Image: Fish Dissection - Pull aside gut

Fish Dissection - Pull aside gut

Here the 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. View an image that shows the long gut of a Carp.

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.

Stuart Humphreys
© Australian Museum



  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.

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Tags fish, blue mackeral, students, Scomber australasicus, dissection, ichthyology, Scombridae,