with major blood vessels in the anterior kidney. Both glucocorticoid and mineralocorticoid are secreted.

The adrenal medullary cells (chromaffin cells) may vary in location. These cells are usually found with the sympathetic ganglia in clumps between the anterior kidney and spine or in the interrenal tissue.

Thyroid Gland

The thyroid follicles are very similar to mammalian thyroid tissue. Thyroid follicles are distributed throughout the connective tissue of the pharyngeal area and may be observed around the eye, ventral aorta, hepatic veins and anterior kidney. It is important to realize that thyroid tissue can be widely distributed. Many times pathologist have erroneously considered this distribution of normal thyroid tissue to represent metastasis from a thyroid follicular cell tumor.

Endocrine Pancreas

The endocrine pancreas is present in most fish as islet of Langerhans and is associated with the exocrine pancreas. In some species the islets are very large and may be grossly visible (Brockman bodies). During the spawning season the size and number of islet will increase in some fish. These should not be confused with an adenoma.

Parathyroid Glands

The parathyroid glands are absent in fish, their function is taken over by other endocrine organs. (Corpuscles of Stannius)

Ultimobranchial Gland

This gland lies ventral to the esophagus in the transverse septum separating the heart from the abdominal cavity. This organ secretes calcitonin (lowers serum calcium levels) which acts with hypocalcin (secreted by the corpuscles of Stannius) to regulate calcium metabolism.

Corpuscles of Stannius

These are islands of eosinophilic granular cells located in paired organs on the ventral surface of the kidney. This organ secretes a protein called hypocalcin (teleocalcin) which acts with calcitonin to regulate calcium metabolism.


This is a neurosecretory organ found on the ventral aspect of the distal end of the spinal cord. These bodies are composed of unmyelinated axons terminating on a capillary wall. The function of the urophysis is unknown.

Pineal Gland

The pineal gland is a light sensitive neuroendocrine structure which lies in the anterior brain and is a well vascularized organ. This gland secretes melatonin which may play a role in controlling reproduction, growth, and migration.


The digestive system of fish is similar to the digestive tract of other animals. Carnivorous fish have short digestive tracts when compared to herbivorous fish. The stomach and intestines contain submucosal eosinophilic granular cells. The function of these cells is unknown. Some species of fish (Salmonids) have pyloric ceca which are occasionally confused with parasites. These ceca secrete the digestive enzymes required to digest some food. Fish without the pyloric cecae have digestive enzyme production in the liver and pancreas. It is not possible to divide the intestine into large and small intestine.

The liver does not have the typical lobular architecture that is present in mammals. In many species of fish there are areas of exocrine pancreas (hepatopancreas) that are present near the small veins off the hepatic portal vein.

The pancreas is scattered in the mesentery, primarily near the pylorus.


Fish do not have lymph nodes. Phagocytic cells are present in the endothelial lining of the atrium of the heart and in the gill lamella. There are no phagocytic cells (Kupffer cells) in the liver. Melanomacrophage centers are present in the liver, kidney and spleen. Melanomacrophage centers increase in number during disease or stress.

The fish thymus is the central lymphoid organ. This organ is located subcutaneously in the dorsal commissure of the operculum.

Fish have the ability to produce specific immunoglobulins (IgM only) and have both delayed and immediate hypersensitivity. Fish have the ability to produce virus neutralizing, agglutinating, and precipitating antibodies. Both B and T lymphocytes are present.


The heart is composed of two chambers; one ventricle and one atrium. Some authors also describe the sinus venosus as the third chamber and bulbus arteriosus as the fourth chamber. Blood flows from the heart through the ventral aorta and the afferent branchial arteries, to the gills for oxygenation. Oxygenated blood returns via the efferent arteries to the dorsal aorta. The dorsal aorta then carries the oxygenated blood to the body. Some oxygenated blood also leaves the dorsal aorta and goes to the pseudobranch to be highly oxygenated and then is sent to the retina which has a high oxygen demand.


The kidneys of fish develop from the pronephros and mesonephros. The function of the kidney is osmoregulation. In freshwater fish, the kidney saves ions and excretes water. In saltwater fish, the kidney excretes ions and conserves water. The majority of nitrogenous waste is excreted through the gills. The other function of the kidney is hematopoiesis with hematopoietic tissue located in the interstitium of the kidney. This function is primarily in the anterior kidney but can be found throughout the entire kidney.


Lateral line system

There are two types of lateral line organs. These are the superficial neuromast and the two lateral line canal organs. There are two types of superficial neuromast, these are located in pits in the epidermis located primarily on the head. Their function is not completely known but is believed to aid in movement and orientation.

The second lateral line organ is the lateral line canal system which runs the entire length of the fish with continuous extensions over the head. This organ is sensitive to hydrostatic stimuli and sound.

Viral Diseases

1. Lymphocystis Disease
  • A) Iridovirus
  • B) Observed in most freshwater and saltwater species.
  • C) Clinically, fish are presented with variably sized white to yellow cauliflower-like growths on the skin fins and occasional gills. Occasionally, this virus may go systemic with white nodules on the mesentery and peritoneum.
  • D) Histopathology: Fibroblast undergoes cytomegaly with many basophilic cytoplasmic inclusion bodies and a thick outer hyalin capsule. The inflammatory response is variable but is usually a chronic lymphocytic inflammatory infiltrate.
  • E) The disease gains entry through epidermal abrasions. The virus infects dermal fibroblasts.
  • F) The disease is self-limiting and refractory to treatment. Nodules may last several months and cause infected fish to be susceptible to secondary bacterial infections. Reinfection can occur.
2. Herpesvirus salmonis (Herpesvirus disease of Salmonids)
  • A) Herpesvirus
  • B) Disease is observed primarily in the fry of Rainbow trout.
  • C) Clinically the fish are lethargic with prominent gill pallor. Mucoid fecal casts are commonly observed trailing from vent.
  • D) Lesions: 
    1) Exophthalmus and ascites
    2) Low hematocrit and numerous immature erythrocytes
    3) Hemorrhage in eyes and base of fins
  • E) Histopathology:
    1) Multifocal areas of necrosis of the myocardium, liver, kidney, and posterior gut (leading to cast formation)
    2) Syncytial cells involving the acinar cells of the pancreas is considered to be a pathognomoni sign.
  • F) Transmission of the virus is believed to be direct.
  • G) Control is by avoiding exposing susceptible trout to the virus. If the disease occurs, raising the water temperature to 15C or more will minimize losses.
3) Channel Catfish Virus
  • A) Herpesvirus
  • B) Observed in fry or fingerling channel catfish (less than 10 gram weight) during the summer when water temperatures are above 22oC.
  • C) Clinically these fish usually show erratic swimming or spiraling followed by terminal lethargy. Mortality is very high.
  • D) Lesions:
    1) Hemorrhage at the base of the fins and skins;
    2) Ascites; exophthalmos; and pale gills;
    3) Kidneys swollen and pale with hemorrhage;
    4) Spleen is enlarged and dark red;
    5) Gills usually pale;
  • E) Histopathology: Multifocal areas of necrosis and hemorrhage
    are observed in the posterior kidney, liver, intestines, and spleen.
    F) Infection is direct with transmission of the virus in the water or feed. Piscivorous birds, snakes, or turtles may mechanically carry the virus from pond to pond. Transovarian transmission has not been conclusively demonstrated but is suspected. Survivors are persistently infected and become carriers for life.
  • G) Control of the disease is by sanitation, purchasing of virus free broodstock and lowering water temperature to less than 19C during an outbreak to lessen the mortality.
4) Epithelioma papillosum (Fish Pox)
  • A) Herpesvirus cyprini
  • B) Non-fatal disease is observed in carp and other
  • C) Lesions: Elevation of the epidermis with the formation of white to yellow plaques over the body of the fish. Healed lesions usually turn black.
  • D) Histopathology: There is epidermal hyperplasia with the epithelial cells occasionally demonstrating intranuclear inclusion bodies.
  • E) Transmission is unknown, however, it is probably direct.
5) Infectious Hematopoietic Necrosis (IHN)
  • A) Rhabdovirus
  • B) The disease is observed in the fry of trout (rainbow) and salmon (chinook and sockeye) with mortality up to 100%.
  • C) Clinical signs and lesions:
    1) Fish become lethargic or hyperactive.
    2) The fish become dark in color.
    3) Exophthalmus, abdominal distension, and fecal cast.
    4) Hemorrhage on skin and viscera primarily at base of fins, behind the skull, and above the lateral line.
    5) Anemia with pale gills.
    6) Surviving fish may develop scoliosis.
  • D) Histopathology: There is prominent necrosis of hematopoietic tissue including melanomacrophages of the kidney, red pulp of the spleen and hepatic parenchyma. Necrosis of the submucosal eosinophilic granular cells is considered pathognomonic for IHN. (This lesion is observed in other systemic viral diseases.) Intranuclear and intracytoplasmic inclusions are occasionally observed in acinar and islet cells of pancreas.
  • E) The virus is transmitted by direct contact with infected survivors or by feeding contaminated feed. The virus is probably shed in contaminated semen and eggs. The disease is most severe at 10C and rare at temperatures above 15C.
6) Viral Hemorrhagic septicemia
  • A) Rhabdovirus
  • B) Widespread and very contagious viral disease of
    rainbow trout. This is a serious disease of trout in
    Europe. Affects both Salmonid in fresh water and sea
    water. Disease occurs in temperatures below 14C.
  • C) Two forms of the Disease --- Acute and Chronic

    1) Acute disease: High mortality in affected fish.
    Fish have pale gills, dark body coloration, ascites, exophthalmus and erratic swimming behavior (spiraling). Hemorrhage is a common finding in the eyes, skin, serosal surfaces of the intestines and muscles. Necrosis of the hematopoietic and lymphoid elements of the anterior kidney and congestion and necrosis of the hepatic parenchyma are histopathologic findings.

    2) Chronic disease: See a slower prolonged mortality.
    Fish become lethargic, have pale anemic gills, darken skin coloration, exophthalmus, and distention of the abdominal cavity. Internal organs are commonly involved with splenomegaly, hepatomegaly, and swollen kidneys Turbot, sea bass, and Atlantic salmon are commonly affected by similar viruses.
  • E) Transmission is believed to be direct with contact of carriers and contaminated water and feed. Vertical transmission via the egg is not reported.
7) Spring Viremia of Carp (SVC) and Swim Bladder Infection virus (SBI)
  • A) Caused by several subtypes of Rhabdovirus carpio.
  • B) Disease occurs in carp and other cyprinids.
  • C) Clinical Signs and Lesions:
    1) Loss of coordination and equilibrium.
    2) Exophthalmus and abdominal distension (ascites).
    3) Inflamed and swollen vent.
    4) Edema and hemorrhage in many organs.
    5) In SBI see pronounced inflammation and hemorrhage of swim bladder.
  • D. Transmission: Virus shed in feces and found in
    contaminated eggs.
8) Infectious Pancreatic Necrosis (IPN)
  • A) Birnavirus
  • B) Affects most salmonids primarily rainbow trout and brook trout. IPN has also been implicated in disease among several nonsalmonid fish.
  • C) Clinical signs and lesions:
    1) IPN is characterized by a sudden explosive outbreak with high mortality.
    2) Affected fish become dark and rotate their bodies while swimming.
    3) Diseased fish usually have distended abdomens and exophthalmus.
    4) The presence of a gelatinous material in the stomach and anterior intestine is highly suggestive of IPN; mucoid fecal casts are common.
    5) Infected fish commonly have a low hematocrit and hemorrhage in gut, primarily in the area of the pyloric ceca.
  • D) Histopathology:
  • Histologically, there is necrosis of the pancreatic acini, gut mucosa, and renal hematopoietic elements. A moderate inflammatory infiltrate is usually observed around the pancreatic acini. Hyalin degeneration of skeletal muscle is also observed.
  • E) Virus can be transmitted vertically in the eggs.

Part II

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