Image: Fish Dissection - Gills exposed
The gills of the Blue Mackerel.
- Gill filaments - the site of gas exchange
- Gill rakers - appendages along the front edge of the gill arch
- Gill arches - support the gills
- Stuart Humphreys
- © Australian Museum
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 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.
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.