The dissection of a Blue Mackerel, Scomber australasicus. The dissection was performed by Year 12 students at St Vincent's College, Potts Point, Sydney in May 1998, and photographed by staff of the Australian Museum Photographic Department.

The aim of the dissection was for students to become familiar with the internal organs of a fish, primarily the swim bladder. This site has been put together to assist students and teachers with similar dissections. Many of the organs that are encountered during a typical fish dissection are named and discussed.

Fish Internal Organs

Fish Dissection - Internal organs

Fish Dissection - Internal organs

Image: Stuart Humphreys
© Australian Museum

Some of the ventrally located internal organs.

  1. heart
  2. liver
  3. pyloric caecae
  4. adipose (fatty) tissue


The circulatory system in fishes is a single circuit, with blood flowing from the heart to the gills and then to the rest of the body. The heart is located a little behind and below the gills.

The typical fish heart has four chambers, however unlike mammals, blood moves through all four in sequence. Venous blood enters the sinus venosus (a thin walled sac) then flows into the atrium, followed by the ventricle (a thick walled pump). Blood then flows into the conus arteriosus (elasmobranchs) or bulbus arteriosus (teleosts) then to the gills and the rest of the body.

The heart of slow moving fishes is comparatively small, whereas active swimming species such as the Blue Mackerel have large hearts.


The liver has many digestive and storage functions. One of these is the production of bile, a solution which emulsifies fats and may assist in changing the acidic conditions of the stomach into the neutral pH of the intestine). The liver is also responsible in some species for the storage of fats, blood sugar, and vitamins A and D. Before it was possible to synthetically create vitamins A and D, sharks were caught for their livers which have high concentrations of these vitamins.

The liver acts as a food reserve and so changes with reproductive condition particularly in viviparous species. The livers of sharks in early pregnancy are large and lightly coloured. Those of individuals that have just given birth tend to be shrunken and darker coloured. Similarly, the livers of males vary depending upon mating activity (J. Stevens, CSIRO pers. comm.).

Pyloric caecae

Pyloric caecae (singular caecum) are fingerlike pouches connected with the alimentary canal (the gut). They are attached to the pylorus, the section of the intestinal tract immediately following the stomach. They range in number from three in a type of scorpionfish (Setarches) to thousands in the tunas (family Scombridae). The Blue Mackerel is a scombrid species with many long, thin pyloric caecae.

Pyloric caecae may have a digestive and/or absorptive function. The enzyme lactase has been found in the pyloric caecae of some fishes such as trout.

In some families, such as the Salmonidae (salmons and trouts), the number of pyloric caecae is an important character used to tell species apart.


  1. Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
  2. Lagler, K.F., J.E. Bardach & Miller R.R. 1962. Ichthyology. John Wiley & sons. Pp. 545.

Pull aside gut

Fish Dissection - Pull aside gut

Here the adipose tissue (1) and gut (2) are pulled aside to expose the swim bladder (3), gonads (4) and kidneys (5).

Image: Stuart Humphreys
© Australian Museum

In the above image see:

the adipose tissue (1)

and gut (2) are pulled aside to expose

the swim bladder (3),

gonads (4) and

kidneys (5).

As a general rule, carnivorous fishes like the Blue Mackerel have relatively short guts. Herbivorous fishes have much longer guts.

The gonads and kidneys are paired. One of each can be seen on both sides of the swim bladder.

The Gonads

The sexes of fishes are usually separate. Males usually have paired testes that produce sperm, and females usually have paired ovaries that produce eggs.

When paired, such as in the Blue Mackerel, the gonads lie on either side of the swim bladder. The method by which the eggs and sperm meet and thus fertilisation occurs varies widely among fishes. Many species are broadcast spawners, shedding their eggs and sperm into the water to fertilise external to the body. Other species such as sharks and rays have internal fertilisation where the sperm are released into the body of the female. Many variations exist, including the seahorse, in which the female deposits her eggs into the pouch of the male where they are fertilised.

The hagfishes and lampreys have a single ovary or testis. Sperm and eggs are shed into the body cavity and out through a urogenital papilla.

The Kidneys

The kidneys are paired organs located in the body cavity ventral to (below) the vertebral column. They are one of the organs involved in excretion and regulation of the water balance within the fish.

Freshwater and marine fishes are faced with different problems with regard to regulating the concentration of salts within the body. Their kidneys differ considerably in structure. Freshwater fishes have larger kidneys than marine fishes. They have a higher concentration of salts in the body tissues than the surrounding water. Conversely marine fishes have a lower concentration of salts in the body tissues than the surrounding water.

The kidneys of freshwater fishes remove water and re-absorb salts and sugars. They produce large amount of very dilute urine. This helps the fish avoid becoming "waterlogged" from the large amounts of water diffusing into the fish.

The kidneys of marine fishes however conserve water. Marine fishes drink water and excrete only a small volume of very concetrated urine.

In most fishes, the gills and gut are largely responsible for the excretion of surplus salts.


  1. Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
  2. Lagler, K.F, J.E Bardach & Miller R.R. 1962. Ichthyology. John Wiley & sons. Pp. 545.

Swim bladder exposed

Fish Dissection - Swim bladder exposed

Fish Dissection - Swim bladder exposed. The other organs have been removed to expose the swim bladder at the top of the body cavity.

Image: Stuart Humphreys
© Australian Museum

Swim Bladder:

The swim bladder (also called the gas bladder or air bladder) is a flexible-walled, gas-filled sac located in the dorsal portion of body cavity. This organ controls the fish's buoyancy and in some species is important for hearing. Most of the swim bladder is not permeable to gases, because it is poorly vascularised (has few blood vessels) and is lined with sheets of guanine crystals.

A fish swimming in the water expends less energy if it is neutrally buoyant (that is, it neither sinks nor floats). If this fish starts to descend, the increased pressure from the water surrounding the fish results in a compression of the gas inside the swim bladder. The fish becomes negatively buoyant and will tend to sink. Conversely, if a fish swims into shallower water, there is a decrease in water pressure and so the gas in the swim bladder expands, and the fish tends to float upwards. The swim bladder helps to solve the problems associated with variations of pressure, and thus buoyancy.

If the fish becomes positively buoyant, and starts to float upwards, gas diffuses out of the swim bladder into the blood. This occurs at a site known as the oval. The gas in the blood is then removed from the body into the surrounding water at the gills.

Conversely if the fish becomes negatively buoyant, and starts to sink, air enters the swim bladder at a region called the gas gland. The way the fish does this involves three processes; the acidification of the blood, an increase in the concentration of lactate and hydrogen ions and the movement of blood through a complex structure called the rete mirabile (literally, the wonderful network). These complex processes are not discussed here. Refer to the reference below for more information.

Not all fishes have a swim bladder. Sharks for example do not have a swim bladder, and many species such as the Greynurse Shark, use a different strategy which includes having a large oily liver and specialised body shape to maintain buoyancy.


  1. Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp 528.

Fish Dissection - The Gills

Fish Dissection - Gills exposed

Fish Dissection - The gills of the Blue Mackerel.

Image: Stuart Humphreys
© Australian Museum

Gill filaments:

Just like the lungs of humans, the gills of fishes are the sites where oxygen is absorbed and carbon dioxide is removed. In addition, the gills are responsible to a varying degree for regulation of the levels of various ions and the pH of the blood.

The gill filaments of bony fishes (also known as a primary lamellae) are complex structures which have a large surface area. Off each are numerous smaller secondary lamellae. Tiny blood capillaries flow through the secondary lamellae of each gill filament. The direction of blood flow is opposite to that of water flow. This ensures that as the blood flows along each secondary lamella, the water flowing beside it always has a higher oxygen concentration than that in the blood. In this way oxygen is taken up along the entire length of the secondary lamellae.

Active swimming fishes, such as the Blue Mackerel have well developed gill filaments to maximise the amount of oxygen that can be absorbed. Less active, bottom-dwelling fishes generally have much smaller gill filament volumes.

Not all fishes rely totally on their gills to breathe. Some species, especially when they are young, absorb a large proportion of their oxygen requirements through the skin. Others species have well developed lungs for breathing air, and will in fact drown if they do not have access to the surface.

Gill rakers:

Gill rakers are bony or cartilaginous projections which point forward and inward from the gill arches. They aid in the fish's feeding. Their shape and number are a good indication of the diet of the fish. Fishes which eat large prey such as other fishes and molluscs have short, widely spaced gill rakers. This type of gill raker prevents the prey item from escaping between the gills. The gill rakers of the Blue Mackerel are like this.

Fishes which eat smaller prey have longer, thinner and more numerous gill rakers. Species which feed on plankton and other tiny suspended matter have the longest, thinnest and most numerous gill rakers, with some species having over 150 on the lower arch alone.

Gill arches:

Most fishes have gill arches. They are the boomerang-shaped bony or cartilaginous structures that support the gills. Each gill arch comprises an upper and a lower limb that are joined posteriorly. Attached to the gill arches are the gill filaments and gill rakers.

The gill arches not only provide support for the gills but also their associated blood vessels. Arteries entering the gills (the afferent branchial arteries) contain blood that has a low concentration of oxygen and a high concentration of wastes. Arteries leaving the gills (the efferent branchial arteries) carry blood rich in oxygen and low in wastes.

Additional information


  1. Helfman G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
  2. Lagler, K.F, J.E Bardach & Miller R.R. 1962. Ichthyology. John Wiley & sons. Pp. 545.
  3. Michael, S.W. 1998. Reef Fishes. Volume 1. A Guide to Their Identification, Behaviour, and Captive Care. Microcosm. Pp. 624.