Clinical anatomy and physiology of fish (Proceedings)


The goal of this session is to highlight the most significant anatomical and physiological differences between fish patients and the terrestrial and avian patients commonly seen by the exotic veterinarian. Many clinicians fear starting to practice aquatic medicine due to these differences in the anatomy and physiology of fish as compared to terrestrial animals.

Objectives of the Presentation

To introduce the clinician to the basics of anatomy and physiology in the fish patient and to highlight clinically significant points.

Key Points

     • Use clinically relevant examples to demonstrate the importance of this knowledge.

     • Encourage clinicians to start seeing fish patients.

Overview of the Issue

The goal of this session is to highlight the most significant anatomical and physiological differences between fish patients and the terrestrial and avian patients commonly seen by the exotic veterinarian. Many clinicians fear starting to practice aquatic medicine due to these differences in the anatomy and physiology of fish as compared to terrestrial animals. However fish are becoming more and more popular as pets and the corresponding need for medical attention for these animals is growing at a fantastic rate. This lecture is aimed at providing a baseline of knowledge in these areas to the clinician who has an interest in starting to see fish as patients.

Basic Anatomical Points

There are approximately 20,000 different fish species currently known to science, and obviously the anatomy and physiology of these many species is accordingly variable. The smallest known fish measures only10 mm (0.4 inches) in length, while the largest fish, the whale shark, measures 14 m (45 ft).

The fish liver is the most familiar organ with much similarity to the mammalian or avian liver. On the other hand, fish actually lack several organs that are found in mammals, e.g., pancreas, distinct adrenal glands, lymph nodes, lungs, bone marrow and parathyroids. Other organs may be present but are distinctively different in form and function to their mammalian equivalent, e.g., kidneys, gonads, skin, heart. Additionally, other anatomical features are present in fish but not in mammals or birds. These include fins, the lateral line organ, swim bladder, and the gills.


The fins are one of the most obvious anatomical features on the fish and they serve a multitude of tasks in addition to locomotion. The dorsal fin is usually used as a stabilizer during swimming but it can also be used during courtship rituals or for defense. Many species have sharp projections protruding from the dorsal fins and it is important to protect these from damage during handling as well as to take care to prevent them from injuring the handler. The pectoral fins are used to counteract forward motion during swimming and act as a 'brake'. The pelvic fins, as well as the anal fins, act as a stabilizers. In males the anal fin can differentiate anatomically into a male sex organ, the gonopodium. This organ is easily observed in many live-bearers e.g., guppies, mollies, and swordfish.


The skin of fish is similar to that of terrestrial animals with some significant differences. Just as in mammals, the skin should be considered an organ, in fish however it fulfills many more roles. In bony fish the scales are produced in the dermal layer and are composed of overlapping plates of bone. Epidermal cells cover these bony plates. The epidermis is approximately 6-8 cell layers thick, and the outermost layer of epithelial cells contain micro-ridges of uncertain function. In some species such as the glass fish, the collagen structure is arranged in a uniform pattern making it translucent, similar to the cornea of the eye. The slime, or mucus layer (glycocalyx), is a vital protective coating produced by glands in the skin. It has both antibiotic and antifungal activity and acts as the initial defensive shield against many pathogens. The skin can secrete nutrients for fry, and even produce cocoons to sleep in! When handling fish, it is therefore of outmost importance to protect this vital slime layer. For the clinician an excessive amount of slime can alert the examiner to the presence of a non-specific stressor to the fish. The skin of some fish also contains other specific cells, such as alarm cells (pheromoes) and taste buds.

Circulatory System

The hematopoetic tissue of fish consists primarily of the kidney, but also includes the spleen and the liver. Blood cells in fish are similar to those of the reptilian or avian patient. Normal blood cell parameters are lacking for many fish species but fortunately more and more data is being published. It is not possible to extrapolate fish normals from terrestrial models as a leukocyte count of only 10% for example might be normal in some fish. In addition some fish lack hemoglobin making the blood appear white! Lymph vessels exist but there are no separate lymph nodes. The heart is positioned just caudo-ventral to the gills. It consists of 4 chambers: the sinous venosus, one atrium, one ventricle and the bulbus arteriosus. These chambers are arranged linearly and blood circulates in a single circulatory pathway. The heart can be accessed for phlebotomy, however the preferred site is the caudal vein.

Urinary system

The gills and the kidney make up the major excretory organs of the fish. The end product of nitrogen metabolism in fish is ammonia. The gills can excrete up to 75% of the ammonia load. Some fish also have a urinary bladder. A more detailed discussion of the gills and kidney is covered in the physiology part of this lecture.

GI System

The barbels which many fish have, including koi, serve a double function. They are a tactile as well as a gustatory organ. Fish lack a muscular tongue; however, teeth and taste buds may be present in the oral cavity. Mouth size is the limiting factor in feeding. This makes the force feeding extremely difficult in certain species, e.g., the seahorse. The anatomy of the stomach varies significantly between species, and many species including the koi, do not have a distinct stomach at all. This is important to be aware of when force feeding a sick fish, as the amount given per feeding in these species needs to be very small. Moreover, while some fish have a fully functional stomach, certain other stomachs lack histological differentiation and serve purely as a storage organ. In general herbivorous fish have longer GI tracts than carnivorous fish. Since fish are poikilothermic digestion is very dependant on the environmental (water) temperature.


In this lecture we will focus on only two physiological processes, respiration and excretion. Understanding these two processes is extremely important to the general understanding of fish physiology. In the fish, respiration and excretion are very tightly linked and the gills are key players in both processes.


As in the terrestrial patient, oxygen exchange is the primary goal of respiration in fish. The physiological process of extracting oxygen from water is much more difficult than extracting oxygen from air. Two major factors are responsible for this. Water is approximately 800 times denser than air and contains only about 3% oxygen. In contrast air usually contains approximately 20 % oxygen. The process of respiration is extremely energy consumptive and the system can only work well if the fish is good physical condition and the environment contains adequate dissolved oxygen. In addition, the surface area of the gills is only about 6-10 times greater than the surface area of the entire body. This is a relatively small difference in comparison to the relative size of the lung as an exchange organ. In contrast, the lung surface is usually 100 times the body surface in mammals. Gas exchange happens in the secondary lamellae of the gills and is extremely efficient. This efficiency is achieved by the countercurrent flow of water and blood. The oxygen-poor venous blood moves opposite to the flow of the relatively oxygen-rich water. In this mechanism, water must flow constantly over the gills in order to keep the respiration effective. About 80% of the environmental oxygen is removed during respiration. In humans only 25% of the oxygen is usually removed from the air during respiration. In fish, anesthesia is achieved using these principles. The anesthetic agent is dissolved in water and anesthesia is maintained by keeping the medicated water flowing across the gills, even if the rest of the fish's body is out of water. The extreme effectiveness of the gills as gas exchange organs also makes them extremely vulnerable to toxic insult. Toxic substances can be accumulated in the fish's body up to 1 million times the concentration of the same substance in the water.


As mentioned above, the gills are also one of the main excretory organs. The gills excrete the majority of the ammonia while the rest of the waste products are excreted via the kidneys. The excretion of metabolic waste products is similar in all fish; however, the kidney and the gills play significantly different roles in fresh water fish in comparison to their roles in salt water fish.

The freshwater fish is hypertonic in comparison to the environment. As a direct consequence, water is constantly entering the fish's body via the gills and diluting the blood. Therefore in fresh water fish, the major role of the kidneys is to eliminate the excess water from the circulatory system. In addition, electrolytes must be conserved during the elimination process. Freshwater fish therefore have relatively large glomeruli within the kidney. The situation is exactly the opposite in marine fish. The salt water fish is hypotonic in comparison to the marine environment. Marine fish constantly need to drink water, as water is constantly lost from the gills into the environment. The major work of the kidney is therefore to conserve water and to eliminate electrolytes. For that reason certain marine fish have aglomerulic kidneys.

These basics concepts are the key to understanding fish physiology and are the key to the supportive care of the sick aquatic patient


While fish initially may appear to be difficult exotic patients due to their unfamiliar environment, this lectures points out how basic understanding of the fish anatomy and physiology make it easy for the clinician to start understanding the concepts of aquatic medicine. Once familiar with the basics, fish medicine offers a great opportunity to expand ones clinical expertise.


Online material available with many beautiful anatomical images (gross and histology):

Gratzek, John The Science of Fish Health Management: Master Volume (Aquariology Series), Tetra press

Related Videos
dvm360 Live! with Dr. Adam Christman
© 2024 MJH Life Sciences

All rights reserved.